PROCEEDINGS of the
23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

Experiencing room acoustics through a library of multichannel
high-resolution room impulse responses
Wieslaw WOSZCZYK and David H. BENSON
Centre for Interdisciplinary Research in Music Media and Technology (CIRMMT)
Schulich School of Music, McGill University, Montreal, Quebec, Canada

ABSTRACTA large room impulse response library being developed at McGill University consists of more than 10,000
individual IRs captured using multichannel microphone techniques in a large variety of acoustic spaces.
The challenge is to maximize the learning value of this resource by maximizing the way the library can beexperienced. To accomplish this a search interface is developed for the library that uses audio signal
features, and statistical and analytical treatment, to add rich metadata describing perceptual qualities,
acoustical parameters, and physical characteristics. The user is able to explore 3D graphic models of venues
and audition different locations of sources and receivers, using a variety of anechoic sources. Moreover, anintegrated convolution reverberation plugin enables the user to employ searched spaces for live music
productions and post-production. Strict criteria are followed to maximize the sonic quality of rendering andof the exploration experience.
Keywords: Sound, Room Impulse Response (RIR), Library.

1. INTRODUCTION
There is a practical need to assess and compare acoustical spaces, such as concert halls. Over anumber of decades, the acoustical community has worked to develop parametric characterization ofroom acoustics that would correlate with perceived room quality, and represent a listening
experience at least to some extent (1, 2). While many parametric measures were adopted (3), theirusefulness meets with skepticism, as halls with similar measures do sound different (4, 5, 6).
Listening is the best manner for anyone to assess the perceived quality of an acoustic space.
Analytical listening during a musical performance or testing may reveal important details, which areabsent in parametric characterization of the acoustics. Listening augmented with parametricassessment of the acoustics allows for subjective validation of parameters and their correlation withperceived quality characteristics. Listening also allows verification of the acoustics using suitablemusical material, instruments, voices, repertoire, which are also absent in parametriccharacterization of rooms. Listening may include any location within the enclosure and reflect
personal preference in balancing attributes to form a judgment.
Listening while being present in a hall during a performance may deliver a strong multisensory
impression of the event and the acoustics, but it is usually based on hearing in a single location, at
the most on two listening perspectives. Comparing listening impressions in multiple locations or indifferent halls is not possible live but can be delivered virtually, using auralization. For auralization
to be truly a comprehensive substitute of listening in situ, it must provide a comparable quality ofexperience but excel in flexibility of use. This requires a carefully constructed database of halls.

2. MCGILL’S SPACEBUILDER
A large room impulse response library is being developed at McGill University as a component ofSpacebuilder. Spacebuilder is a convolution reverberation system, which has been in development for
many years at CIRMMT (7, 8, 9). It consists of three components: a large impulse response (IR) library(the Spacebuilder Library), a search interface for the library, which is implemented as a web application7919

(the Spacebuilder Webapp), and a convolution reverb plugin (the Spacebuilder Plugin). The Plugin
includes an embedded web browser, which can display a version of the Webapp‘s search interface. WhenIRs are selected in this interface they are seamlessly downloaded into Plugin’s convolution engine, which
affords Plugin users access to the entire IR library without requiring that it be preinstalled on their machine.
The Library and the Plugin allow for a meaningful and systematic aural comparison of perceived
characteristics in different spaces. They offer comparison in different locations of each venue, enable studyof the interactions between different sources and enclosures, and permit subjective evaluation of audible
alterations made to each virtual space.
Two modes of listening. The Library and the Plugin allow for two modes of listening:stereophonic - directly in the Spacebuilder Webapp; and 3D immersive – via the Plugin, installed
within a digital workstation (DAW), and using a suitable 3D monitoring environment.
2.1

The Library
At present, two interfaces are provided for searching and browsing the library. These are the Venue
View and Feature View. The former displays IRs from one venue at a time and emphasizes information
related to how these IRs were measured. This includes the size and shape of the venue, and the positions of
the IRs’ sources and receivers within it. The latter interface displays various signal features calculated fromthe IRs. Signal features in this context refer to standard room acoustic parameters such as the Early Decay
Time (EDT) and Clarity (C80), as well as lesser-known measures. The two interfaces are each described in
more detail below.
2.2 Venue View
The Venue View is designed to allow a spatial exploration of the acoustics of each venue. In thisview, venues are visualized as 3D models, and each IR is visualized by a pair of icons representing
its source and receiver position. Box-shaped yellow icons indicate sources while round blue icons
indicate receivers. IRs can be selected for listening by clicking on a source and receiver icon, or by
selecting the IR in a textual list (Figure 1). Either of these actions loads the IR into a convolution
engine, where it can be convolved with a sound source and auditioned. The currently loadedsource-receiver pair is always highlighted in red.

Figure 1 – The Venue View showing the IR list, with one IR selected

7920

The Venue View also displays architectural and cultural information related to each venue. The
3D model is rendered in WebGL and may be freely rotated, zoomed and repositioned in real-time.
Additionally, buttons allow the visual perspective to be quickly changed to a “floorplan” view (fromdirectly above) or a “profile” view (from the side). Information about fine details of the venue, suchas the nature of wall or floor coverings, can be gathered from a set of included photographs. Finally,a short text explains the venue’s noteworthy acoustic qualities and its cultural significance (Figure
2).

Figure 2 – The Venue View showing a venue photo and descriptive text
2.3
Feature View
Unlike the Venue View, which focuses on communicating information related to where an IR was
measured, the Feature View is instead focused on conveying physical and perceptual qualities
inherent to the IR’s signal. In this view, each IR is displayed as a point on a two-dimensional
scatterplot. The axes of the plot represent features calculated from the IR signals. Any of a large
collection of features, described below, can be assigned to the X or Y axis.
While the display shows the entire library by default, functionality also exists to “filter” the data,or to hide IRs according to certain criteria. IRs can be filtered according to feature values, using a setof range sliders, for example to hide IRs outside of a narrow range of EDT values. Finally, theinterface also allows for filtering by venue, permitting investigations of inter-venue feature
differences (Figure 3).
2.4 Signal features
The features available in the Feature View belong to three broad categories. First are features
calculated directly from the impulse response. These features generally originate from the room
acoustics community. Second are features from the room acoustics community that are calculated noton impulse responses, but rather on reverberant audio signals. (A reverberant audio signal is taken
here to mean the convolution output of an IR and an anechoic musical signal.) Third are features thatoriginate from outside of the room acoustics community.

7921

2.4.1 Impulse-response based features
The first group includes many of the standard room acoustic measures described in the ISO
3382-1 document (10) such as the T30, the EDT and the C80. These measures are available in 8
octave bands, as well as in A, C and linearly weighted versions, as calculated by the AcMus Matlab
library (11). This first group also includes the Interaural Cross Correlation measure (IACC) as
described by Hidaka, Beranek and Okano (12), as well as measures of IR spectral balance, such as
the Early Bass Level and Treble Ratio described by Bradley and Soulodre (13).
Also included are several impulse response-based features that are either more recently developedor whose relationship with perceptual attributes of reverberation is less well understood. These
include the Deviation of Level (14), the Normalized Echo Density mixing time of Abel and Huang(15), and the Number of Peaks measure of Jeon and Vörlander (16).

Figure 3 – The Feature View showing the 1 kHz EDT and PASW for three selected venues
2.4.2 Room acoustic (spatial) reverberant audio features
The second group of features, like the first, has also been designed to predict perceptual attributesof reverberation. Unlike the first group, however, the second group are features calculated onreverberation signals rather than on impulse responses. To compute the features in this group, the IRs
in the library were first convolved with a set of four representative monophonic musical signals (solosinging, choir singing, drum kit, and orchestra) to generate four reverberant audio signals. Features
were calculated on these four convolution outputs.
The four features in this group developed from recent work towards predicting room acoustic
attributes using binaural modeling of van Dorp Schuitman (17). The features P REV , P CLAR , P ASW , andP LEV are designed to correlate, respectively, with perceived reverberance, clarity, apparent source
width and listener envelopment. The features are computed over short frames of the audio signals;
the average values over the duration of each signal are recorded in the system.

7922

2.4.3 Magnitude spectrum (timbre) reverberant audio features
The third and final group differs from group one in being calculated on reverberant audio signals,
and differs from group two in not having been designed specifically to analyze reverberation. Thesefeatures all describe timbral aspects of the signal’s magnitude spectrum, namely the Spectral Slope,
Spectral Skew, Spectral Decrease and Spectral Flatness. All are calculated via the open source
Timbre Toolbox (18). As with the second group, only the average value over the all signal windows
is recorded in the system.
2.5
The Spacebuilder Plugin
The Spacebuilder audio-plugin is software that transforms the input signal via several
audio-processing algorithms. It contains multiple “multi-channel processing units” (MCPUs), which
are able to perform certain digital signal processing tasks: convolution, linear decay-slope
transformation (RT adjustment), frequency-dependent (linearly distorted) decay slope transformation,
time delay, equalization, dynamic processing, switchable between Compressor/Expander, oravailable simultaneously, tremolo (amplitude) modulation at sub-audio modulation rates (slow andvery slow oscillations), and diffusion (applied at the front end – to input signals).
Multiple MCPUs can be engaged simultaneously to allow comparing and combining several
reverberation streams, including adjustments of their positions in the rendering space. The
adjustability of each stream offers an ability to fine-tune reverb quality, including gentle time
variance and motion. The editing, trimming the IR in time, is used to aurally evaluate temporal
sections of the IR. Segmentation of IRs has proven to be useful for building parts of hybrid IRs (19).
As stated earlier, the Plugin is integrated with the Spacebuilder Library and the Spacebuilder
Webapp via an embedded web browser. Both components of the Webapp’s search interface (i.e., theVenue View and the Feature View) are available to Plugin users for selecting IRs.
2.5.1 The Intended Plugin Applications
When installed in a DAW (digital audio workstation), the plugin may output convolution streams
to multiple loudspeakers or binaural processors. Its primary fields of application are post-production
for film and music recording, and live sound applications, including concerts in virtual acoustics andauralization of architectural spaces. A unique feature of the library is to allow 3D rendering of virtual
acoustics with immersive characteristics including height. Different reverberant streams can be
combined into a common spherical space image, and hybrid designs of new spaces having
customized perceptual characteristics can be built. Architectural designers and acousticians may findthe coordinated aural and parametric approach to be useful for research and development.
2.6
The Quality of Room Impulse Responses
One of the challenges in measuring a representative room impulse response involves the choice
and arrangement of electroacoustic radiators emanating analytical signal within the room. Can aloudspeaker properly substitute for voices and musical instruments having different frequency anddynamic range, varied dimensions, and complex variations in directivity as solo, group, or orchestra?
Musical instruments are arranged on stage in depth, height, and width, and listeners too are broadly
distributed in the hall. Changes in directivity, pitch, dynamics, articulation, gestures, and movement
during performance are difficult to represent in a time-invariant capture of a room response triggeredby a fixed loudspeaker’s signal. Occlusion, diffraction and absorption of sound radiated amongmusicians on stage are another complicating factor, as is the acoustic impact of audience members inproximity to individual listeners. All of these factors affect audibility of music performance (20).
This abundance of factors cannot be precisely accommodated and accounted for in a measured IR,even less so in an IR of a modeled space. So, the complexity of musical radiation must be somehow
approximated in the measurement, and expanded upon later during the rendering process forauralization (21, 22). When choosing and adjusting microphone arrays, the focus must be on
capturing the clarity and aural intelligibility of the room, conveying its spatial, temporal, anddynamic uniqueness. This, in principle, is similar to a photographic rendering of a room.

7923

2.6.1 Source representation
Every effort has been made to articulate key characteristics of real acoustic sources (voices,
instruments, groups) radiating energy into multiple directions. During IR measurements, multiple
radiators were set up to emulate musical sources with two goals in mind: to engage room reflections
cultivating directions that enhance room perception (lateral and vertical sound energy) (23), and todeliver a convincing source image to listeners conducting the measurements. Ten sources(bidirectional, unidirectional, omnidirectional) were adjusted in situ to produce a compelling imageof voice, trumpet, chorus, French horn, clarinet, acoustic guitar, and percussion, using playback ofanechoic monophonic recordings. Radiation synthesis of other virtual sources was previously tested
in the lab, including grand piano, strings, and woodwinds. The grouped radiators covered afrequency range from 20Hz to 40kHz to assure adequate representation of physical dimensions andfrequency characteristics of all instruments. To minimize interference, the sources were spaced apartand overlaps in frequency and direction were avoided. The 10 sources managed to significantly
increase the number of reflections as they represented a volumetric source, not a point source.
2.6.2
Receiver representation
The capture of room response requires a 3D microphone array to record the arrivals of reflections
and reverberation from all directions. A channel-based method of capture was employed withomnidirectional and bidirectional high quality microphones receiving room response at three heights,
2m, 3m, and 4m, from near the audience level to higher up in the free field. Channel-based methods
were developed over decades to record music, and are accepted as being able to deliver to a humanlistener a believable image of musical performance in a room. The richness of time- andintensity-based stereophonic cues captured in three dimensions gives compelling auralization resultsin both loudspeaker and headphone listening (24, 25).
2.6.3 Capturing musical sources for auralization
To take advantage of the 3D room impulse responses, the input signal sent to the convolution
engine also needs to be captured in three dimensions, including width, depth, and height. The
simplest receiver of spherical sound field at the source is a tetrahedral microphone containing fournearly-coincident capsules arranged in 3 dimensions (26). The Spacebuilder plugin allows thecapsule signals to be routed into appropriate convolution channels, so that e.g. the upward radiationof the source is convolved with the elevation channels of the IRs.

3. CONCLUSIONS
Experiencing halls aurally via the Library containing a large number of measured IRs provides
educational and research value, as it allows for comparisons of acoustical properties, finding
examples of halls with specific perceptual and parametric characteristics, while auditioning a variety
of music sources. For example, one may study the aural effect of the shape of the enclosure on its
acoustic response, the influence of source and listener location, assess the sound quality on stage andin the auditorium, and simulate reverberation time changes due to air volume, absorption from seatcoverings, presence of audience, scattering from acoustic finishes, and even the effect of temperatureand humidity.
Independent multichannel convolution engines contained in the plugin may be used to study
audibility of aural fusion and segregation (27) of sources when changing parameters of auralization.
What room characteristics improve perceptual separation and blending of sources? How do different
sources uniquely engage the room and determine an overall spatial image perceived by a listener? To
what extent do sources and rooms depend on each other, functionally and aesthetically? We hope tostudy such questions using these Library resources.
Can auralization replace the experience of hearing live musicians who interact with the acoustics
and the audience? Perhaps. Virtual acoustics immerses both the audience and the musicians in acommon simulated space, and allows the spaces to be changed. It can sustain an emotional
connection between musicians and the audience, and between musicians and the room (28). The
comprehensive toolset of the library, the search engine, and the convolution plugin, may be
employed to create an acoustic space that articulates the music and enhances meaningful interactions.
Virtual acoustics is becoming a formidable tool for designing rooms by experience.

7924

ACKNOWLEDGEMENTS
The authors would like to thank Vanille Patier-Debray for her help curating the Library’s
metadata. Numerous collaborators have helped to build the McGill’s Spacebuilder system and the
Library, and we graciously thank them for their commitment. The financial assistance of the Social
Sciences and Humanities Research Council and the Natural Sciences and Engineering ResearchCouncil of Canada are acknowledged. We thank all musicians and artists who engage to express theirmusic in virtual acoustics.

REFERENCES
1. Schroeder M, Gottlob G, Siebrasse K. Comparative study of European concert halls: Correlation ofsubjective preference with geometric and acoustic parameters. The Journal of the Acoustical Society ofAmerica 56, (1974) 1195-1201.
2. Beranek LL. Concert hall acoustics: Recent findings. The Journal of the Acoustical Society ofAmerica 139, Issue 4, 1548 – 1556 (2016).
3. ISO 3382-1:2009(en) Acoustics — Measurement of room acoustic parameters — Part 1: Performance
spaces. International Organization for Standardization, Geneva, Switzerland. 2009.
4. Kirkegaard L, Gulsrund T. In search of a new paradigm: How do our parameter and measurement
techniques constrain approaches to concert hall design? Acoustics Today 7(1), (2011), 7-14.
5. Lokki T. Throw away that standard and listen: your two ears work better. Journal of Building Acoustics
20(4), (2013), 283-294.
6. Hochgraf KA. The Art of Concert Hall Acoustics: Current Trends and Questions in Research and Design.
Acoustics Today, Spring 2019, volume 15, issue 1, p. 28-36.
7. Woszczyk W, Leonard B, Benson D. Spatial sound design tool for 22.2 channel 3-D audio productions,with height. Audio Engineering Society Convention Paper Number: 8753, in Proceedings of the 133rd
Convention, San Francisco, October 26-29, (2012).
8. Benson D, Woszczyk W. Comparing search interfaces for room impulse response libraries. AudioEngineering Society Paper Number: 8758, in Proceedings of the 133rd Convention, San Francisco,
October 26-29, (2012).
9. Ko D, Woszczyk W, Chon S.H. Evaluation of a new active acoustics system in performances of five
string quartets”. Audio Engineering Society Convention Paper Number: 8603, in Proceedings of the132nd Convention, Budapest, Hungary, April 26–29, 2012.
10. ISO 3382-1:2009(en) Acoustics — Measurement of room acoustic parameters — Part 1: Performance
spaces. International Organization for Standardization, Geneva, Switzerland.
11. Masiero BS. AcMus - Room Acoustic Parameters, São Paulo; 2007. [cited May 31, 2019].
Matlab Central, The MathWorks Inc.
https://www.mathworks.com/matlabcentral/fileexchange/11392-acmus-room-acoustic-parameters
12. Hidaka T, Beranek LL, Okano T. Interaural cross-correlation, lateral fraction, and low- and highfrequency sound levels as measures of acoustical quality in concert halls. J Acoust Soc of Am. 1995;
98(2): 988-1007.
13. Soulodre G, Bradley J. Subjective evaluation of new room acoustic measures. J Acoust Soc of Am.
1995; 98(1): 294-301.
14. Takahashi D, Togawa K, Hotta Tetsuo. Objective measures for evaluating tonal balance of sound fields.
Acoust. Sci. & Tech. 2008; 29(1):1-8.
15. Abel J, Huang P. A Simple, Robust Measure of Reverberation Echo Density. In AES Convention 121;
2006 Oct 5-8; San Francisco, California, United States. Available from: AES E-Library.
16. Jeon JY, Jang HS, Kim YH, Vorländer M. Influence of wall scattering on the early fine structures ofmeasured room impulse responses. J Acoust Soc Am. 2015;137(3):1108-1116.
17. van Dorp Schuitman J, de Vries D, Lindau A. Deriving content-specific measures of room acoustic
perception using a binaural, nonlinear auditory model. J Acoust Soc Am. 2013; 133(3):1572-1585.
18. Peeters G, Giordano BL, Susini P, Misdariis N, McAdams S. The Timbre Toolbox: Extracting audio
descriptors from musical signals. 2011;130(5):2902-2916.
19. Woszczyk W, Ko D, Leonard B. Convolution-based virtual concert hall acoustics using aural
segmentation and selection of multichannel impulse responses, The Proceedings of INTER-NOISE
2009, The 38 International Congress and Exposition on Noise Control Engineering, Ottawa, Canada,
August 23-26, 2009.
20. Meyer J. Acoustics and the Performance of Music: Manual for Acousticians, Audio Engineers,Musicians, Architects and Musical Instrument Makers, 5th ed. Springer-Verlag, New York. 2009.

7925

21. Sacks J, Kleiner M. Front-to-rear movement of reverberation in a concert hall. The 148th Meeting of the
Acoust. Soc. Am. San Diego, California, 15-19 November 2004.
22. Hong J, Woszczyk W, Begault D, Benson D. Synthesis of moving reverberation using active
acoustics—preliminary report”, 138th Convention of Audio Engineering Society Paper Number: 9326,
Warsaw, Poland. (May 2015).
23. Pätynen J, Lokki T. Concert halls with strong and lateral sound increase the emotional impact oforchestra music. J. Acoust. Soc. Am. 139 (3), March 2016, p. 1214-1224.
24. Ko D, Woszczyk W. Virtual acoustics for musicians: subjective evaluation of a virtual acoustic system
in performance of string quartets. J. Audio Eng. Soc., vol. 66, no. 09, pp. 1–12, (2018 September).
25. Kaushik S, Woszczyk W. Investigation of impulse response recording techniques in binaural renderingof virtual acoustics. 141st Convention of Audio Engineering Society Convention Paper 9609, Los
Angeles, USA. (October 2016)
26. Woszczyk W, Nulsen J, Hahn E, Degtyareva K, Castillo J, Nagata H. Tetrahedral microphone: aversatile “spot” and ambience receiver for 3D mixing and sound design”.
Audio Engineering Society
Conference e-Brief 57, published and presented at the International Conference on Spatial Reproduction– Aesthetics and Science, 2018 August 7–9, Tokyo, Japan (2018)27. Bregman, A. S. Auditory Scene Analysis. The MIT Press, Cambridge, MA. 1990.
28. Abel JS, Woszczyk W, Ko D, Levine S, Hong J, Skare T, Wilson MJ, Coffin S, Lopez-Lezcano F.
Recreation of the acoustics of Hagia Sophia in Stanford’s Bing Concert Hall for the concert
performance and recording of Cappella Romana, ISRA 2013, Proceedings of the International
Symposium on Room Acoustics, Toronto, Canada, 2013 June 9-11.

7926