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

A case study on the new reverberation room built in
University of Technology Sydney
Xiaojun QIU1 ; Qiaoxi ZHU1 ; Shuping WANG1 ; Jiaxin ZHONG11 Centre for Audio, Acoustics and Vibration, Faculty of Engineering and IT,

University of Technology Sydney, Australia

ABSTRACTA new reverberation room has just been built at Centre for Audio, Acoustics and Vibration in
University of Technology Sydney. This paper reports some key parameters of the room, which
include the background noise and its spectrum, the number of modes in low frequency bands, thecut-off frequency, the standard deviation of the spatial variations of the reverberant field, the decay
curves of the sound pressure level versus time, the reverberation times, and the absorption
coefficients. The volume of the room is approximately 232 m3 with a total surface area ofapproximately 247 m2 . The averaged overall background noise level inside the room is 23.1 dB and
18.1 dBA, the number of modes is greater than 30 in one-third-octave bands with centre frequencies
of 160 Hz and above, the standard deviation of the sound pressure levels is less than 1.5 dB in
one-third-octave bands with centre frequencies of 125 Hz and above, and the decay curves ofsound-pressure level versus time is linear in all one-third-octave bands. The reverberation time of
the room can be as long as 20 s in the low frequency range around 100 Hz, and the shortest
reverberation time is about 3.1 s in the high frequency range around 5000 Hz. The equivalentsound absorption areas of the empty room in all one-third-octave bands are smaller than thethreshold values specified in the standard, and the curve is smooth without any dips or peaks which
differ by more than 15% from the mean values of two adjacent one-third-octave bands. The soundabsorption coefficients in all frequency bands of the empty reverberation room are less than 0.02.
Keywords: Reverberation, decay curve

1. INTRODUCTION
A reverberation room is a room w ith low abs orption (i. e. the total room abs orption is smallin comparison with the actual surface area) and large volume (i.e. the smalles t dimens ion islarge in comparis on with the sound wavelength), des igned to produc e a nearly diffus ereverberant sound field (1). It can be used to meas ure sound absorption of materials andstructures (2) and to determination sound power levels of noise s ources us ing sound pressure
prec is ion methods (3). There is no s trict dimens ion requirement for reverberation rooms, but it
is recommended in the standards that the volume of the reverberation room shall be at least 150
m3 (2). For new constructions, the volume is strongly recommended to be at least 200 m 3 , butwhen the volume is greater than about 500 m3 , the sound absorption at high frequenc ies might
be hard to be measured accurately due to air absorption (4).
The shape of the reverberation room shall fulfill certain requirements (2). For example,
the length of the longest straight line which fits within the boundary of the room should beless than 1.9 times of the cube root of the volume of the room. To achieve a uniform
distribution of natural frequenc ies, es pec ially in the low frequency bands, no two dimens ionsof the room shall be in the ratio of small whole numbers . Stationary or suspended diffus ers
are often randomly ins talled (w ith random orientations) throughout the room to make thedecaying sound field suffic iently diffus e. The s ize of the diffusers ranges fromapproximately 0.8 m2 to 3 m2 in area (for one s ide), and they can be plane or s lightly curved
sheets w ith low sound absorption and w ith a mass per unit area of about 5 kg/m 2 . In
1

xiaojun.qiu@uts.edu.au4051

rectangular rooms, the area (both s ides) of diffus ers usually needs to be 15-25% of the total
surface area of the room to achieve satisfactory diffusion (2).
The equivalent sound abs orption area of the empty room shall not exceed certain values(2). For a room with a volume of 200 m3 , this value is 6.5 m2 for the one-third-octave bands
from 100 Hz to 800 Hz. For the bands from 1000 Hz to 1600 Hz, it increas es from 7.0 m2 to
8.0 m2 w ith a step of 0.5 m2 . At the bands of 2000 Hz and 2500 Hz, the values are 9.5 m2 and10.5 m2 respectively. For the bands from 3150 Hz to 5000 Hz, it increas es from 12. 0 m2 to14.0 m2 w ith a s tep of 1. 0 m2 . If the volume of the room V differs from 200 m3 , thes e values
shall be multiplied by (V/200)2/3 . T he curve of the equivalent sound abs orption area of theempty room versus the frequency shall be smooth w ithout any dips or peaks which differ bymore than 15% from the mean values of two adjacent one-third-octave bands.
The prediction accuracy us ing the diffuse -field formulae is related to the degree ofsound-field diffuseness in the reverberation room (5). In some standards, there is a procedure
to ass ist in ensuring that reverberation rooms create highly diffus e fields. For example, inAnnex A of ISO 354-2006, a procedure is introduced to achieve acceptable diffus ivity by
us ing fixed diffus ers. The idea is to check the change of the mean value of the s oundabsorption coeffic ients from 500 Hz to 5000 Hz w ith the number of diffusers until it
approaches a maximum for a standard test spec imen whic h has a sound absorption
coeffic ient greater than 0.9 over the frequency range from 500 Hz to 4000 Hz (2). Even w ith
this highly controlled environment in a reverberation room created and commiss ioned basedon the standard, s ignificant deviations in meas ured quantities have been found in manyround-robin tests (6).
When measuring highly absorptive spec imen of a certain size, the sound field in areverberation room might not be diffus e. Furthermore, those panel diffusers introduce anumber of unc ertainties (7). Due to all thes e randomly placed panels, the acoustical behavior
in the room is much more complicated, and it’s not easy to determine the real acoustic aleffective volume and the total boundary surface of the reverberation room. Volume diffusors
such as fixed reinforced concrete spheric al diffusers have been used w ith an irregular shapeand tilted walls to enhanc e the diffus ivity in a reverberation room w ith the advantage ofknowing the real volume and boundary area of the room (8).
Three reverberation rooms of different s izes (82 m3 , 125 m3 , and 150 m3 ) and shapesprescribed by standards were studied by analyzing their capacity to approximate a diffus esound field in terms of cut-off-frequenc y, number of modes, spatial uniformity of reverberant
sound field, curvature of energy decay curves, accuracy of measured reverberation time andabsorption coeffic ient (5). It is found that the measurement s tarting from the 160-Hz
third-octave band us ing the 150 m 3 oblique-s haped reverberation room w ith the shortest
vertic al dimens ion gives a better reverberation time prediction accuracy and smaller s oundpressure level spatial variation than the other reverberation rooms of different volum es andshapes.
A new reverberation room has just been built at Centre for Audio, Acoustics andVibration in Univers ity of Technology Sydney (UT S). This paper reports some key
parameters of the room, whic h inc lude the background nois e and its spectrum, the number ofmodes in low frequency bands, the cut-off frequency, the standard deviation of the spatial
variations of the reverberant field, the decay curves of the sound pressure level versus time,
the reverberation times, and the absorption coefficients.

2. THE REVERBERATION ROOM AT UTS
2.1 Dimensions
The reverberation room at UTS compris es an inner shell of concrete walls forming four
walls and a roof, all mounted on a des igned concrete floating s lab floor whic h in turn is floatedon correctly loaded anti-vibration mounts to reduce the inference from s urround sound andvibration. T he room has two double leaf triple seal access doors with the dimens ion of 1.30 m
wide and 2.54 m wide, respectively. The structural opening for meas uring the transmiss ion
loss with a receiving room is about 3.83 m wide and 2.99 m high w ith a clear opening 3.37 m
wide and 2. 99 m high. For the tes ts that only use the reverberation room, the opening is c losedwith a two double-leaf triple-seal thic k door. Four randomly placed transparent perspex plan e4052

panels are s uspended as diffusers from the roof of the room to tune the acous tics to meet therequirements of ISO 354. The s ize of the panel is 1.50 m u 1.20 m u 0. 01 m. Figure 1 shows
the geometry of the reverberation room, where HT means the height of wall from floor level
and all units are in millimeter. Figure 2 shows 2 photographs inside the reverberation room.

Figure 1 – Geometry of the reverberation room, where HT means the height of the wall fromfloor level with the default units being millimeter

Figure 2 – Photographs of the reverberation room at UTS
Measurements were taken in 18 one-third-octave bands with the centre frequenc ies of 100
Hz, 125 Hz, 160 Hz, 200 Hz, 250 Hz, 315 Hz, 400 Hz, 500 Hz, 630 Hz, 800 Hz, 1000 Hz,
1250 Hz, 1600 Hz, 2000 Hz, 2500 Hz, 3150 Hz, 4000 Hz, and 5000 Hz according to ISO 354.
The background sound pressure level was meas ured by us ing a Brüel & Kjær hand-held
analyzer (Type 2250), and the other parameters were meas ured by Brüel & Kjær 4191 1/2"
microphones with a Brüel & Kjær 4292-L high-power omnidirectional loudspeaker. The
pos itions of the microphone and the sound source used in the measurements are listed inTable 1, and the origin of the coordinate is at the right bottom corner of the floor of the room
as shown in Figure 1. The temperature and relative humidity in the room during themeasurements are 22.8°C and 68%, respectively.
2.2 Background NoiseThe background noise was meas ured at the 6 microphone pos itions s hown in Table 1
follow ing the requirements of ISO 354. The averaged measured values (arithmetic meanvalue of the values at the 6 microphone pos itions) in the 18 one-third-octave bands and theabsolute criteria for background noise spec ified in ISO 3741 are show n in Figure 3. The
averaged overall background nois e level of the sound press ure level in the whole fre quency
band is 23.1 dB and 18.1 dBA.

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Table 1 – The positions of the microphone and sound source in the measurements
Items
x (m)
y (m)
z (m)
Microphone 1
3.88
4.23
1.23
Microphone 2

5.14

2.66

1.23

Microphone 3

3.66

1.22

1.23

Microphone 4

1.08

3.62

1.23

Microphone 5

2.65

4.61

1.23

Microphone 6

2.27

1.01

1.23

Source 1

4.79

5.84

1.45

Source 2

5.50

0.66

1.45

Source 3

0.41

1.80

1.45

Figure 3 – The average background noise levels in the reverberation room at UTS measured
with a Brüel & Kjær Type 2250 hand-held analyzer and the criteria for background noise
specified in ISO 3741
2.3 Number of Modes in Low Frequency Bands
Figure 4 shows a typical frequency response measured at one corner of the room (0. 80,
0.75, 1. 23) m w ith a loudspeaker at the oppos ite corner of the room (5. 80, 4.80, 1.45) m
us ing the sweep s ine waves, and the number of modes in the firs t 4 one-third-octave bands inthe low frequency range are shown in Table 2. Because 20 is the minimum number of modes
required for s uffic ient field diff useness, it s eems that the reverberation room at UTS meetsthe requirements for one-third-octave bands with centre frequencies of 160 Hz and above.
2.4 Spatial Variations of the Reverberant Field
The standard deviation of the sound pressure levels measured at s ix microphone pos itions
for a sound source pos ition in a one-third-octave band can be calculated with Eq. (1)following Chapter 8.4.1 of ISO 3741,

sM( Li  LM )2¦
NM  1i 1
NM(1)

where Li is time-averaged sound pressure level in the one-third-octave band measured at theith microphone pos ition, LM is the arithmetic mean value of the time -averaged s oundpressure levels in one-third-octave band measured at the s ix microphone pos itions, and NM =6 is the number of microphone pos itions. Table 3 lists the averaged s tandard deviation of thesound pressure levels for the 3 sound sources, which should not exceed 1. 5 dB in any
one-third-octave band. In this room, the averaged s tandard deviation of the s ound pressurelevels at 100 Hz exceeds 1.5 dB, so more diffuse panels might be needed in the room.

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Figure 4 – A typical frequency response in the reverberation room at UTS, where thevertical lines are used to illustrate the first 4 one-third-octave bands with centre frequencies
100 Hz, 125 Hz, 160 Hz and 200 Hz.
Table 2 – Number of modes in the first 4 one-third-octave bands in the reverberation room at UTS
Frequency Band (Hz)

100

125

160

200

Number of Modes

17

17

33

54

Frequency (Hz)

Table 3 – The parameters of the reverberation room at UTS
Averaged
Mean RT (s)standard
Sound absorption Sound absorption(interrupted noise
deviation of thearea (m2 )
coefficients
method)SPL (dB)

100

2.2

15.0

2.43

0.0099

125

1.2

19.3

1.86

0.0075

160

1.2

14.4

2.47

0.010

200

1.1

12.9

2.73

0.011

250

0.8

12.0

2.87

0.012

315

0.6

11.4

2.91

0.012

400

0.5

10.7

3.01

0.012

500

0.5

10.3

2.99

0.012

630

0.3

9.6

3.06

0.012

800

0.2

8.9

3.19

0.013

1000

0.3

8.9

2.99

0.012

1250

0.3

8.1

3.20

0.013

1600

0.2

7.3

3.44

0.014

2000

0.3

6.3

3.80

0.015

2500

0.2

5.6

4.03

0.016

3150

0.3

4.7

4.40

0.018

4000

0.3

3.9

4.75

0.019

5000

0.4

3.1

5.01

0.020

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2.5 Decay Curves of the Sound Pressure Level versus Time
Degree of linearity of temporal-decay curves can be a good indic ator of field diffuseness.
The decay curves of sound-pressure level versus time should be linear to c onform to theassumption of the diffuse-field theory. For each microphone/loudspeaker pos ition, the decay
curve is obtained by averaging 3 meas urements as required by Chapter 7.2.2 of ISO 354. The
decay curves recorded at different microphone/louds peaker pos itions shall not be averaged.
Figure 5 s hows the decay curves of the s ound pressure level versus time at all microphone
pos itions in the one-third-octave bands of 100 Hz, 500 Hz, 1000 Hz and 5000 Hz w ith thesound s ource at pos ition 1. The decay curves over 12.5 s are pres ent. Follow ing Chapter
7.4.1 of ISO 354, the evaluation of each dec ay curve for each frequency band s hall start at 5dB below the initial sound pressure level, the evaluation range shall be 20 dB and the bottomof the evaluation range shall be at least 10 dB above the overall background nois e of themeasuring system.

(a)

(c)

(b)

(d)

Figure 5 – Decay curves of the sound pressure level versus time for the 6 microphones withthe sound source at position 1, (a) 100 Hz, (b) 500 Hz, (c) 1000 Hz, (d) 5000 Hz.
2.6 Reverberation TimeThe interrupted noise method is chosen to measure the reverberation time as spec ified in
Chapter 7. 2 of ISO 354. The exc itation time is set to 20 s in the experiments which is at least
half of the estimate of the expected reverberation time in order to obtain steady -state
conditions according to Chapter 7. 2.1 of ISO 354. The number of spatially indep endentmeasured decay curves shall be at leas t 12. Six microphone pos itions and 3 sound source
pos itions have been used, and their pos itions follow the guidelines s pec ified in Chapters
7.1.2 and 7.1.3 of ISO 354. The mean reverberation time of the room in each frequency bandis express ed by the arithmetic mean of the reverberation time measured in the frequency
band as specified in Chapter 8.1.1 of ISO 354.

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According to Chapter 3.2, the reverberation time (the time w ith a sound pressure level
decrease by 60 dB) can be fulfilled by linear extrapolation of shorter evaluation ranges. The
evaluation ranges for reverberation time (RT) calculation are 20 dB for lower frequency
bands and 30 dB for higher frequency bands. Table 3 lists the mean RT values us ing theinterrupted noise method as spec ified in ISO 354. The reverberation time can be as long as
20 s in the low frequency range around 125 Hz, and the shortest reverberation time is about
3.1 s in the high frequency range around 5000 Hz.
2.7 Sound Absorption Areas and Absorption Coefficients
As defined in Chapter 6.1.4 of ISO 354, the equivalent sound abs orption area of theempty room shall not exceed certain values. For this room w ith a volume of 232.0 m 3 , thisvalue is 7. 15 m2 for the one-third-octave bands from 100 Hz to 800 Hz. For the bands from1000 Hz to 1600 Hz, it increas es from 7.7 m2 to 8.8 m2 w ith a step of 0.55 m2 . At the bandsof 2000 Hz and 2500 Hz, the values are 10.5 m2 and 11. 6 m2 , res pectively. For the bands
from 3150 Hz to 5000 Hz, it increases from 13.2 m2 to 15.4 m2 with a step of 1.1 m2 .
The equivalent s ound absorption areas lis ted in Table 3 are calc ulated w ith Eq. (5) in ISO
354 us ing the mean measured reverberation time of the room in each frequency band in
Section 2. 6. In the c alculation, the volume V = 232.0 m3 , the sound s peed c = 344.7 m/s, andthe power attenuation coeffic ient m1 = D /[10ulog10 (e)], where D is the attenuation coeffic ientdue to atmos pheric absorption calc ulated according to ISO 9613-1 (9). Figure 6 shows thecurve of the equivalent s ound absorption area of this empty room versus frequency. The
curve is smooth w ithout any dips or peaks whic h differ by more than 15% from the meanvalues of two adjacent one-third-octave bands.

Figure 6 – The equivalent sound absorption area of this empty room versus the frequency
The sound abs orption coeffic ient of the empty reverberation room in each frequency bandis c alculated by dividing the equivalent sound absorption area of the empty room with thetotal surfac e area (walls, ceiling, floor and two s ides of each diffuse panels) in the room. In
the calc ulation, the areas of the four walls are 42.34, 34.32, 38. 74 and 40.62 m2 respectively;
the area of the ceiling is 41.22 m2 ; the area of the floor is 35.43 m2 , and areas of the panel
diffuses are 1.80 u 8=14.40 m2 , so the total surface area of the room is 247.2 m2 . T he results
are also listed in Table 3. The absorption coeffic ients in the low frequency range below 160
Hz is less than 0.01. In the frequency range from 200 Hz to 2000 Hz, it increases from 0.01
to 0. 015. About 2000 Hz, the abs orption coeffic ients are greater than 0.015, and the
maximum value is approximately 0.02 at 5000 Hz. The absorption coeffic ients in all
frequency band are less than 0.02.
Future work inc ludes meas uring the s ound absorption coeffic ient w ith sound absorption
materials, comparing the tes t results with that from other acoustics labs in the world (roundrobin tests), studying the effects of the diffuse panels and optimizing the loc ation andnumber of the diffusers, exploring new materials and structures for better sound diffus ion,and building up a fast measurement system.

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3. CONCLUSIONS
A new reverberation room has just been built at Centre for Audio, Ac oustics and Vibrationin Univers ity of Technology Sydney. Th is paper reports some key parameters of the room,
which inc lude the background nois e and its spectrum, the number of modes in low frequency
bands, the cut-off frequenc y, the standard deviation of the spatial variations of the reverberant
field, the decay curves of the sound press ure level versus time, the reverberation times, and theabsorption coeffic ients. T he volume of the room is approximately 232 m3 with a total surfac earea of approximately 247 m2 . The averaged overall background noise level ins ide the roomis 23. 1 dB and 18. 1 dBA, the number of modes is greater than 30 in one-third-octave bands
with centre frequenc ies of 160 Hz and above, the s tandard deviation of the s ound pressurelevels is less than 1.5 dB in one-third-octave bands w ith centre frequenc ies of 125 Hz andabove, and the decay curves of sound-pressure level versus time is linear in all
one-third-octave bands. The reverberation t ime of the room can be as long as 20 s in the low
frequency range around 100 Hz, and the shortest reverberation time is about 3. 1 s in the highfrequency range around 5000 Hz. The equivalent sound absorption areas of the empty room
in all one-third-octave bands are smaller than the threshold values specified in the standard
ISO 354, and the curve is smooth without any dips or peaks whic h differ by more than 15%
from the mean values of two adjacent one-third-octave bands. The sound absorption
coefficients in all frequency bands of the empty reverberation room are less than 0.02.

ACKNOWLEDGEMENTS
This research is supported under the Australian Res earch Counc il’s Linkage Projectfunding scheme (LP160100616).

REFERENCES
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