tim-brookes

Dr Tim Brookes


Senior Lecturer in Audio, Director of Research (IoSR)
BSc, MSc, DPhil, MAES

Academic and research departments

Department of Music and Media.

Biography

University roles and responsibilities

  • Director of Research and Admissions Tutor for Postgraduate Research Programmes in the IoSR
  • Assistant Senior Professional Training Tutor for the Tonmeister Programme
  • IoSR Webmaster

Research

Research interests

My teaching

Supervision

Postgraduate research supervision

My publications

Publications

The IoSR is responsible for world-class research in audio-related subject areas, and offers postgraduate research-based MPhil and PhD programmes, as well as being home to the world-famous Tonmeister" BMus undergraduate degree course in Music & Sound Recording. Since the creation of the Institute of Sound Recording (IoSR) in 1998 it has become known internationally as a leading centre for research in psychoacoustic engineering, with world-class facilities and with significant funding from research councils (in particular EPSRC) and from industry (we have successfully completed projects in collaboration with Adrian James Acoustics, Bang & Olufsen, BBC R&D, Genelec, Harman-Becker, Institut für Rundfunktechnik, Meridian Audio, Nokia, Pharos Communications and Sony BPE). Additionally, the IoSR was a founding partner in the EPSRC-funded Digital Music Research Network (DMRN) and Spatial Audio Creative Engineering Network (SpACE-Net). We are interested in human perception of audio quality, primarily of high-fidelity music signals. Our work combines elements of acoustics, digital signal processing, psychoacoustics (theoretical and experimental), psychology, sound synthesis, software engineering, statistical analysis and user-interface design, with an understanding of the aesthetics of sound and music. One particular focus of our work is the development of tools to predict the perceived audio quality of a given soundfield or audio signal. If, for example, a new concert hall, hi-fi or audio codec is being designed, it is important to know how each candidate prototype would be rated by human listeners and how it would compare to other products which may be in competition. Traditional acoustic and electronic measurements (e.g. RT60, SNR, THD) can give some indication but a truly representative assessment requires lengthy listening tests with a panel of skilled human listeners. Such tests are time-consuming, costly and often logistically difficult. The tools that we are developing will describe the quality of the prototype without the need for human listeners. An introduction to our research will be given by the Director of research, Dr.Tim Brookes, followed by demonstrations and posters from our postgraduate researchers. We welcome those working in industry and academia to attend the presentation and to discuss our recent findings and overall research goals.
Brookes T (2003) The Psychoacoustics of Sound Recording,
Neher T, Brookes Tim, Mason Russell (2006) Musically Representative Signals for Use in Interaural Cross-Correlation Coefficient Measurements, Acta Acustica United with Acustica 92 (5) pp. 787-796 Hirzel Verlag
Typically, measurements that aim to predict perceived spatial impression of music signals in concert halls are performed by calculating the interaural cross-correlation coefficient (IACC) of a binaurally-recorded impulse response. Previous research, however, has shown that this can lead to results very different from those obtained if a musical input signal is used. The reasons for this discrepancy were investigated, and it was found that the overall duration of the source signal, its onset and offset times, and the magnitude and rate of any spectral fluctuations, have a very strong effect on the IACC. Two test signals, synthesised to be representative of a wide range of musical stimuli, can extend the external validity of traditional IACC-based measurements.
Williams D, Brookes T (2010) Perceptually-Motivated Audio Morphing: Warmth, Convention Proceedings, Paper 8019 Audio Engineering Society
A system for morphing the warmth of a sound independently from its other timbral attributes was coded, building on previous work morphing brightness only (1), and morphing brightness and softness (2). The new warmth-softness-brightness morpher was perceptually validated using a series of listening tests. A Multidimensional Scaling analysis of listener responses to paired-comparisons showed perceptually orthogonal movement in two dimensions within a warmth-morphed and everything-else-morphed stimulus set. A verbal elicitation experiment showed that listeners? descriptive labeling of these dimensions was as intended. A further ?quality control? experiment provided evidence that no ?hidden? timbral attributes were altered in parallel with the intended ones. A complete timbre morpher can now be considered for further work, and evaluated using the tri-stage procedure documented here.
Mason R, Brookes T (2004) Perception, measurement and synthesis of spatial impression,
Brookes T, Tyrrell A, Howard D (2000) On the differences between conventional and auditory spectrograms of English consonants, Logopedics Phoniatrics Vocology 25 (2) pp. 72-79 Taylor & Francis
A new tool for speech analysis is presented, operating in real-time and incorporating the analysing power of a contemporary auditory model to produce the familiar display of the speech spectrograph. This ?auditory spectrograph? is used to analyse English consonant sounds and the results are compared with conventional wide and narrow band spectrograms. The auditory analyses are found to attach more visual weight to the acoustic cues associated with speech production and perception, and features that are either difficult or impossible to distinguish on conventional spectrograms are clarified.
Stokes T, Brookes Tim, Hummersone C (2012) Improving the Quality of Separated Audio: What Works?,
Howard DM, Tyrrell AM, Brookes TS (1997) Spectrography by Human Hearing Modelling, prepri
Francombe J, Brookes T, Mason R (2017) Evaluation of Spatial Audio Reproduction Methods (Part 1): Elicitation of Perceptual Differences, Journal of the Audio Engineering Society 65 (3) pp. 198-211 Audio Engineering Society
There are a wide variety of spatial audio reproduction systems available, from a single loudspeaker to many spatially distributed loudspeakers. An important factor in the selection, development, or optimization of such systems is listener preference, and the important perceptual characteristics that contribute to this. An experiment was performed to determine the attributes that contribute to listener preference for a range of spatial audio reproduction methods. Experienced and inexperienced listeners made preference ratings for combinations of seven program items replayed over eight reproduction systems, and reported the reasons for their judgments. Automatic text clustering reduced redundancy in the responses by approximately 90%, facilitating subsequent group discussions that produced clear attribute labels, descriptions, and scale end-points. Twenty-seven and twenty-four attributes contributed to preference for the experienced and inexperienced listeners respectively. The two sets of attributes contain a degree of overlap (ten attributes from the two sets were closely related); however, the experienced listeners used more technical terms whilst the inexperienced listeners used more broad descriptive categories.
Ashby T, Brookes Tim, Mason Russell (2014) Towards a head-movement-aware spatial localisation model: Elevation, 21st International Congress on Sound and Vibration 2014, ICSV 2014 4 pp. 2808-2815
A multiple-microphone-sphere-based localisation model has been developed that predicts source location by modelling the cues given by head movement. In order to inform improvements to this model, a series of experiments was devised to investigate the impact of head movement cues on the localisation response accuracy of human listeners. It was shown that head movements improve elevation localisation response accuracy for noise sources. When pinna cues are impaired the significance of head movement cues increases. The improved localisation resulting from head movement is due to dynamic cues available during the period of movement, and not to improved static cues available once the head is turned to face the sound source. Head movements improve elevation localisation to a similar degree for band- limited sources with differing centre frequencies (500 Hz, 2 kHz and 6 kHz), which indicates that both dynamic ILDs and dynamic ITDs are used. Head movements do not improve elevation response accuracy for programme items with less than an octave bandwidth. Head movements improve elevation response accuracy to a greater degree for sources further away from the equatorial plane.
Brookes TS (2007) Audio Perception, Measurement & Synthesis,
Kassier R, Lee HK, Brookes T, Rumsey F (2005) An informal comparison between surround-sound microphone techniques,
Mason R, Brookes T, Rumsey F (2003) Creation and verification of a controlled experimental stimulus for investigating selected perceived spatial attributes, Audio Engineering Society Preprint 5771
In order to undertake controlled investigations into perceptual effects that relate to the interaural cross-correlation coefficient, experiment stimuli that meet a tight set of criteria are required. The requirements of each stimulus are that it is narrow band, normally has a constant cross-correlation coefficient over time, and can be altered to cover the full range of values of cross-correlation coefficient, including specified variations over time if required. Stimuli created using a technique based on amplitude modulation are found to meet these criteria, and their use in a number of subjective experiments is described.
Kim C, Mason R, Brookes T (2008) Initial investigation of signal capture techniques for objective measurement of spatial impression considering head movement, Audio Engineering Society Preprint 7331 Audio Engineering Society Preprint
In a previous study it was discovered that listeners normally make head movements attempting to evaluate source width and envelopment as well as source location. To accommodate this finding in the development of an objective measurement model for spatial impression, two capturing models were introduced and designed in this research, based on binaural technique: 1) rotating Head And Torso Simulator (HATS), and 2) a sphere with multiple microphones. As an initial study, measurements of interaural time difference (ITD), level difference (ILD) and cross-correlation coefficient (IACC) made with the HATS were compared with those made with a sphere containing two microphones. The magnitude of the differences was judged in a perceptually relevant manner by comparing them with the just-noticeable differences (JNDs) of these parameters. The results showed that the differences were generally not negligible, implying the necessity of enhancement of the sphere model, possibly by introducing equivalents of the pinnae or torso. An exception was the case of IACC, where the reference of JND specification affected the perceptual significance of its difference between the two models.
Mason R, Brookes T, Rumsey F (2004) Integration of measurements of interaural cross-correlation coefficient and interaural time difference within a single model of perceived source width, Audio Engineering Society Preprint 6137
A measurement model based on the interaural cross-correlation coefficient (IACC) that attempts to predict the perceived source width of a range of auditory stimuli is currently under development. It is necessary to combine the predictions of this model with measurements of interaural time difference (ITD) to allow the model to provide its output on a meaningful scale and to allow integration of results across frequency. A detailed subjective experiment was undertaken using narrow-band stimuli with a number of centre frequencies, IACCs and ITDs. Subjects were asked to indicate the perceived position of the left and right boundaries of a number of these stimuli by altering the ITD of a pair of white noise comparison stimuli. It is shown that an existing IACC-based model provides a poor prediction of the subjective results but that modifications to the model significantly increase its accuracy.
Kim C, Mason R, Brookes T (2008) Improvements to a Spherical Binaural Capture Model for Objective Measurement of Spatial Impression with Consideration of Head Movements, Audio Engineering Society Preprint 7579 Audio Engineering Society
This research aims, ultimately, to develop a system for the objective evaluation of spatial impression, incorporating the finding from a previous study that head movements are naturally made in its subjective evaluation. A spherical binaural capture model, comprising a head-sized sphere with multiple attached microphones, has been proposed. Research already conducted found significant differences in interaural time and level differences, and cross-correlation coefficient, between this spherical model and a head and torso simulator. It is attempted to lessen these differences by adding to the sphere a torso and simplified pinnae. Further analysis of the head movements made by listeners in a range of listening situations determines the range of head positions that needs to be taken into account. Analyses of these results inform the optimum positioning of the microphones around the sphere model.
Brookes T (2005) The Spatial Audio Creative Engineering Network,
Pike C, Brookes T, Mason R (2013) Auditory adaptation to loudspeaker and listening room acoustics, 135th Audio Engineering Society Convention 2013 pp. 116-125
Timbrai qualities of loudspeakers and rooms are often compared in listening tests involving short listening periods. Outside the laboratory, listening occurs over a longer time course. In a study by Olive et al. (1995) smaller timbrai differences between loudspeakers and between rooms were reported when comparisons were made over longer versus shorter time periods. This is a form of timbrai adaptation, a decrease in sensitivity to timbre over time. The current study confirms this adaptation and establishes that it is not due to response bias but may be due to timbrai memory, specific mechanisms compensating for transmission channel acoustics, or attentional factors. Modifications to listening tests may be required where tests need to be representative of listening outside of the laboratory.
Mason Russell, Brookes Tim, Rumsey F (2005) The effect of various source signal properties on measurements of the interaural cross-correlation coefficient, Acoustical Science and Technology 26 (2) pp. 102-113 Acoustical Society of Japan
Measurements that attempt to predict the perceived spatial impression of musical signals in concert halls typically are conducted by calculating the interaural cross-correlation coefficient (IACC) of an impulse response. The causes of interaural decorrelation are investigated and it is found that this is affected by frequency dependent interaural time and level differences and variations in these over time. It is found that the IACC of impulsive and of narrowband tonal signals can be very different from each other in a wide range of acoustical environments, due to the differences in the spectral content and the duration of the signals. From this, it is concluded that measurements made of impulsive signals are unsuitable for attempting to predict the perceived spatial impression of musical signals. It is suggested that further work is required to develop a set of test signals that is representative of a wide range of musical stimuli
Brookes TS, Howard DM, Tyrrell AM (1995) T800/T9000 Communications: A Programmer's View, pp. 18-28 IOS Press (Oxford)
Kim C, Mason R, Brookes T (2010) Investigation into and modelling of head movement for objective evaluation of the spatial impression of audio, Journal of the Acoustical Society of America 127 (3) pp. 1886-1886 Acoustical Society of America
Research was undertaken to determine the nature of head movements made when judging spatial impression and to incorporate these into a system for measuring, in a perceptually relevant manner, the acoustic parameters which contribute to spatial impression: interaural time and level differences and interaural cross?correlation coefficient. First, a subjective test was conducted that showed that (i) the amount of head movement was larger when evaluating source width and envelopment than when judging localization and timbre and (ii) the pattern of head movement resulted in ear positions that formed a sloped area. These findings led to the design of a binaural signal capture technique using a sphere with multiple microphones, mounted on a simulated torso. Evaluation of this technique revealed that it would be appropriate for the prediction of perceived spatial attributes including both source direction and aspects of spatial impression. Reliable derivation of these attributes across the range of ear positions determined from the earlier subjective test was shown to be possible with a limited number of microphones through an appropriate interpolation and calculation technique. A prototype capture system was suggested as a result, using a sphere with torso, with 21 omnidirectional microphones on each side. [Work supported by the Engineering and Physical Sciences Research Council (EPSRC), UK, Grant No. EP/D049253.]
Williams D, Brookes T (2010) Testing a prototype timbre morpher,
Stokes T, Hummersone C, Brookes T Audio Un-mixing Dataset, University of Surrey
Pike C, Mason RD, Brookes T (2014) The effect of auditory memory on the perception of timbre, Audio Engineering Society Preprint 9028
Listeners are more sensitive to timbral differences when comparing stimuli side-by-side than temporally-separated. The contributions of auditory memory and spectral compensation to this effect are unclear. A listening test examined the role of auditory memory in timbral discrimination, across retention intervals (RIs) of up to 40 s. For timbrally complex music stimuli discrimination accuracy was good across all RIs, but there was increased sensitivity to onset spectrum, which decreased with increasing RI. Noise stimuli showed no onset sensitivity but discrimination performance declined with RIs of 40 s. The difference between program types may suggest different onset sensitivity and memory encoding (categorical vs non-categorical). The onset bias suggests that memory effects should be measured prior to future investigation of spectral compensation.
Francombe J, Brookes T, Mason R, Flindt R, Coleman P, Liu Q, Jackson PJB (2015) Production and reproduction of programme material for a variety of spatial audio formats, Proc. AES 138th Int. Conv. (e-Brief), Warsaw pp. 4-4
For subjective experimentation on 3D audio systems, suitable programme material is needed. A large-scale recording session was performed in which four ensembles were recorded with a range of existing microphone techniques (aimed at mono, stereo, 5.0, 9.0, 22.0, ambisonic, and headphone reproduction) and a novel 48-channel circular microphone array. Further material was produced by remixing and augmenting pre-existing multichannel content. To mix and monitor the programme items (which included classical, jazz, pop and experimental music, and excerpts from a sports broadcast and a lm soundtrack), a flexible 3D audio reproduction environment was created. Solutions to the following challenges were found: level calibration for different reproduction formats; bass management; and adaptable signal routing from different software and fille formats.
Brookes T, Treble C (2005) The effect of non-symmetrical left/right recording pinnae on the perceived externalisation of binaural recordings,
Font F, Brookes Tim, Fazekas G, Guerber M, La Burthe A, Plans D, Plumbley MD, Shaashua M, Wang W, Serra X (2016) Audio Commons: bringing Creative Commons audio content to the creative industries, AES E-Library Audio Engineering Society
Significant amounts of user-generated audio content, such as sound effects, musical samples and music pieces, are uploaded to online repositories and made available under open licenses. Moreover, a constantly increasing amount of multimedia content, originally released with traditional licenses, is becoming public domain as its license expires. Nevertheless, the creative industries are not yet using much of all this content in their media productions. There is still a lack of familiarity and understanding of the legal context of all this open content, but there are also problems related with its accessibility. A big percentage of this content remains unreachable either because it is not published online or because it is not well organised and annotated. In this paper we present the Audio Commons Initiative, which is aimed at promoting the use of open audio content and at developing technologies with which to support the ecosystem composed by content repositories, production tools and users. These technologies should enable the reuse of this audio material, facilitating its integration in the production workflows used by the creative industries. This is a position paper in which we describe the core ideas behind this initiative and outline the ways in which we plan to address the challenges it poses.
Kim C, Mason RD, Brookes T (2009) The role of head movement in the analysis of spatial impression, Engineering and Physical Sciences Research Council
Neher T, Rumsey F, Brookes TS, Craven P (2003) Unidimensional simulation of the spatial attribute 'ensemble width' for training purposes, preprint 5769
Francombe J, Brookes T, Mason RD (2015) Perceptual evaluation of spatial quality: where next?,
From the early days of reproduced sound, engineers have sought to reproduce the spatial properties of sound fields, leading to the development of a range of technologies. Two-channel stereo has been prevalent for many years; however, systems with a higher number of discrete channels
(including rear and height loudspeakers) are becoming more common and, recently, there has been a move towards loudspeaker-agnostic methods using audio objects. Perceptual evaluation, and perceptually-informed objective measurement, of alternative reproduction systems can inform
further development and steer future innovations. It is important, therefore, that any gaps in the field of perceptual evaluation and measurement are identified and that future work aims to fill those gaps. A standard research paradigm in the field is identification of the perceptual attributes of a stimulus set, facilitating controlled listening tests and leading to the development of predictive models. There have been numerous studies that aim to discover the perceptual attributes of reproduced spatial sound, leading to more than fifty descriptive terms. However, a literature review revealed the following key problems: (i) there is little agreement on exact definitions, nor on the relative importance of each attribute; (ii) there may be important attributes that have not yet been identified (e.g. attributes arising from differences between real and reproduced audio, or pertaining to new 3D or object-based methods); and (iii) there is no model of overall spatial quality based directly on the important attributes. Consequently, the authors contend that future
research should focus on: (i) ascertaining which attributes of reproduced spatial audio are most important to listeners; (ii) identifying any important attributes currently missing; (iii) determining the relationships between the important attributes and listener preference; (iv) modelling overall
spatial quality in terms of the important perceptual attributes; and (v) modelling these perceptual attributes in terms of their physical correlates.
Francombe J, Brookes T, Mason RD (2015) Elicitation of the differences between real and reproduced audio, Audio Engineering Society Preprint 9307
To improve the experience of listening to reproduced audio, it is beneficial to determine the differences between listening to a live performance and a recording. An experiment was performed in which three live performances (a jazz duet, a jazz-rock quintet, and a brass quintet) were captured and simultaneously replayed over a nine-channel with-height surround sound system. Experienced and inexperienced listeners moved freely between the live performance and the reproduction and described the difference in listening experience. In subsequent group discussions, the experienced listeners produced twenty-nine categories using some terms that are not commonly found in the current spatial audio literature. The inexperienced listeners produced five categories that overlapped with the experienced group terms but that were not as detailed.
Brookes TS, Howard DM, Tyrrell AM (1994) Simulations of the Human Peripheral Hearing System: A Comparison of T800 and T9000, pp. 17-24
Mason R, Brookes T, Rumsey F (2004) Spatial impression: measurement and perception of concert hall acoustics and reproduced sound, Proceedings of the International Symposium on Room Acoustics
Auditory width measurements based on the interaural cross-correlation coefficient (IACC) are often used in the field of concert hall acoustics. However, there are a number of problems with such measurements, including large variations around the centre of a room and a limited range of values at low frequencies. This paper explores how some of these problems can be solved by applying the IACC in a more perceptually valid manner and using it as part of a more complete hearing model. It is proposed that measurements based on the IACC may match the perceived width of stimuli more accurately if a source signal is measured rather than an impulse response, and when factors such as frequency and loudness are taken into account. Further developments are considered, including methods to integrate the results calculated in different frequency bands, and the temporal response of spatial perception
Howard DM, Hirson A, Brookes TS, Tyrrell AM (1995) Spectrography of Disputed Speech Samples by Peripheral Human Hearing Modelling, Forensic Linguistics 2 (1) pp. 22-38
Ashby T, Mason RD, Brookes T (2013) Head movements in three-dimensional localisation, Proceedings of the 134th Audio Engineering Society Convention 8881 Audio Engineering Society Preprint
Previous studies give contradicting evidence as to the importance of head movements in localisation. In this study head movements were shown to increase localisation response accuracy in elevation and azimuth. For elevation, it was found that head movement improved localisation accuracy in some cases and that when pinna cues were impeded the significance of head movement cues was increased. For azimuth localization, head movement reduced front-back confusions. There was also evidence that head movement can be used to enhance static cues for azimuth localisation. Finally, it appears that head movement can increase the accuracy of listeners? responses by enabling an interaction between auditory and visual cues.
Neher T, Brookes T, Rumsey F (2006) A Hybrid Technique for Validating Unidimensionality of Perceptual Variation in a Spatial Auditory Stimulus Set, Journal of the Audio Engineering Society 54 (4) pp. 259-275 Audio Engineering Society
Signal-processing algorithms that are meant to evoke a certain subjective effect often have to be perceptually equalized so that any unwanted artifacts are, as far as possible, eliminated. They can then be said to exhibit ?unidimensionality of perceived variation.? Aiming to design a method that allows unidimensionality of perceived variation to be verified, established sensory evaluation approaches are examined in terms of their suitability for detailed, undistorted profiling and hence reliable validation of an algorithm?s subjective effects. It is found that a procedure combining multidimensional scaling with supplementary verbal elicitation constitutes the most appropriate approach. In the context of validating a signal-processing method intended to produce a specific spatial effect, this procedure is evaluated and some shortcomings are identified. However, following refinements, it is concluded that these can be overcome through additional data collection and analysis, resulting in a multistage hybrid validation technique.
Brookes T, Neher T (2005) Perceptually Unidimensional Control of Spatial Audio: a Pilot study,
Ashby T, Mason R, Brookes T (2011) Prediction of perceived elevation using multiple psuedo-binaural microphones, Audio Engineering Society Preprint Audio Engineering Society
Computational auditory models that predict the perceived location of sound sources in terms of azimuth are already available, yet little has been done to predict perceived elevation. Interaural time and level differences, the primary cues in horizontal localisation, do not resolve source elevation, resulting in the ?Cone of Confusion?. In natural listening, listeners can make head movements to resolve such confusion. To mimic the dynamic cues provided by head movements, a multiple microphone sphere was created, and a hearing model was developed to predict source elevation from the signals captured by the sphere. The prototype sphere and hearing model proved effective in both horizontal and vertical localisation. The next stage of this research will be to rigorously test a more physiologically accurate capture device.
Kim C, Mason R, Brookes T (2010) A quasi-binaural approach to head-movement-aware evaluation of spatial acoustics, Proceedings of the International Symposium on Room Acoustics General papers 4 (1) pp. 292-300 The International Congress on Acoustics (ICA)
This research incorporates the nature of head movement made in listening activities, into the development of a quasibinaural
acoustical measurement technique for the evaluation of spatial impression. A listening test was conducted
where head movements were tracked whilst the subjects rated the perceived source width, envelopment, source
direction and timbre of a number of stimuli. It was found that the extent of head movements was larger when
evaluating source width and envelopment than when evaluating source direction and timbre. It was also found that
the locus of ear positions corresponding to these head movements formed a bounded sloped path, higher towards the
rear and lower towards the front. This led to the concept of a signal capture device comprising a torso-mounted
sphere with multiple microphones. A prototype was constructed and used to measure three binaural parameters
related to perceived spatial impression - interaural time and level differences (ITD and ILD) and interaural crosscorrelation
coefficient (IACC). Comparison of the prototype measurements to those made with a rotating Head and
Torso Simulator (HATS) showed that the prototype could be perceptually accurate for the prediction of source
direction using ITD and ILD, and for the prediction of perceived spatial impression using IACC. Further
investigation into parameter derivation and interpolation methods indicated that 21 pairs of discretely spaced
microphones were sufficient to measure the three binaural parameters across the sloped range of ear positions
identified in the listening test.
Brookes T, Mason Russell (2013) Auditory source width of paired vs single narrow-band signals in terms of centre-frequency, loudness level and interaural cross-correlation coefficient, Journal of the Acoustical Society of America Acoustical Society of America
Pearce A, Brookes TS, Dewhirst M (2015) Validation of Experimental Methods to Record Stimuli for Microphone Comparisons, 139th International AES Convention papers Audio Engineering Society
Test recordings can facilitate evaluation of a microphone's characteristics but there is currently no standard or experimentally validated method for making recordings to compare the perceptual characteristics of microphones. This paper evaluates previously used recording methods, concluding that, of these, the most appropriate approach is to record multiple microphones simultaneously. However, perceived differences between recordings made with microphones in a multi-microphone array might be due to (i) the characteristics of the microphones and/or (ii) the different locations of the microphones. Listening tests determined the maximum acceptable size of a multi-microphone array to be 150 mm in diameter, but the diameter must be reduced to no more than 100 mm if the microphones to be compared are perceptually very similar.
Mason R, Brookes T, Rumsey F (2002) The perceptual relevance of extant techniques for the objective measurement of spatial impression, Proceedings of the Institute of Acoustics 24
Supper B, Brookes T, Rumsey F (2006) An auditory onset detection algorithm for improved automatic source localization, IEEE Transactions on Audio, Speech and Language Processing 14 (3) pp. 1008-1017 IEEE
An algorithm is described which detects auditory onsets quickly in arbitrary binaural audio streams. Aspects of the precedence effect are implemented to speed up computation, and to increase the usability of the output. The onset detector is tested with a number of binaural signals. Onsets that are suitable for spatial auditory processing are found reliably. This will allow spatial feature extraction to be performed.
Kim C, Mason R, Brookes T (2010) Validation of a simple spherical head model as a signal capture device for head-movement-aware prediction of perceived spatial impression, Proceedings of the 40th International AES Conference Audio Engineering Society
In order to take head movement into account in objective evaluation of perceived spatial impression (including source direction), a suitable binaural capture device is required. A signal capture system was suggested that consisted of a head-sized sphere containing multiple pairs of microphones which, in comparison to a rotating head and torso simulator (HATS), has the potential for improved measurement speed and the capability to measure time varying systems, albeit at the expense of some accuracy. The error introduced by using a relatively simple sphere compared to a more physically accurate HATS was evaluated in terms of three binaural parameters related to perceived spatial impression ? interaural time and level differences (ITD and ILD) and interaural cross-correlation coefficient (IACC). It was found that whilst the error in the IACC measurements was perceptually negligible when the sphere was mounted on a torso, the differences in measured ITD and ILD values between the sphere-with-torso and HATS were not perceptually negligible. However, it was found that the sphere-with-torso could give accurate predictions of source location based on ITD and ILD, through the use of a look-up table created from known ITD-ILD-direction mappings. Therefore the validity of the multi-microphone sphere-with-torso as a binaural signal capture device for perceptually relevant measurements of source direction (based on ITD and ILD) and spatial impression (based on IACC) was demonstrated.
Brookes TS, Howard DM, Tyrrell AM (1994) A T9000 Simulation of the Human Peripheral Hearing System, 208 pp. 4.1-4.4
Brookes TS, Tyrrell AM, Howard DM (1996) Musical Analysis using a Real-Time model of Peripheral Hearing, pp. 79-82
Ashby T, Mason Russell, Brookes Tim (2014) Elevation localisation response accuracy on vertical planes of differing azimuth, Audio Engineering Society Preprint 9046
Head movement has been shown to significantly improve localisation response accuracy in elevation. It is unclear from previous research whether this is due to static cues created once the head has reached a new stationary position or dynamic cues created through the act of moving the head. In this experiment listeners were asked to report the location of loudspeakers placed on vertical planes at four different azimuth angles (0°, 36°, 72°, 108°) with no head movement. Static elevation response accuracy was significantly more accurate for sources away from the median plane. This finding, combined with the statement that listeners orient to face the source when localising, suggests that dynamic cues are the cause of improved localisation through head movement.
Mason R, Brookes T, Rumsey F (2004) Development of the interaural cross-correlation coefficient into a more complete auditory width prediction model, Proceedings of the 18th International Congress on Acoustics IV pp. 2453-2456 International Congress on Acoustics
Auditory width measurements based on the interaural cross-correlation coefficient (IACC) are often used in the field of concert hall acoustics. However, there are a number of problems with such measurements, including large variations around the centre of a room and a limited range of values at low frequencies. This paper explores how some of these problems can be solved by applying the IACC in a more perceptually valid manner and using it as part of a more complete hearing model. It is proposed that measurements based on the IACC may match the perceived width of stimuli more accurately if a source signal is measured rather than an impulse response, and when factors such as frequency and loudness are taken into account. Further developments are considered, including methods to integrate the results calculated in different frequency bands, and the temporal response of spatial perception
Brookes TS (1997) A Real-Time Auditory Spectrograph,
Brookes T, Paul M (2006) Controlling the Perceived Spatial Orientation of a Reproduced Human Voice Source,
Brookes TS (2007) Engineering the Senses: Psychoacoustics,
Pearce Andrew, Brookes Tim, Mason Russell, Dewhirst M (2016) Measurements to determine the ranking accuracy of perceptual models, 140th Convention Proceedings Audio Engineering Society
Linear regression is commonly used in the audio industry to create objective measurement models that predict subjective data. For any model development, the measure used to evaluate the accuracy of the prediction is important. The most common measures assume a linear relationship between the subjective data and the prediction, though in the early stages of model development this is not always the case. Measures based on rank ordering (such as Spearman?s test), can alternatively be used. Spearman?s test, however, does not consider the variance of the subjective data. This paper presents a method of incorporating the subjective variance into the Spearman?s rank ordering test using Monte Carlo simulations, and shows how this can be beneficial in the development of predictive models.
Pike C, Mason Russell, Brookes Tim (2014) Auditory adaptation to static spectra,
Auditory adaptation is thought to reduce the perceptual impact of static spectral energy and increase sensitivity to spectral change. Research suggests that this adaptation helps listeners to extract stable speech cues across different talkers, despite inter-talker spectral variations caused by differing vocal tract acoustics. This adaptation may also be involved in compensation for distortions caused by transmission channels more generally (e.g. distortions caused by the room or loudspeaker through which a sound has passed). The magnitude of this adaptation and its ecological importance has not been established. The physiological and psychological mechanisms behind adaptation are also not well understood. The current research aimed to confirm that adaptation to transmission channel spectrum occurs when listening to speech produced though two types of transmission channel: loudspeakers and rooms. The loudspeaker is analogous to the vocal tract of a talker, imparting resonances onto a sound source which reaches the listener both directly and via reflections. The room-affected speech however, reaches the listener only via reflections ? there is no direct path. Larger adaptation to the spectrum of the room was found, compared to adaptation to the spectrum of the loudspeaker. It appears that when listening to speech, mechanisms of adaptation to room reflections, and adaptation to loudspeaker/vocal tract spectrum, may be different.
Williams D, Brookes TS (2007) Perceptually-Motivated Audio Morphing: Brightness, Conven Audio Engineering Society
A system for morphing the brightness of two sounds independently from their other perceptual or acoustic attributes was coded, based on the Spectral Modelling Synthesis additive/residual model. A Multidimensional Scaling analysis of listener responses showed that the brightness control was perceptually independent from the other controls used to adjust the morphed sound. A Timbre Morpher, adjusting additional timbral attributes with perceptually meaningful controls, can now be considered for further work.
Hummersone Christopher, Stokes T, Brookes Tim (2014) On the Ideal Ratio Mask as the Goal of Computational Auditory Scene Analysis, In: Naik GR, Wang W (eds.), Blind Source Separation: Advances in Theory, Algorithms and Applications (12) 12 pp. 349-368 Springer
The ideal binary mask (IBM) is widely considered to be the benchmark for time?frequency-based sound source separation techniques such as computational auditory scene analysis (CASA). However, it is well known that binary masking introduces objectionable distortion, especially musical noise. This can make binary masking unsuitable for sound source separation applications where the output is auditioned. It has been suggested that soft masking reduces musical noise and leads to a higher quality output. A previously defined soft mask, the ideal ratio mask (IRM), is found to have similar properties to the IBM, may correspond more closely to auditory processes, and offers additional computational advantages. Consequently, the IRM is proposed as the goal of CASA. To further support this position, a number of studies are reviewed that show soft masks to provide superior performance to the IBM in applications such as automatic speech recognition and speech intelligibility. A brief empirical study provides additional evidence demonstrating the objective and perceptual superiority of the IRM over the IBM.
Neher T, Brookes T, Rumsey F (2003) Unidimensional simulation of the spatial attribute 'ensemble depth' for training purposes. Part 1: pilot study into early reflection pattern characteristics, pp. 123-137
Hummersone Christopher, Mason Russell, Brookes Tim (2013) A Comparison of Computational Precedence Models for Source Separation in Reverberant Environments, Journal of the Audio Engineering Society 61 (7/8 (J) pp. 508-520 Audio Engineering Society
Reverberation is a problem for source separation algorithms. Because the precedence effect allows human listeners to suppress the perception of reflections arising from room boundaries, numerous computational models have incorporated the precedence effect. However, relatively little work has been done on using the precedence effect in source separation algorithms. This paper compares several precedence models and their influence on the performance of a baseline separation algorithm. The models were tested in a variety of reverberant rooms and with a range of mixing parameters. Although there was a large difference in performance among the models, the one that was based on interaural coherence and onset-based inhibition produced the greatest performance improvement. There is a trade-off between selecting reliable cues that correspond closely to free-field conditions and maximizing the proportion of the input signals that contributes to localization. For optimal source separation performance, it is necessary to adapt the dynamic component of the precedence model to the acoustic conditions of the room.
Kim C, Mason Russell, Brookes Tim (2011) Head-movement-aware signal capture for evaluation of spatial acoustics, Building Acoustics 18 (1) pp. 207-226 Multi Science Publishing
This research incorporates the nature of head movement made in listening activities, into the development of a quasi- binaural acoustical measurement technique for the evaluation of spatial impression. A listening test was conducted where head movements were tracked whilst the subjects rated the perceived source width, envelopment, source direction and timbre of a number of stimuli. It was found that the extent of head movements was larger when evaluating source width and envelopment than when evaluating source direction and timbre. It was also found that the locus of ear positions corresponding to these head movements formed a bounded sloped path, higher towards the rear and lower towards the front. This led to the concept of a signal capture device comprising a torso-mounted sphere with multiple microphones. A prototype was constructed and used to measure three binaural parameters related to perceived spatial impression - interaural time and level differences (ITD and ILD) and interaural cross- correlation coefficient (IACC). Comparison of the prototype measurements to those made with a rotating Head and Torso Simulator (HATS) showed that the prototype could be perceptually accurate for the prediction of source direction using ITD and ILD, and for the prediction of perceived spatial impression using IACC. Further investigation into parameter derivation and interpolation methods indicated that 21 pairs of discretely spaced microphones were sufficient to measure the three binaural parameters across the sloped range of ear positions identified in the listening test.
Brookes T, Mason R (2004) Perceptually Motivated Measurement and Control of Digital Music,
Brookes T (2001) A speech-based frequency scale,
Kassier R, Brookes T, Rumsey F (2006) Designing a spatial audio attribute listener training system for optimal transfer,
Hummersone C, Mason R, Brookes T (2010) A comparison of computational precedence models for source separation in reverberant environments, Audio Engineering Society Preprint 7981 Audio Engineering Society
Mason R, Kim C, Brookes T (2009) Perception of head-position-dependent variations in interaural cross-correlation coefficient, Audio Engineering Society Preprint 7729 Audio Engineering Society
Experiments were undertaken to elicit the perceived effects of head-position-dependent variations in the interaural cross-correlation coefficient of a range of signals. A graphical elicitation experiment showed that the variations in the IACC strongly affected the perceived width and depth of the reverberant environment, as well as the perceived width and distance of the sound source. A verbal experiment gave similar results, and also indicated that the head-position-dependent IACC variations caused changes in the perceived spaciousness and envelopment of the stimuli.
Supper B, Brookes T, Rumsey F (2003) A new approach to detecting auditory onsets within a binaural stream,
Mason RD, Kim C, Brookes T (2008) Taking head movements into account in measurement of spatial attributes, Proceedings of the Institute of Acoustics Reproduced Sound Conference 30 (6) pp. 239-246 Institute of Acoustics
Measurements of the spatial attributes of auditory environments or sound reproduction systems commonly only consider a single receiver position. However, it is known that humans make use of head movement to help to make sense of auditory scenes, especially when the physical cues are ambiguous. Results are summarised from a three-year research project which aims to develop a practical binaural-based measurement system that takes head movements into account. Firstly, the head movements made by listeners in various situations were investigated, which showed that a wide range of head movements are made when evaluating source width and envelopment, and minimal head movements made when evaluating timbre. Secondly, the effect of using a simplified sphere model containing two microphones instead of a head and torso simulator was evaluated, and methods were derived to minimise the errors in measured cues for spatial perception that were caused by the simplification of the model. Finally, the results of the two earlier stages were combined to create a multi-microphone sphere that can be used to measure spatial attributes incorporating head movements in a perceptually-relevant manner, and which allows practical and rapid measurements to be made.
Williams D, Brookes T (2009) Perceptually-motivated audio morphing: softness, Convention Proceedings, Paper 7778 Audio Engineering Society
A system for morphing the softness and brightness of two sounds independently from their other perceptual or acoustic attributes was coded. The system is an extension of a previous one that morphed brightness only, that was based on the Spectral Modelling Synthesis additive/residual model. A Multidimensional Scaling analysis, of listener responses to paired comparisons of stimuli generated by the morpher, showed movement in three perceptually-orthogonal directions. These directions were labelled in a subsequent verbal elicitation experiment which found that the effects of the brightness and softness controls were perceived as intended. A Timbre Morpher, adjusting additional timbral attributes with perceptually-meaningful controls, can now be considered for further work.
Conetta R, Brookes Tim, Rumsey F, Zielinski S, Dewhirst M, Jackson P, Bech S, Meares D, George S (2015) Spatial Audio Quality Perception (Part 2): A Linear Regression Model, JOURNAL OF THE AUDIO ENGINEERING SOCIETY 62 (12) pp. 847-860 AUDIO ENGINEERING SOC
Previously-obtained data, quantifying the degree of quality degradation resulting from a range of spatial audio processes (SAPs), can be used to build a regression model of perceived spatial audio quality in terms of previously developed spatially and timbrally relevant metrics. A generalizable model thus built, employing just five metrics and two principal components, performs well in its prediction of the quality of a range of program types degraded by a multitude of SAPs commonly encountered in consumer audio reproduction, auditioned at both central and off-center listening positions. Such a model can provide a correlation to listening test data of r = 0.89, with a root mean square error (RMSE) of 11%, making its performance comparable to that of previous audio quality models and making it a suitable core for an artificial-listener-based spatial audio quality evaluation system.
Auralisation is the process of rendering virtual sound fields. It is used in areas including: acoustic design, defence, gaming and audio research. As part of a PhD project concerned with the influence of loudspeaker directivity on the perception of reproduced sound, a fully-computed auralisation system has been developed. For this, acoustic modelling software is used to synthesise and extract binaural impulse responses of virtual rooms. The resulting audio is played over headphones and allows listeners to experience the excerpt being reproduced within the synthesised environment. The main advance with this system is that impulse responses are calculated for a number of head positions, which allows the listeners to move when listening to the recreated sounds. This allows for a much more realistic simulation, and makes it especially useful for conducting subjective experiments on sound reproduction systems and/or acoustical environments which are either not available or are even impractical to create. Hence, it greatly increases the range and type of experiments that can be undertaken at Surrey. The main components of the system are described, together with the results from a validation experiment which demonstrate that this system provides similar results to experiments conducted previously using loudspeakers in an anechoic chamber.
Kassier R, Brookes T, Rumsey F (2006) A Comparison Between Spatial Audio Listener Training and Repetitive Practice,
Hummersone Christopher, Mason Russell, Brookes Tim (2010) Dynamic precedence effect modeling for source separation in reverberant environments, IEEE Transactions on Audio, Speech and Language Processing 18 (7) pp. 1867-1871 IEEE
Reverberation continues to present a major problem for sound source separation algorithms. However, humans demonstrate a remarkable robustness to reverberation and many psychophysical and perceptual mechanisms are well documented. The precedence effect is one of these mechanisms; it aids our ability to localize sounds in reverberation. Despite this, relatively little work has been done on incorporating the precedence effect into automated source separation. Furthermore, no work has been carried out on adapting a precedence model to the acoustic conditions under test and it is unclear whether such adaptation, analogous to the perceptual Clifton effect, is even necessary. Hence, this study tests a previously proposed binaural separation/precedence model in real rooms with a range of reverberant conditions. The precedence model inhibitory time constant and inhibitory gain are varied in each room in order to establish the necessity for adaptation to the acoustic conditions. The paper concludes that adaptation is necessary and can yield significant gains in separation performance. Furthermore, it is shown that the initial time delay gap and the direct-to-reverberant ratio are important factors when considering this adaptation.
Francombe J, Brookes TS, Mason R, Melchior F (2015) Loudness matching multichannel audio programme material with listeners and predictive models, 139th International AES Convention papers
Loudness measurements are often necessary in psychoacoustic research and legally required in broadcasting. However, existing loudness models have not been widely tested with new multichannel audio systems. A trained listening panel used the method of adjustment to balance the loudnesses of eight reproduction methods: low-quality mono, mono, stereo, 5-channel, 9-channel, 22-channel, ambisonic cuboid, and headphones. Seven programme items were used, including music, sport, and a lm soundtrack. The results were used to test loudness models including simple energy-based metrics, variants of ITU-R BS.1770, and complex psychoacoustically motivated models. The mean differences between the perceptual results and model predictions were statistically insignificant for all but the simplest model. However, some weaknesses in the model predictions were highlighted.
Brookes TS (2007) Spatial Psychoacoustics & Creative Engineering,
Hermes K, Brookes TS, Hummersone C (2015) The influence of dumping bias on timbral clarity ratings, 139th International AES Convention papers Audio Engineering Society
When listening test subjects are required to rate changes in a single attribute, but also hear changes in other attributes, their ratings can become skewed by ?dumping bias.? To assess the influence of dumping bias on timbral ?clarity? ratings, listeners were asked to rate stimuli: (i) in terms of clarity only; and (ii) in terms of clarity, warmth, fullness, and brightness. Clarity ratings of type (i) showed (up to 20%) larger interquartile ranges than those of type (ii). It is concluded that in single-attribute timbral rating experiments, statistical noise?potentially resulting from dumping bias?can be reduced by allowing listeners to rate additional attributes either simultaneously or beforehand.
Kim C, Mason R, Brookes T (2007) An investigation into head movements made when evaluating various attributes of sound, Audio Engineering Society Preprint 7031 Audio Engineering Society
This research extends the study of head movements during listening by including various listening tasks where the listeners evaluate spatial impression and timbre, in addition to the more common task of judging source location. Subjective tests were conducted in which the listeners were allowed to move their heads freely whilst listening to various types of sound and asked to evaluate source location, apparent source width, envelopment, and timbre. The head movements were recorded with a head tracker attached to the listener?s head. From the recorded data, the maximum range of movement, mean position and speed, and maximum speed were calculated along each axis of translational and rotational movement. The effects of various independent variables, such as the attribute being evaluated, the stimulus type, the number of repetition, and the simulated source location were examined through statistical analysis. The results showed that whilst there were differences between the head movements of individual subjects, across all listeners the range of movement was greatest when evaluating source width and envelopment, less when localising sources, and least when judging timbre. In addition, the range and speed of head movement was reduced for transient signals compared to longer musical or speech phrases. Finally, in most cases for the judgement of spatial attributes, head movement was in the direction of source direction.
Kassier R, Brookes T, Rumsey F (2005) A pilot study into listener training for spatial audio evaluation,
Supper B, Brookes T, Rumsey F (2004) A lateral angle tool for spatial auditory analysis,
Hummersone Christopher, Mason Russell, Brookes Tim (2011) Ideal Binary Mask Ratio: a novel metric for assessing binary-mask-based sound source separation algorithms, IEEE Transactions on Audio, Speech and Language Processing 19 (7) pp. 2039-2045 IEEE
A number of metrics has been proposed in the literature to assess sound source separation algorithms. The addition of convolutional distortion raises further questions about the assessment of source separation algorithms in reverberant conditions as reverberation is shown to undermine the optimality of the ideal binary mask (IBM) in terms of signal-to-noise ratio (SNR). Furthermore, with a range of mixture parameters common across numerous acoustic conditions, SNR?based metrics demonstrate an inconsistency that can only be attributed to the convolutional distortion. This suggests the necessity for an alternate metric in the presence of convolutional distortion, such as reverberation. Consequently, a novel metric?dubbed the IBM ratio (IBMR)?is proposed for assessing source separation algorithms that aim to calculate the IBM. The metric is robust to many of the effects of convolutional distortion on the output of the system and may provide a more representative insight into the performance of a given algorithm.
Tyrrell AM, Howard DM, Brookes T (1998) Transputer-based human hearing simulation, Simulation Practice and Theory 6 (5) pp. 479-491
Simulations of the human hearing system can help in a number of research fields; including work with the speech and hearing impaired as well as improving the accuracy of speech recognition systems and the naturalness of speech synthesis technology. The results from psychoacoustic experiments carried out over the last few decades have enabled models of the human peripheral hearing system to be developed. Conventionally, analyses such as the Fast Fourier Transform are used to analyze speech and other sounds to establish the acoustic cues which are important for human perception. Such analyses can be shown to be inappropriate in a number of ways. Additional insights could be gained into the importance of various acoustic cues if analyses based on hearing models were used. This paper describes an implementation of a real-time spectrograph based on a contemporary model of the peripheral human hearing system, executing on a network of T9000 transputers. The differences between it and conventional spectrographs are illustrated by means of test signals and speech sounds. © 1998 Elsevier Science B.V. All rights reserved.
Pike C, Mason RD, Brookes T (2014) Auditory compensation for spectral coloration, Audio Engineering Society Preprint 9138
The ?spectral compensation effect? (Watkins, 1991) describes a decrease in perceptual sensitivity to spectral modifications caused by the transmission channel (e.g., loudspeakers, listening rooms). Few studies have examined this effect: its extent and perceptual mechanisms are not confirmed. The extent to which compensation affects the perception of sounds colored by loudspeakers and other channels should be determined. This compensation has been mainly studied with speech. Evidence suggests that speech engages special perceptual mechanisms, so compensation might not occur with non-speech sounds. The current study provides evidence of compensation for spectrum in nonspeech tests: channel coloration was reduced by approximately 20%.
Conetta R, Brookes Tim, Rumsey F, Zielinski S, Dewhirst M, Jackson P, Bech S, Meares D, George S (2015) Spatial Audio Quality Perception (Part 1): Impact of Commonly Encountered Processes, JOURNAL OF THE AUDIO ENGINEERING SOCIETY 62 (12) pp. 831-846 AUDIO ENGINEERING SOC
Spatial audio processes (SAPs) commonly encountered in consumer audio reproduction
systems are known to generate a range of impairments to spatial quality. Two listening tests
(involving two listening positions, six 5-channel audio recordings, and 48 SAPs) indicate that
the degree of quality degradation is determined largely by the nature of the SAP but that
the effect of a particular SAP can depend on program material and on listening position.
Combining off-center listening with another SAP can reduce spatial quality significantly
compared to auditioning that SAP centrally. These findings, and the associated listening test
data, can guide the development of an artificial-listener-based spatial audio quality evaluation
system.
Stokes T, Hummersone C, Brookes TS (2013) Reducing Binary Masking Artefacts in Blind Audio Source Separation, Proceedings of the 134th Audio Engineering Society Convention paper 8853 Audio Engineering Society
Binary masking is a common technique for separating target audio from an interferer. Its use is often
justi ed by the high signal-to-noise ratio achieved. The mask can introduce musical noise artefacts, limiting
its perceptual performance and that of techniques that use it. Three mask-processing techniques, involving
adding noise or cepstral smoothing, are tested and the processed masks are compared to the ideal binary mask
using the perceptual evaluation for audio source separation (PEASS) toolkit. Each processing technique's
parameters are optimised before the comparison is made. Each technique is found to improve the overall
perceptual score of the separation. Results show a trade-o between interferer suppression and artefact
reduction.
Tyrrell AM, Brookes TS, Howard DM (1997) T9000 and T800 transputers: A real-time application, JOURNAL OF SYSTEMS ARCHITECTURE 43 (1-5) pp. 341-344 ELSEVIER SCIENCE BV
Kassier R, Brookes T, Rumsey F (2004) A simplified scene-based paradigm for use in spatial audio listener training applications,
Mason R, Brookes T, Rumsey F (2004) Evaluation of an auditory source width prediction model based on the interaural cross-correlation coefficient, Journal of the Acoustical Society of America 116
A model based on the interaural cross-correlation coefficient (IACC) has been developed that aims to predict the perceived source width of a wide range of sounds. The following factors differentiate it from more commonly used IACC-based measurements: the use of a running measurement to quantify variations in width over time; half-wave rectification and low pass filtering of the input signal to mimic the breakdown of phase locking in the ear; compensation for the frequency and loudness dependency of perceived width; combination of a model of perceived location with a model of perceived width; and conversion of the results to an intuitive scale. Objective and subjective methods have been used to evaluate the accuracy and limitations of the resulting measurement model.
Brookes Tim, Hummersone Christopher (2010) Machine Listening for Sound Quality Evaluation, Machine Listening Workshop 2010
Kim C, Mason R, Brookes T (2010) Development of a head-movement-aware signal capture system for the prediction of acoustical spatial impression, Proceedings of the 20th International Congress on Acoustics 4 pp. 2768-2775 International Congress of Acoustics (ICA)
This research introduces a novel technique for capturing binaural signals for objective evaluation of spatial impression; the technique allows for simulation of the head movement that is typical in a range of listening activities. A subjective listening test showed that the amount of head movement made was larger when listeners were rating perceived source width and envelopment than when rating source direction and timbre, and that the locus of ear positions corresponding to the pattern of head movement formed a bounded sloped path ? higher towards the rear and lower towards the front. Based on these findings, a signal capture system was designed comprising a sphere with multiple microphones, mounted on a torso. Evaluation of its performance showed that a perceptual model incorporating this capture system is capable of perceptually accurate prediction of source direction based on interaural time and level differences (ITD and ILD), and of spatial impression based on interaural cross-correlation coefficient (IACC). Investigation into appropriate parameter derivation and interpolation techniques determined that 21 pairs of spaced microphones were sufficient to measure ITD, ILD and IACC across the sloped range of ear positions.
Stokes T, Hummersone C, Brookes T, Mason A (2014) Perceptual quality of audio separated using sigmoidal masks, 137th Audio Engineering Society Convention 2014 pp. 167-173
Separation of underdetermined audio mixtures is often performed in the Time-Frequency (TF) domain by masking each TF element according to its target-to-mixture ratio. This work uses sigmoidal functions to map the target-to-mixture ratio to mask values. The series of functions used encompasses the ratio mask and an approximation of the binary mask. Mixtures are chosen to represent a range of different amounts of TF overlap, then separated and evaluated using objective measures. PEASS results show improved interferer suppression and artifact scores can be achieved using softer masking than that applied by binary or ratio masks. The improvement in these scores gives an improved overall perceptual score; this observation is repeated at multiple TF resolutions.
The perceptual characteristics of a microphone are not always clear from its technical specification. This thesis documents a first step towards creating more perceptually relevant measures.

Consideration of relevant criteria revealed that the most appropriate method for recording stimuli for perceptual microphone comparisons is to use all microphones under test simultaneously. Experiments determined that a maximum array size of 150 mm will ensure that the perceptual differences between the recorded stimuli are predominantly due to the characteristics of the microphones and not artefacts of the spacing between them.

It was established that there are eight standard physical differences that exist between microphones which may impact the perceived characteristics of a recording. These differences, supplemented with expert opinions, indicated that recording five programme items with eight studio and two MEMS microphones would allow for determination of the most prominent inter-microphone perceptual differences. A combination of indirect and direct elicitation experiments on the resulting 50 recordings identified a hierarchy of 40 perceptual attributes that describe the differences between microphones. A novel attribute contribution experiment conducted on the 31 lowest-level attributes in the hierarchy showed that brightness contributes the most overall to the inter-microphone difference.

The spectral centroid and ratios comparing the relative level of high frequencies were previously used to predict brightness; however, these metrics did not predict subjective ratings of microphone-related brightness as well as a newly proposed combination metric: the product of the spectral centroid above 3 kHz, and the ratio of energy above 3 kHz compared to all energy. This model performed well on training data (r = 0.909). Validating it on independent microphones and programme items suggested that improvements may be necessary for error-free prediction of programme-related aspects of brightness, but showed good correlation with each programme item and overall (r = 0.854), indicating that the model predicts microphone-related brightness well.

Interest in spatial audio has increased due to the availability of multichannel reproduction systems for the home and
car. Various timbral ear training systems have been presented, but relatively little work has been carried out into
training in spatial attributes of reproduced sound. To demonstrate that such a training system is truly useful, it is
necessary to show that learned skills are transferable to different settings. Issues relating to the transfer of training
are examined; a recent study conducted by the authors is discussed in relation to the level of transfer shown by
participants, and a new study is proposed that is aimed to optimise the transfer of training to different environments.
Mason RD, Brookes TS, Rumsey F (2005) Frequency dependency of the relationship between perceived auditory source width and the interaural cross-correlation coefficient for time-invariant stimuli., Journal of the Acoustical Society of America 117 (3 Pt 1) pp. 1337-1350 Acoustical Society of America
Previous research has indicated that the relationship between the interaural cross-correlation coefficient (IACC) of a narrow-band sound and its perceived auditory source width is dependent on its frequency. However, this dependency has not been investigated in sufficient detail for researchers to be able to properly model it in order to produce a perceptually relevant IACC-based model of auditory source width. A series of experiments has therefore been conducted to investigate this frequency dependency in a controlled manner, and to derive an appropriate model. Three main factors were discovered in the course of these experiments. First, the nature of the frequency dependency of the perceived auditory source width of stimuli with an IACC of 1 was determined, and an appropriate mathematical model was derived. Second, the loss of perceived temporal detail at high frequencies, caused by the breakdown of phase locking in the ear, was found to be relevant, and the model was modified accordingly using rectification and a low-pass filter. Finally, it was found that there was a further frequency dependency at low frequencies, and a method for modeling this was derived. The final model was shown to predict the experimental data well. (c) 2005 Acoustical Society of America.
Francombe J, Brookes TS, Mason RD, Woodcock J (2016) Determining and Labeling the Preference Dimensions of Spatial Audio Replay, QoMEX2016
There are many spatial audio reproduction systems currently in domestic use (e.g. mono, stereo, surround sound, sound bars, and headphones). In an experiment, pairwise pref-erence magnitude ratings for a range of such systems were collected from trained and untrained listeners. The ratings were analysed using internal preference mapping to: (i) uncover the principal perceptual dimensions of listener preference; (ii) label the dimensions based on the important perceptual attributes; and (iii) observe differences between trained and untrained listeners. To aid with labelling the dimensions, perceptual attributes were elicited alongside the preference ratings and were analysed by: (i) considering a metric derived from the frequency of use of each attribute and the magnitude of the related preference judgements; and (ii) observing attribute use for comparisons between speci?c methods. The ?rst preference dimension accounted for the vast majority of the variance in ratings; it was related to multiple important attributes, including those associated with spatial capability and freedom from distortion. All participants exhibited a preference for reproduction methods that were positively correlated with the ?rst dimension (most notably 5-, 9-, and 22-channel surround sound). The second dimension accounted for only a very small proportion of the variance, and appeared to separate the headphone method from the other methods. The trained and untrained listeners generally showed opposite preferences in the second dimension, suggesting that trained listeners have a higher preference for headphone reproduction than untrained listeners.
Francombe J, Brookes T, Mason R, Woodcock J (2017) Evaluation of Spatial Audio Reproduction Methods (Part 2): Analysis of Listener Preference, Journal of the Audio Engineering Society 65 (3) pp. 212-225 Audio Engineering Society
It is desirable to determine which of the many different spatial audio reproduction systems listeners prefer, and the perceptual attributes that are most important to listener experience, so that future systems can be perceptually optimized. A paired comparison preference rating experiment was performed alongside a free elicitation task for eight reproduction methods (consumer and professional systems with a wide range of expected quality) and seven program items (representative of potential broadcast material). The experiment was performed by groups of experienced and inexperienced listeners. Thurstone Case V modeling was used to produce preference scales. Both listener groups preferred systems with increased spatial content; nineand five-channel systems were most preferred. The use of elicited attributes was analyzed alongside the preference ratings, resulting in an approximate hierarchy of attribute importance: three attributes (amount of distortion, output quality, and bandwidth) were found to be important for differentiating systems where there was a large preference difference; sixteen were always important (most notably enveloping and horizontal width); and seven were used alongside small preference differences.
Kim CJ, Mason RD, Brookes TS (2013) Head movements made by listeners in experimental and real-life listening activities, Journal of the Audio Engineering Society 61 (6 (Jun) pp. 425-438
Understanding the way in which listeners move their heads must be part of any objective model for evaluating and reproducing the sonic experience of space. Head movement is part of the listening experience because it allows for sensing the spatial distribution of parameters. In the first experiment, the head positions of subjects was recorded when they were asked to evaluate perceived source location, apparent source width, envelopment, and timbre of synthesis stimuli. Head motion was larger when judging source width than when judging direction or timbre. In the second experiment, head movement was observed in natural listening activities such as concerts, movies, and video games. Because the statistics of movement were similar to that observed in the first experiment, laboratory results can to be used as the basis of an objective model of spatial behavior. The results were based on 10 subjects.
Hermes K, Brookes TS, Hummersone C (2016) The harmonic centroid as a predictor of string instrument timbral clarity, Audio Engineering Society proceedings
Spectrum is an important factor in determining timbral clarity. An experiment where listeners rate the changes in timbral clarity resulting from spectral equalisation (EQ) can provide insight into the relationship between EQ and the clarity of string instruments. Overall, higher frequencies contribute to clarity more positively than lower ones, but the relationship is programme-item-dependent. Fundamental frequency and spectral slope both appear to be important. Change in harmonic centroid (or dimensionless spectral centroid) correlates well with change in clarity, more so than octave band boosted/cut, harmonic number boosted/cut, or other variations on the spectral centroid.
Neher T, Brookes TS (2002) Training of Listeners for the Evaluation of Spatial Sound Reproduction, AES Convention Papers AES
This paper presents some preliminary results from an ongoing study into methods for the training of listeners in subjective evaluation of spatial sound reproduction. Exemplary stimuli were created illustrating two spatial attributes: individual source width and source distance. Changes in each of the two attributes were highly controlled in an attempt to allow uni-dimensional variation of their perceptual effects. The stimuli were validated with the help of an experienced listening panel and then used to instruct naïve listeners. By comparing the listeners' performances at ranking a number of stimuli before and after the training sessions the effectiveness of the adopted method was quantified.
Francombe J, Mason RD, Jackson PJB, Brookes TS, Hughes R, Woodcock J, Franck A, Melchior F, Pike C (2017) Media Device Orchestration for Immersive Spatial Audio Reproduction, Audio Mostly conference on Augmented and Participatory Sound and Music Experiences Proceedings ACM
Whilst it is possible to create exciting, immersive listening experiences with current spatial audio
technology, the required systems are generally difficult to install in a standard living room. However,
in any living room there is likely to already be a range of loudspeakers (such as mobile phones,
tablets, laptops, and so on). \Media device orchestration" (MDO) is the concept of utilising all
available devices to augment the reproduction of a media experience. In this demonstration, MDO is
used to augment low channel count renderings of various programme material, delivering immersive
three-dimensional audio experiences.
Pearce A, Brookes TS, Mason RD (2017) Timbral attributes for sound effect library searching, AES E-Library Audio Engineering Society
To improve the search functionality of online sound effect libraries, timbral information could be extracted using
perceptual models, and added as metadata, allowing users to filter results by timbral characteristics. This paper
identifies the timbral attributes that end-users commonly search for, to indicate the attributes that might usefully be
modelled for automatic metadata generation. A literature review revealed 1187 descriptors that were subsequently
reduced to a hierarchy of 145 timbral attributes. This hierarchy covered the timbral characteristics of source types
and modifiers including musical instruments, speech, environmental sounds, and sound recording and reproduction
systems. A part-manual, part-automated comparison between the hierarchy and a freesound.org search history
indicated that the timbral attributes hardness, depth, and brightness occur in searches most frequently.
Mixing music is the process of combining tracks of recorded audio to an overall piece. This is a complicated process and, hence, automatic mixing or metering tools would be useful. The aim of the current research project was to work towards measuring the perceived quality of music mixes by establishing predictors for one important perceptual attribute of high- quality mixes (spectral clarity).

A review of academic and non-academic literature revealed that the high-level parameters that are responsible for determining the perceived quality of a music mix are ?clarity and separation?, ?balance?, ?impact and interest? and ?freedom from technical faults?, alongside context-specific parameters. A further in-depth literature review established that clarity and separation?the chosen focus for this research?depend on spectral, spatial and intensity factors, and temporal changes in these factors. Spectral factors play an important role across all areas of literature consulted (namely timbral clarity, clarity in concert halls, masking, loudness, auditory scene analysis and speech intelligibility), and so the impact of mix EQ on spectral clarity was investigated in a series of experiments.

These experiments determined that two important factors contribute to the spectral clarity of single sounds. These are the harmonic centroid (spectral centroid divided by the sound?s average fundamental frequency) and mid-range spectral peakiness (related to sharp peaks in the frequency spectrum). For sounds modified by simple spectral filtering, these two factors are sufficient to model clarity changes with a Spearman correlation ranging from 0.631 (bass and vocal stimuli) to 0.848 (string stimuli). For sounds in a mix, however, other factors become important. Adding a peak audibility measure proved useful. This measure determined whether the audibility of peaks in the spectra of the target sounds was increased or decreased through EQ. Target and overall mix harmonic centroids and mid- range spectral peakiness, combined with peak audibility, correlated positively with target spectral clarity (r=0.568).

Findings could contribute to the development of marketable products such as a piece of software able to judge the overall sound quality of a mix, automatic mixers or sonically improved music production software. Further work will allow a more comprehensive and generalizable model to be developed.

Francombe J, Brookes T, Mason R (2017) Automatic text clustering for audio attribute elicitation experiment responses, AES 143rd Convention Audio Engineering Society
Collection of text data is an integral part of descriptive analysis, a method commonly used in audio quality evaluation experiments. Where large text data sets will be presented to a panel of human assessors (e.g., to group responses that have the same meaning), it is desirable to reduce redundancy as much as possible in advance. Text clustering algorithms have been used to achieve such a reduction. A text clustering algorithm was tested on a dataset for which manual annotation by two experts was also collected. The comparison between the manual annotations and automatically-generated clusters enabled evaluation of the algorithm. While the algorithm could not match human performance, it could produce a similar grouping with a significant redundancy reduction (approximately 48%).
Francombe J, Brookes T, Mason R (2018) Determination and validation of mix parameters for modifying envelopment in object-based audio, Journal of the Audio Engineering Society 66 (3) pp. 127-145 Audio Engineering Society
Envelopment is an important attribute of listener preference for spatial
audio reproduction. Object-based audio offers the possibility of altering
the rendering of an audio scene in order to modify or maintain perceptual
attributes - including envelopment - if the relationships between attributes
and mix parameters are known. In a method of adjustment experiment,
mixing engineers were asked to produce mixes of four program items at low,
medium, and high levels of envelopment, in 2-channel, 5-channel, and 22-channel reproduction systems. The participants could vary a range of level,
position, and equalization parameters that can be modified in object-based
audio systems. The parameters could be varied separately for different semantic
object categories. Nine parameters were found to have significant
relationships with envelopment; parameters relating to the horizontal and
vertical spread of sources were shown to be most important. A follow-on
experiment demonstrated that these parameters can be adjusted to produce
a range of envelopment levels in other program items.
Francombe J, Woodcock J, Hughes R, Mason R, Franck A, Pike C, Brookes T, Davies W, Jackson P, Cox T, Fazi F, Hilton A (2018) Qualitative evaluation of media device orchestration for immersive spatial audio reproduction, Journal of the Audio Engineering Society 66 (6) pp. 414-429 Audio Engineering Society
The challenge of installing and setting up dedicated spatial audio systems
can make it difficult to deliver immersive listening experiences to the general
public. However, the proliferation of smart mobile devices and the rise of
the Internet of Things mean that there are increasing numbers of connected
devices capable of producing audio in the home. \Media device orchestration"
(MDO) is the concept of utilizing an ad hoc set of devices to deliver
or augment a media experience. In this paper, the concept is evaluated by
implementing MDO for augmented spatial audio reproduction using objectbased
audio with semantic metadata. A thematic analysis of positive and
negative listener comments about the system revealed three main categories
of response: perceptual, technical, and content-dependent aspects. MDO
performed particularly well in terms of immersion/envelopment, but the
quality of listening experience was partly dependent on loudspeaker quality
and listener position. Suggestions for further development based on these
categories are given.
Jackson Philip, Plumbley Mark D, Wang Wenwu, Brookes Tim, Coleman Philip, Mason Russell, Frohlich David, Bonina Carla, Plans David (2017) Signal Processing, Psychoacoustic Engineering and Digital Worlds: Interdisciplinary Audio Research at the University of Surrey,
At the University of Surrey (Guildford, UK), we have brought together research groups in different disciplines, with a shared interest in audio, to work on a range of collaborative research projects. In the Centre for Vision, Speech and Signal Processing (CVSSP) we focus on technologies for machine perception of audio scenes; in the Institute of Sound Recording (IoSR) we focus on research into human perception of audio quality; the Digital World Research Centre (DWRC) focusses on the design of digital technologies; while the Centre for Digital Economy (CoDE) focusses on new business models enabled by digital technology. This interdisciplinary view, across different traditional academic departments and faculties, allows us to undertake projects which would be impossible for a single research group. In this poster we will present an overview of some of these interdisciplinary projects, including projects in spatial audio, sound scene and event analysis, and creative commons audio.
Woodcock James, Franombe Jon, Franck Andreas, Coleman Philip, Hughes Richard, Kim Hansung, Liu Qingju, Menzies Dylan, Simón Gálvez Marcos F, Tang Yan, Brookes Tim, Davies William J, Fazenda Bruno M, Mason Russell, Cox Trevor J, Fazi Filippo Maria, Jackson Philip, Pike Chris, Hilton Adrian (2018) A Framework for Intelligent Metadata Adaptation in Object-Based Audio, AES E-Library pp. P11-3 Audio Engineering Society
Object-based audio can be used to customize, personalize, and optimize audio reproduction depending on the speci?c listening scenario. To investigate and exploit the bene?ts of object-based audio, a framework for intelligent metadata adaptation was developed. The framework uses detailed semantic metadata that describes the audio objects, the loudspeakers, and the room. It features an extensible software tool for real-time metadata adaptation that can incorporate knowledge derived from perceptual tests and/or feedback from perceptual meters to drive adaptation and facilitate optimal rendering. One use case for the system is demonstrated through a rule-set (derived from perceptual tests with experienced mix engineers) for automatic adaptation of object levels and positions when rendering 3D content to two- and ?ve-channel systems.
Hummersone Christopher, Mason Russell, Brookes Tim (2010) A Perceptually?Inspired Approach to Machine Sound Source Separation in Real Rooms,
Automated separation of the constituent signals of complex mixtures of sound has made significant progress over the last two decades. Unfortunately, completing this task in real rooms, where echoes and reverberation are prevalent, continues to present a significant challenge. Conversely, humans demonstrate a remarkable robustness to reverberation. An overview is given of a project that set out to model some of the aspects of human auditory perception in order to improve the efficacy of machine sound source separation in real rooms. Using this approach, the models that were developed achieved a significant improvement in separation performance. The project also showed that existing models of human auditory perception are markedly incomplete and work is currently being undertaken to model additional aspects that had previously been neglected. Work completed so far has shown that an even greater improvement in separation performance will be possible. The work could have many applications, including intelligent hearing aids and intelligent security cameras, and could be incorporated in to many other products that perform automated listening tasks, such as speech recognition, speech enhancement, noise reduction and medical transcription.
Pearce A, Brookes Tim, Dewhirst M, Mason Russell (2016) Eliciting the most prominent perceived differences between microphones, Journal of the Acoustical Society of America 139 (5) pp. 2970-2981 ACOUSTICAL SOC AMER AMER INST PHYSICS
The attributes contributing to the differences perceived between microphones (when auditioning recordings made with those microphones) are not clear from previous research. Consideration of technical specifications and expert opinions indicated that recording five programme items with eight studio and two MEMS microphones could allow determination of the attributes related to the most prominent inter-microphone differences. Pairwise listening comparisons between the resulting 50 recordings, followed by multi-dimensional scaling analysis, revealed up to five salient dimensions per programme item; seventeen corresponding pairs of recordings were selected exemplifying the differences across those dimensions. Direct elicitation and panel discussions on the seventeen pairs identified a hierarchy of 40 perceptual attributes. An attribute contribution experiment on the 31 lowest-level attributes in the hierarchy allowed them to be ordered by degree of contribution and showed brightness, harshness, and clarity to always contribute highly to perceived inter-microphone differences. This work enables the future development of objective models to predict these important attributes.