Reverberation Chamber Design 1x3t6l

Acoustics Instruments and Measurements

July 2013, Caseros, Buenos Aires Province, Argentina

REVERBERANTION CHAMBER DESIGN AGUSTÍN Y. ARIAS 1 1

Universidad Nacional de Tres de Febrero, Buenos Aires, Argentina. [email protected]

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 diffs 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 lowfrequency 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 2 2 Frequency [Hz] A1 [m ] Frequency [Hz] A1 [m ] 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

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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- 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 weatherstripping 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 (leftside wall removed).

Figure 3. Reverberation chamber views. Top: cross-section view. Middle: plane view. Bottom: front view.

Figure 4. Double steel gate. 2

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 6. Side walls of the cover structure. Table 2. Side walls materials of the cover structure. Item 4a 4 3 2

4. SOUND INSULATION: FLOORS

WALLS

AND

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.

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Material

Thickness [mm]

Weight 2 [k/m ]

Plasterboard

10

8.0

Fiberglass

25

1.7

-

20

-

-

-

4.0

Ceramic

120

180

175

194

R [dB]

Figure 5. 3D model of the control room.

Wall structure Plasterboard lining “ISOVER” Calibel Air chamber Gripping paste Solid brick partition Total

Frequency [Hz] Figure 7. Acoustic Reduction Index of the cover structure partition. (- - -) without acoustic treatment. (---) with “ISOVER” Calibel.

3

Double wall

Figure 8. Side walls of the reverberation chamber.

3 2 1

Material

Thickness [mm]

Weight [k/m2]

Ceramic

80

-

Glass wool

40

-

Ceramic

35

-

Plaster

10

-

165

140

Felting “ISOVER” FF Figure 12mm 10. Reverberation chamber walls and floor construction.

5.

INDOOR ENVIRONMENT OF REVERBERATION CHAMBER

THE

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.

R [dB]

4

Wall structure Double hollow brick wall “ISOVER” PV Simple hollow brick Laying of plaster Total

Tile

Reinforced concrete 100mm

Concrete

Table 3. Side walls materials of the reverberation chamber. Item

Terrazo

Figure 11. Absorption values of Tile

Frequency [Hz] Figure 9. Acoustic Reduction Index of the reverberation chamber walls.

Figure 12. Absorption values of Terrazzo 4

Figure 16. Chamber model in EASE. Figure 13. MDF convex board.

The absorption coefficients of the double- window and the steel gate are shown in Figures 14 and 15 respectively.

The results obtained are shown in Figure 17. It is observed the high influence of the air attenuation at high frequencies, so the diff 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 14. Absorption values of double- window

Figure 17. Reverberation time obtained according to Sabine’s equation.

7. BUILDING

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.

In addition to the reverberant chamber and the control room, the building has two istrative 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.

V: volume of the chamber [m3] Atot: total absorption of the chamber [m2] m: attenuation sound constant in air 5

Bathroom

Control room

Dinning room istrative rooms

Steel gate

Reverberation chamber

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.

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