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SOUND FIELD SIMULATION
ABSTRACT
This document explains the basics behind acoustic
computer simulation. It includes details on how human
hearing uses several techniques to localize sound
sources, how we can simulate factors that influence
human auditory perception with computer software,
and how we can reproduce the listening experience for
a space that has not been built.
auralization, an analogy to vizualization. Through
auralization, it is possible to identify the objective
parameters that correspond to certain subjective
reactions experienced by listeners.
ACOUSTIC SIMULATION
To determine the location of a sound source, our brain
uses a number of different cues, among the following
[Malham 1998]:
It also includes a case study that analyzes numerical
parameters and creates a sound simulation of a space
that allows the listener to sujectively “grade” the
acoustical qualities.
a. Sound reaches an individual’s two ears at
different times. As long as the source of the sound is
not directly behind or in front of a listener, the sound
will arrive at one ear before it arrives at the other. The
time difference is known as “Interaural Time Delay”
(ITD). This effect occurs only at frequencies where the
wavelength is less than twice the distance between the
ears; by this technique alone, humans are unable to
determine the location of sound with longer
wavelengths.
INTRODUCTION
Over the last several decades, the process of predicting
the acoustics of a room in advance of its construction
has advance from an art into a controlled and exact
process. In the last few years computer models have
grown from being just a supplemental tool to becoming
a full substitute of earlier techniques and a superior
design method.
Devices capable of realistically simulating typical room
sound fields will prove to be a useful tool for
understanding the acoustical qualities of spaces. They
will help the acoustician (as well as people outside of
the field) make decisions on projects that address
acoustical issues.
Results can be visualized and analyzed much better
than before because the computer model contains more
information than a set of measurements done in a scale
model or than a single number rendered by formulae.
Today, computer models have become reliable and
efficient design tools for the acoustic consultant, and
“the results of a simulation can be presented not only
for the eyes but also for the ears with techniques for
auralization” [Rindell 2000] .
b. Sound reaches an individual’s two ears at
different levels. When a sound source is located to the
side of a receiver, the sound reaches one ear directly.
The other ear receives sound only after it has diffracted
around the head. Sound arriving at the occluded ear
will therefore be quieter.
c. The human brain can distinguish sound
position relative to a phenomenon called the Head
Related Transfer Function (HRTF). HRTF is a
frequency dependent response that varies with source
location, and is based on the shape of the head and the
external part of the ears. “When the source gives an
ambiguous ITD, this is the brain’s main position-
sensing mechanism” [Malham 1998].
d. By moving one’s head, a listener can vary the
ITD and adjust the HRTF between the ears, giving the
brain more information to determine the sound source
location.
By modeling the significant acoustical parameters of a
design, we can preview a proposed acoustical solution.
The technique of using computer software to predict
and recreate sound in a given room has been called
The best system to recreate the acoustic qualities of a
space is a surround sound alternative known as
Ambisonics. It offers features impossible to realize
through other methods. With this system it is possible
to capture a sound event (such as a musical
performance) and replay it such that, as far as possible,
the original sound and acoustical environment of the
original performance is faithfully recreated.
Ambisonics satisfies simultaneously as many as
possible of the mechanisms used by the brain/ear to
localize sound.
yet built. This software allows the acoustician to define
plane surfaces in order to build a three-dimensional
model of a space in a computer. The acoustician then
assigns to each surface certain material characteristics:
the degree to which it absorbs, reflects and deflects
sound waves. A source can then be defined: where it is
located, how its directivity is shaped, and how powerful
the source signal is.
An additional practical benefit is that the realistic
listening area for Ambisonic Surround Sound is larger
than that of conventional stereo [Elen 2001].
Ambisonics equipment generates a 4-channel signal,
known as the B-Format, that contains all the
information in the soundfield (direction, delay, sound
intensity, etc). These four channels record the event
into left-right, front-back and up-down information
plus a mono reference signal [Elen 2001]. Figure 1
shows the B-Format signal.
The code computes the path of sound from a source to a
receiver. As the sound travels from the source to the
receiver, it may reflect off of walls, defract around
edges, or arrive at a receiver position directly. “These
paths are utilized to simulate and predict the acoustic
qualities of the space” [Markham 2002].
CATT-Acoustic software relies on the real geometric
layout of the space and specific properties (sound
absorption and diffusion) of materials within that space.
The software uses a technique similar to ray tracing
known as “randomized tail-corrected cone tracing” in
order to perform acoustics calculations and simulations
[Dalenback 2002]. Simulations are run with one source
sending its signal through the room. Each point receiver
shows the values for reverberation time (the time in
seconds that it takes for a sound to reduce in sound
pressure level by 60dB after the sound source has been
silenced) and other variables at a sampled listening
post. CATT-Acoustic also illustrates the decay curve at
each receiver point, which indicates how the sound died
out.
Figure 1 B-Format signal
When replayed the ambisonic signal is processed and
fed from a decoder to each speaker independently to
create a horizontal surround. (A minimum of four
speakers are required.) Each signal contains all the
elements of the original recording but in different
ratios, working together to recreate the ambience and
acoustics of the original space.
SIMULATION OF A BUILT SPACE
Before aurally simulating the acoustical environment of
an unbuilt space it is important to affirm the acuracy of
the software. To do this we created and processed a
model of an existing space to compare the numerical
parameters against actual measurments taken at the
physical space.
The Ruth Shapiro Theater is located at Brandeis
Univeristy in Waltham Massachusetts. This 250-seat
hall will accommodate productions by the
Undergraduate Theater Collective and lectures, among
other events. Figures 2 and 3 show an interior image of
the space and a section of the computer model,
respectively.
The important thing to note is that there is no need to
consider the actual details when doing B-Format
recording, synthesis or reproduction: if B-Format
specifications are followed and suitable
loudspeaker/decoder setups are used, “all will be well”
[Malham 1998].
CATT-Acoustic [Dalenback 2002], an acoustical
prediction software, is capable of generating a B-
Format room response of a room which has not been
recommendations from CATT-Acoustic. The values
used derive from personal judgment from visual
analysis of the surfaces.
The computer predictions differ from the average
measurement results by the same magnitude as any two
individual field measurements. All are within the 5%
subjective difference limen range [Gibbs & Oldham]
except in the 1KHz frequency band. Thus, we can
deduce that the accuracy of the best computer
calculations is roughly as good as two consecutive
measurement results and can be considered acceptable.
See graph 1.
Reverberation Time Comparison Graph
Figure 2 Interior image
1.3
Real
Predicted
1.2
5% Limen
1.1
1.0
0.9
0.8
0.7
Figure 3 Ruth Shapiro Theater computer model
125
250
500
1K
2k
4K
Hertz
Graph 1 Reverberation comparison.
The computer model was initially set up for verifing the
results from the reverberation time calculations.
Generally, larger rooms have longer reverberation
times simply because it takes longer for the sound to
travel around the room. Rooms with shorter
reverberation times tend to be better suited for speech,
while rooms with reverberation times around 2 to 3
seconds are good for solo or orchestral music. For
organ music and chant, Rooms with longer
reverberation times are preferred [Cowan 2000].
SIMULATION OF AN UNBUILT SPACE
After validating the reverberation time computer
prediction, the next step it to generate a simulation of
the environment of a space which has not yet been
built; in simple terms, an acoustic test drive.
The need for an “acoustic test drive” arose in the
development of a large atrim space. This enormous
glass structure of 800,000 sqft. is planned as a multi-
purpose space, where large dining events with music
can be held. The lack of absorptive materials in the
original concept, resulting in an unacceptably long
reverberation time, was a concern shared by the owner,
architect and acoustic consultant from the early stages
of the design procsess.
Reverberation time (RT) is far from the only metric
used to judge the acoustics of spaces. However, the RT
is a good starting point since it is a central parameter in
many applications of room acoustics. In order for a
room to achieve appropriate room acoustics conditions,
most acousticians would agree than a room must have
an appropriate RT.
The single most important variable that influences the
acoustics in the atrium is the number of sound
absorbing units, or sabins, in the space. A sabin can be
defined as a totally absorptive area of 1 sqft. The
absorption coefficient ranges from 0.01 to 1; fuzzy
porous materials (velvet, glass fiber insulation) are on
the higher region of the range while hard and dense
materials such as marble have a low coefficient. This
The material characteristics are needed for recreating
the acoustical environment through the computer. The
absorption performance data of the materials were
taken from Cavanaugh [1999] “Acoustical control in
Buildings”. The data on diffusion were deduced from
physical reasoning following the scattering coefficient
study provided the tools to establish objective goals for
the acoustical treatment in the proposed design.
T-1
: Inclusion of sound absorptive treatments on one of
the walls and some structural elements, average
absorption coefficient of 0.18 and a reverberation time
of 3.2 seconds.
CATT-Acoustic was used to calculate the room
response and acoustic signature of three different
versions of the modeled space. Figure 4 shows the
acoustical treatment in color orange on each version.
T-2
: treatments as proposed in Scheme T-1, plus an
additional area of sound absorption, around 4,300
sabins, to represent the amount of treatment that is
believed will achieve the goals of the Owner. This
version has an average absorption coefficient of 0.25
and a reverbaration time of 2.7 seconds. The actual
location of this extra material is on the wall located
directly in front of the previously treated wall.
T-0
Multivolver, a software application that works in
conjunction with CATT-Acoustic, processes the room
impulse response from the computer model, the source
material (eg. music or speech) and the loudspeaker
layout of the test room where the auralization will be
reproduced. An Ambisonic reproduction of the
complete sound field one would experience during an
actual event is generated and allowes reasonable
comparisons of different acoustical conditions,
replicating what a listener would hear in the projected
space for each of the given conditions.
T-1
As source material, recordings of the sound of groups
in a dining environment are used. The recording was
done in a small, rather non-reverberant restaurant
environment so that the room acoustics of the recording
space did not influence the final auralization. The
recordings were processed into the acoustic computer
model so that it simulates the environment of 500
diners, situated at tables around the main volume.
The particular auralization technique developed for this
project allows the listener to hear the various conditions
in the laboratory test room without having to wear
headphones. The sound surrounds the listener as the
sound would in the real space.
T-2
At the acoustic consultant´s laboratory, seven different
types of events in the three differently treated spaces
were developed. The laboratory is a semi-anechoic
room treated with absorptive ceiling and wall panels as
well as carpeted floor; the room characteristics prevent
any reflection from the real space to hamper the virtual
simulation.
Figure 4 Treatment schemes
T-0
: all hard surfaces, no acoustical treatment, average
absorption coefficient (total number of sabins evenly
distributed over all surfaces) of 0.03 and a
reverberation time of ~14 seconds (never actually
considered, but useful for comparison).
For the simulation, four B-Format decoded signals
were fed to four loudspeakers situated in a horizontal
square array. This created a pantophonic (360°) system
that recreated the sound stage of the future atrium. This
helped key persons participating in this project make
their own judgment of the relative effect of different
amounts of sabins in the space. Table 1 is a table given
to the participants to select any of the 21 auralization
available.
T-0
T-1
T-2
subjectively judge acoustics in space, participants must
be able to listen to how the space will sound.
Estimated Reverberation Time (sec) ~ 14.0
3.2
2.7
Estimated Average Absorption
0.03
0.18
0.25
The auralization techniques offer the possibility to use
the ears and listen to the acoustics of a room during the
design process. Several acoustical problems can be
detected by the ears, whereas they may be difficult to
express with a parameter that can only be calculated.
Using these tools the acoustician can communicate the
acoustic consequences of a design to the client/architect
effectively. This technique can be used very early in the
project to achieve the desired results.
Auralization Choices:
Banquet, 500 diners only
Banquet, 500 diners, plus classical
Banquet, 500 diners, plus jazz
Banquet, 500 diners, plus rock
Banquet, 500 diners, plus solo
Banquet, 500 diners, amplified
speech
Typical use, just passers-by
Table 1 Auralization table
The result was a fair and accurate representation of the
various acoustical conditions and ambience. A clear
difference was perceived by the listeners; some even
experimented by having conversations while in the
laboratory simulation such as they would have at a
cocktail party. With versions T-0 and T-1 it was
difficult to perceive the music playing in the
background, and it was very uncomfortable to carry out
a conversation. Although T-2 was by no means quiet
(remember it is 500 diners plus music) it was much
more comfortable to speak to individuals seated on the
opposite side of the table, and the music can be clearly
distinguished. It also has the added advantage the extra
absorption has of noticeably reducing the intensity of
the ambient noise levels.
Nevertheless, it is important to note that the technique
described in this paper relies on a model. With
listeners focusing on acoustical tests in a visually
artificial environment, the experience can obviously
never be totally representative. The acoustician must
always remind the participants of the physical and
psycho-acoustic limitations of the model.
ACKNOWLEDGMENT
This work was supported by Acentech Incorporated.
Ther authors wish to thank Carl Rosenberg and Leslie
Norford for their guidance and support on this research.
REFERENCES
ACENTECH and Cowan, James. Architectural
Acoustics Design Guide, McGraw Hill, 2000.
Markham, Benjamin. “Renovation of Sound.” Thesis,
Bachelor of Science in Engineering, Princeton
2002
The feedback from the listeners was very positive and
they commented favorably on the help this simulation
rendered for the decision-making process. Version T-
2 with the extra 4,300 sabins of treatment was selected
as the most desirable option and is now incorporated on
the design.
Cavanaugh, William and. Wilkes, Joseph.
“Architectural Acoustics – Principles and
Practice”, Wiley & sons, 1999.
CONCLUSION
Previously, acoustics could be measured, quantified
and charted, but with the help of auralization the
subjective character of sound can now be simulated and
judged.
Dalenbach, B.I., “CATT-Acoustic User’s Manual”,
2002.
Elen, Richard, “Ambisonics: The Surround
Alternative” 2001, [article on-line], available from
http://www.ambisonic.net/pdf/ambidvd2001.pdf;
Internet; accessed on 28 March 2004.
In the past, acoustical consultants could only try to
convince the client/architect that with calculations and
geometrical plots that they could create an acoustically
exceptional space.
Gibbs, Barry and Oldham, David, “building
Acoustics”, Multi-Science 2004, [article on-line],
available from http://www.mulit-
science.co.uk.lba_09.htm; Internet; accessed on 28
March 2004.
In the simulation of the unbuilt space discussed in this
paper, architects and acousticians collaborated to create
iterations of the material treatment until the acoustic
goals were achieved. The client was able to identify the
influence of sound reflections and different material
treatement schemes and relate them to a subjective
impression of the sound in the proposed space. To
Malham, D.J., “Spatial Hearing Mechanisms and Sound
Reproduction”, University of York 1998, [article
on-line], available from
http://www.york.ac.uk/inst/mustech/3d_audio/ambi
s2.htm; Internet; accessed on 9 March 2004.
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