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UNTREF, Sound Engineering, Acoustic’s Instruments & Measurements May 2013, Argentina

DESIGN OF A REVERBERATION CHAMBER

RAMÓN FACUNDO1

1 Univesidad Nacional de Tres de Febrero, Sound Engineering, Caseros, Argentina. facundo.ramon@gmail.com

1. INTRODUCTION

A reverberation chamber is an acoustically controlled room where the sound energy distribution is homogeneous throughout the space and a diffuse field is achieved [1]. Ideally, the sound energy inside the chamber decays only due to air absorption and at any point of the room the incident sound comes from all directions; in other words, the walls have to be as much smooth and solid as possible to reflect all the sound energy and symmetry has to be avoid to help the sound wave to travel in every possible direction.

It is used whenever diffuse sound field is needed, mainly to measure absorption coefficient of surfaces or to measure the sound power of a source.

ISO 345:2003 standardized methods for sound absorption measurements in a reverberation room and also describes constructive characteristics and acoustical parameters a reverberation chamber must achieve to fit the standards [2].

This paper reports a design of a reverberation chamber in accordance to ISO 345:2003 and considering the limitations given by real-state’s regulations of Ciudad Autónoma de Buenos Aires in Argentina [3]. 2. PRE-ESTABLISHED CONDITIONS

In order to make a realistic and functional design, the conditions established by real-state’s regulations and ISO 345:2003 are taken in care.

2.1. Terrain

According to urban planning code of Buenos Aires city [4], every terrain has a minimum width of 10 m and its area should contain a rectangle which sizes are related in proportion 2.5 to 1 as minimum. Maximum height is defined for each district. It is considered that limit height is 4 m for this design.

Dividing wall between buildings has to be 0.15 m thickness.

2.2. ISO 345:2003 conditions

ISO 345:2003 specifies the following conditions a reverberation chamber must achieve:

- Volume shall be between 150 m3 and 500m3 - The shape of the reverberation chamber shall be

such that the following condition is fulfilled

!!"# < 1.9  !! ! (1) where !!"# is the length of the longest straight line which fits with the boundary of the room, in meters and ! is the volume in m3

- No two dimensions of the room shall be in the ratio of small whole numbers

- The use of stationary or suspended diffusers or rotating vanes is, in general, required

- The absorption of the surfaces shall not exceed the following values

Table 1: Absorption coefficients per third octave band

[Hz] A [Hz] A 100 6.5 800 6.5 125 6.5 1000 7.0 160 6.5 1250 7.5 200 6.5 1600 8.0 250 6.5 2000 9.5 315 6.5 2500 10.5 400 6.5 3150 12.0 500 6.5 4000 13.0 630 6.5 5000 14.0

where A is the equivalent sound absorption area of the empty reverberation room defined as follow:

! = !!.!  !

!  !− 4!" (2)

where V is the volume of the room, c is the speed of sound, T is the reverberation time of the room and m is the power attenuation coefficient.

! = !!"  !"#  (!)

(3)

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Coefficients in the table are normalized for a room volume of 200 m3, if the rooms volume differs, coefficients shall be multiplied by (V/200m!)! !.

3. DESIGN

The objective of the design is to satisfy all the needs of a laboratory, therefore, not only the chamber is considered; there is room for scientists to work comfortably, a place to store equipment and a reception for customers to wait and being attended (see fig. 1).

3.1. Building in general

The chosen terrain is a rectangle of 10 m by 25 m. It fits the minimum size established by law (see section 2.1). The terrain is divided in two, one part is for the chamber and laboratory and the other for reception and facilities (see fig. 2). The first part is one meter below ground level to give more height to the reverberation chamber. Since the entrance to the building is on the front, there are stairs and a ramp to facilitate the transportation of the stuff that is going to be measure.

Dividing walls are made of solid bricks of 0.15 m thickness.

As shown in figures 1 and 2, there is plenty of space for circulation, working and storing instrumentation.

3.2. Chamber

The chamber is made with solid concrete of 0.3 m of thickness. Its volume is 198 m3 and its total interior surface is 212.6 m2. Dimensions and plan are shown in figures 3 and 4.

There is no symmetry between walls to avoid generation of resonance modes and to help the distribution of sound.

Interior surfaces are made with revoked concrete. The door is 2.3 m high and 1.3 m wide. There is a window of 2 m wide and 0.5 m high made of double glass.

It is important to mention that inside height is 4.3 m, the chamber’s floor and roof are as thick as the walls.

The length of the longest straight line inside the room is 10.8 m; therefore, the condition established by ISO is accomplished (equation 1).

4. ACOUSTICAL PREDICTIONS

The estimated reverberation time of the chamber is 9.3 s. It was obtained with Sabine’s equation [1]

!" = !.!"!  !

!  ! (4)

where V is the volume in cubic meters, S is the total surface of the chamber and ! is the average of absorption coefficient shown in table 1.

Reverberation time per third octave band is shown in table 1.

The equivalent sound absorption areas given by the ISO were corrected for the volume of the chamber (A (ISO) corrected) and compared with the calculated (eq. 2) equivalent sound absorption areas of the design (A Estimated).

Table 2: calculated RT and Absorption coefficients.

As shown in table 2, the standards established by

ISO 345 are accomplished.

5. REFERENCES

[1] Kutrruff H., “Acoustics, an introduction”. E-book version, pp 281-282. Taylor and Francis. New York. USA. 2006. [2] ISO 345:2003(E) “Measurement of sound absorption in a reverberation room”. [3] Ley 449. BOCBA N° 1044. Buenos Aires. Argentina. 2000. [4] Urban Planning code of Buenos Aires City. Link: http://www.ssplan.buenosaires.gov.ar/CPU2010/textos2010.pdf

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Figure 1: General sketch of the building

Figure 2: Plan of the building

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Figure 3: Plan of the reverberation chamber

Figure 4: Isometric view of the reverberation chamber