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Acoustics Instruments and Measurements July 2013, Caseros, Buenos Aires Province, Argentina

REVERBERANTION CHAMBER DESIGN

AGUSTÍN Y. ARIAS

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1 Universidad Nacional de Tres de Febrero, Buenos Aires, Argentina.

agustin.arias@outlook.com

1. INTRODUCTION

A reverberation chamber is, basically, a room that

has a long reverberation time and is designed as

diffuse as possible. The construction of the room

should realize a high performance of sound insulation

from any noise that comes from outside, since the

interior of the room is used primarily for acoustics

characteristics of material testing, which requires

complete independence of any unwanted outside

sound. Furthermore, the materials on the surface of

the inner walls must be carefully chosen, for

minimum absorption of sound energy. Reducing the

sound energy absorption means to increase the

energy of the reflections, which leads to achieve a

totally diffuse field and a long reverberation time.

Thus the factors that dominate the sound attenuation

are: air absorption, which is considerable regarding

the size of the chamber, especially at high

frequencies, and the low absorption coefficient of the

room surfaces.

In this report the design of a reverberation

chamber is presented according to the requirements

of ISO-354 “Acoustics - Measurement of sound

absorption in a reverberation room” [1]. The

construction details are specified and finally the

simulation results are shown for evaluating

reverberation time within the chamber.

2. ISO 354 REQUIREMENTS

As mentioned above, the reverberant chamber

design must meet certain essential characteristics

defined in the Standard ISO-354. The most important

are:

The minimum volume of the chamber should be

approximately 200 m3.

The room should allow a large diffusion of the

sound field, for which suspended diffusers are

needed (large plates that hang from the ceiling to

improve the sound diffusion).

The relative humidity in the chamber should be

greater than 40%, and temperature above 10 º C.

The shape of the reverberation room shall be

such that the following condition is fulfilled:

(1)

Where is the length of the longest straight

line which fits within the boundary of the room

(e.g. in a rectangular room it is the major

diagonal), in meters. V is the volume of the

room, in cubic meters.

In order to achieve a uniform distribution of

natural frequencies, especially in the low-

frequency bands, no two dimensions of the room

shall be in the ratio of small whole numbers.

The equivalent sound absorption area of the

empty room, A1 determined in one-third octave

bands, shall not exceed the values given in Table

1. If the volume V of the room differs from 200

m3, the values given in Table 1 shall be

multiplied by (V/200 m3)

2/3.

Table 1. Maximum equivalent sound absorption areas for

room volume V = 200 m3

Frequency [Hz] A1 [m2] Frequency [Hz] A1 [m

2]

100 6,5 800 6,5

125 6,5 1000 7

160 6,5 1250 7,5

200 6,5 1600 8

250 6,5 2000 9,5

315 6,5 2500 10,5

400 6,5 3150 12

500 6,5 4000 13

630 6,5 5000 14

3. DESIGN

3.1. Reverberation chamber

There is no ideal way to build reverberant

chambers, but it's better to select non-uniform

asymmetrical. In this manner, the reverberant field

produced indoor will be as diffuse as possible. Figure

1 shows a 3D model of the reverberation chamber

from an external and internal view.

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Figure 1. 3D model.

The volume of the chamber is 399.52 m3 and the

total surface is 337.37 m2. The constructive details

will be described below. From Figures 2 and 3, it is

possible to observe that the walls and ceiling of the

chamber are asymmetrical. There is a double-panel

window type “fishbowl” that communicates the

chamber interior to the control room. The access to

the chamber is through a double steel gate (Figure 4).

This gate must be carefully installed with weather-

stripping in order to minimize the external noise

transmission into the room. The size of the gate

allows access industrial machinery for measurement

of acoustic power. Figure 3 shows the front view,

plan view and cross-section view of the chamber

indicating the main dimensions. To avoid any type of

background noise and to prevent vibration

transmission within the chamber, it was located

inside of a big structure of solid brick, as it can be

observed in Figure 2. The left-side wall of the

enclosure was removed for a better understanding.

Also it can be observed the side hall conducting to

the control room behind the chamber.

Figure 2. Solid brick structure covering the chamber (left-

side wall removed).

Figure 3. Reverberation chamber views. Top: cross-section

view. Middle: plane view. Bottom: front view.

Figure 4. Double steel gate.

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3.2. Control room

The control room is placed behind the back wall

of the reverberation chamber. A double glassed

window allows a direct vision to the interior of the

chamber. The control room is used to install the

external equipment necessary to perform the

measurements (desktop and personal computers,

power amplifiers, mixer, cables patching (XLR-TRS

¼”), etc. Figure 5 shows a 3D model of the control

room.

Figure 5. 3D model of the control room.

4. SOUND INSULATION: WALLS AND

FLOORS

As it was mentioned, besides achieving a

complete diffuse sound field within the chamber, it is

necessary a complete insulation to any external noise.

This requirement leads to the design of the surface

structure. The walls of this structure are designed as

indicated in Figure 6 [2]. In addition to this structure

design, it may be added a metal mesh in the air gap

between the walls to avoid electromagnetic

interferences (Faraday Cage).

The Acoustic Reduction Index for that partition is

shown in Figure 7. The Acoustic Reduction Index

weighted is Rw = 57 dBA.

Regarding the construction of the reverberation

chamber, a double wall was designed. In addition, a

floating floor is required to avoid any type of

vibrations transmitted to the chamber interior. Figure

8 shows the wall structure of the chamber.

The Acoustic Reduction Index for that partition is

shown in Figure 9. The Acoustic Reduction Index

weighted is Rw = 47 dBA.

Figure 10 shows the floating floor of the chamber

and the walls design. Finally, the ceiling of the

reverberation chamber is made of a reinforced

concrete slab of 140 mm thickness. The space

between the cover structure and the chamber is

coated with glass wool “ISOVER” PV 40 mm

thickness.

Figure 6. Side walls of the cover structure.

Table 2. Side walls materials of the cover structure.

Item Wall

structure Material

Thickness [mm]

Weight [k/m

2]

4a Plasterboard

lining Plasterboard 10 8.0

4 Panel

“ISOVER” Calibel

Fiberglass 25 1.7

3 Air chamber - 20 -

2 Gripping

paste - - 4.0

1 Solid brick partition

Ceramic 120 180

Total 175 194

Figure 7. Acoustic Reduction Index of the cover structure

partition. (- - -) without acoustic treatment. (---) with Panel “ISOVER” Calibel.

R [

dB

]

Frequency [Hz]

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Figure 8. Side walls of the reverberation chamber.

Table 3. Side walls materials of the reverberation chamber.

Item Wall

structure Material

Thickness [mm]

Weight [k/m2]

4 Double

hollow brick wall

Ceramic 80 -

3 Panel

“ISOVER” PV Glass wool 40 -

2 Simple

hollow brick Ceramic 35 -

1 Laying of plaster

Plaster 10 -

Total 165 140

Figure 9. Acoustic Reduction Index of the reverberation

chamber walls.

Figure 10. Reverberation chamber walls and floor

construction.

5. INDOOR ENVIRONMENT OF THE

REVERBERATION CHAMBER

As shown in Figure 10, the indoor surfaces of the

chamber consist of tile walls, and terrazzo floor.

These materials were chosen because of their low

sound absorption coefficient, which are detailed in

Figures 11 and 12. To improve sound diffusion inside

the chamber and thus increase the reverberation time

especially at high frequencies, the installation of

fixed and removable diffusing surfaces is

recommended. For example, fixed diffusing surfaces

may be convex wooden plates MDF (medium-density

fiberboard) as seen in Figure 13.

Figure 11. Absorption values of Tile

Figure 12. Absorption values of Terrazzo

Double wall Terrazo Tile

Concrete

Felting “ISOVER” FF

12mm

Reinforced

concrete 100mm

R [

dB

]

Frequency [Hz]

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Figure 13. MDF convex board.

The absorption coefficients of the double-panel

window and the steel gate are shown in Figures 14

and 15 respectively.

Figure 14. Absorption values of double-panel window

Figure 15. Absorption values of steel gate.

6. SIMULATION

The reverberant chamber was modeled on EASE

to predict the reverberation time with the surface

absorptions mentioned above. The reverberation time

was calculated according to Sabine’s equation.

V: volume of the chamber [m3]

Atot: total absorption of the chamber [m2]

m: attenuation sound constant in air

Figure 16. Chamber model in EASE.

The results obtained are shown in Figure 17. It is

observed the high influence of the air attenuation at

high frequencies, so the diffuser installation is highly

recommend. At low frequencies the reverberation

time remains above 10 s, which indicates an excellent

performance of the chamber in those frequencies

bands.

Figure 17. Reverberation time obtained according to

Sabine’s equation.

7. BUILDING

In addition to the reverberant chamber and the

control room, the building has two administrative

rooms, a bathroom and a dining room, as shown in

Figure 18. The total terrain area is 227.27 m2. The

principal dimensions are: width 9.38m, 24.23m long

and 6.34 m height. These dimensions were adjusted

to the urban planning code of Buenos Aires [3]. In

addition, the urban planning code establishes that the

dividing wall between two adjacent buildings must be

at least 150 mm thickness.

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Figure 18. Building rooms.

8. REFERENCES

[1]ISO-354 “Acoustics - Measurement of sound

absorption in a reverberation room

[2]ISOVER “Manual de Aislamiento”.

[3]Ley 449. BOCBA N° 1044. Buenos

Aires.Argentina. 2000.

Administrative rooms

Dinning room

Bathroom

Reverberation

chamber

Steel gate

Control room