Applied Acoustics 29 (1990) 139-149

A c o u s t i c D e s i g n o f a n A n e c h o i c C h a m b e r

Xiang Duanqi, Wang Zheng & Chen Jinjing

Acoustic Laboratory, Beijing Institute of Architectural Design, Beijing 100045, People's Republic of China

(Received 13 March 1989; accepted 16 September 1989)

ABSTRACT

This paper describes the acoustic design of an anechoic chamber built mainly for the measurement of the acoustic properties of electrical equipment. In order to reduce background noise the anechoic chamber was built on a 'box within a box' principle and mounted on an isolating foundation which consisted of 50 spring vibration isolators. To extend the free field and to obtain the lowest possible low cut-off frequency, absorbing wedges of 1150mm long were used in the chamber, spaced 150ram from the wall. Measurements conducted after the completion of the anechoic chamber indicated that all the original acoustic design criteria had been reached, some even surpassed. The anechoic chamber showed excellent acoustic character- istics during its use after completion in the following year.

INTRODUCTION

The anechoic chamber of Hangzhou Electrisound Factory, which is located in Hangzhou city, the capital of Zhejiang Province, China, is built mainly for the measurement of acoustic properties of loudspeakers, loudspeaker cabinets and other electrical equipment, such as television sets, recorders, etc. Therefore, the anechoic chamber must have excellent acoustic character- istics. The design requirements are as follows:

• Low-frequency cut-off, fo < 63 Hz • Measuring radius of free field, r > 2.5 m • Maximum deviation from ideal free field, V < + 1"0 dB • Background noise level, L < 20dB(A) and PNC-15 curve

139 Applied Acoustics 0003-682X/90/$03.50 © 1990 Elsevier Science Publishers Ltd, England. Printed in Great Britain

140 Xiang Duanqi, Wang Zheng, Chen Jinjing

The dimensions of the anechoic chamber are 11.46 x 7"38 x 6"48 m, the effective volume is 549 m 3, and the total surface area is 414 m 2.

In order to reduce background noise the anechoic chamber is built on a 'box within a box' principle and mounted on vibration isolators which consist of 50 springs. To extend the free field and to obtain the lowest low cut-off frequency, absorbing wedges of 1150 mm in length are used in the chamber, spaced 150mm from the wall.

The door of the anechoic chamber is divided into two parts, the insulation door and absorption door on which the wedges are fixed for ease of movement.

To reduce the relative humidity in the chamber, which is very necessary for the preservation of the wedges' sound absorption and for rust prevention of springs, a ventilating system is incorporated.

Both plan and cross-sectional views of the anechoic chamber are shown in Fig. 1. The acoustical measurement conducted after completion of the anechoic chamber indicates that all the design requirements have been satisfied.

ACOUSTIC DESIGN

Design of sound insulation and vibration isolation

The anechoic chamber of Hangzhou Electrisound Factory is situated in an industrial area near a military airport, where the noise level is up to 85 dB(A) during taking-off and landing of aircraft. To keep out the noise, a 'room within a room' design has been adopted in the construction of the anechoic chamber; to be precise, the anechoic chamber is in fact built in a larger room which consists of a brick wall 360 mm thick and a reinforced concrete slab roof. The roof of the inner chamber is made of reinforced concrete. The gap between the inner chamber and outer room is 730-750 mm (see Fig. 1). For both the purpose of absorption and insulation, the doors of anechoic chambers are generally very heavy, which as a result are difficult to move. To solve this problem, the door of our chamber is built in two parts, the wedge door for sound absorbing and the insulation door for sound insulating. The wedge door is further divided into two parts, push-pull and rotation doors, the former is for personnel and the latter for large test equipment. There are two insulation doors to keep noise out of the chamber. In the space between, two small simple insulating doors are mounted to avoid the noise in the gap entering the chamber. The layout and details of the door are shown in Fig. 2.

To condition the air in both chamber and space, a ventilating system is incorporated. The intake is installed between the inner and outer roofs. In

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142 Xiang Duanqi, Wang Zheng, Chen Jinjing

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Acoustic design o f an anechoic chamber 143

the intake, there is a small insulation door which is operated in the machine room. An absorptive silencer is installed in the duct to the space to prevent the outdoor noise intruding into the space.

In order to eliminate vibration due to punch pulses in the nearby workshop and other vibration sources, the inner chamber sits on 50 vibration isolators made up of steel springs and rubber cushions. In order to take shock into consideration the design requirements of the isolators were as follows:

• Low natural frequency, < 2"5 Hz • Higher damping to prevent possible resonance • Easier installation and replacement of springs and rubber cushions • Low price and simple structure

According to the requirements stated above, the authors designed an isolator made up of a series of springs and rubber cushions (see Fig. 3).

Design of wedges

One of the most important acoustic criteria for an anechoic chamber is the sound absorption ability of each surface. In an ideal anechoic chamber the sound wave travelling into the surface should be fully absorbed. According to the theories of room acoustics, the ratio of reflected energy density E, and direct sound energy density Ed at an arbitrary point in a room is

Er 16r - (1)

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where r is distance from measuring point to a source, S is the total surface area of the room (m 2) and ~ is the absorption coefficient of the surface of the room.

For free field conditions in a chamber, the value of E,/Ed has to be as low as possible. According to eqn (1), ~ should approach 1"0, otherwise the range of free field would be reduced. Generally, ~ should be greater than 0.99.

To ensure that ~ is greater than 0-99, there should be a special structure on the surface of the anechoic chamber. At present, absorbing wedges made up of porous materials are used in most anechoic chambers, of which the absorption is sufficiently large at high and medium frequencies (above 300 Hz), for ~ to reach 0-99, but is not sufficient at low frequencies where ~ is generally lower than 0"99. The absorption in the low-frequency range can be increased by means of an air space between the wedges and wall. The sound absorbing performance of wedges can be expressed by their low frequency cut-offfo, which means that in the frequency range above it the absorption coefficient a is equal to or greater than 0-99.

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144 Xiang Duanqi, Wang Zheng, Chen Jinjing

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Acoustic design of an anechoic chamber 145

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The absorbing wedges in the chamber discussed above are designed on the basis of the following requirements:

• Low frequency cut-off, f0 < 63 Hz • Absorbing materials with large absorption and durability • Possible shortest length with possible largest absorption.

On the basis of contrasting tests, it was decided that each wedge should be l l 5 0 m m long with a flat end, the density of the material in the wedges should be 85 kg/m 3, and that the thickness of the air space should be 150 mm.

ACOUSTIC CHARACTERISTICS OF THE ANECHOIC C H A M B E R

After completion of the anechoic chamber in October 1987, acoustical measurements on a background noise, sound insulation, vibration isolation, and sound field features were conducted.

146 Xiang Duanqi, Wang Zheng, Chen Jinjing

T A B L E 1 (Minimum value

Background Noise Level \ M aximum ~ j

Position T#ne Octare band pressure level (dB)

31"5 63 125 250 500 lk 2k 4k 8k .4

In the 20-8 16-2 1l-0 10-0 17-0 9:00-22:00 <10 <10 <10 <10 <10

chamber 26-5 18-4 li-5 11"0 17-3

Outside the 46-7 48.3 38"5 36 .8 33'6 27,1 23.5 23'8 25"6 33.9 9:00-22:00

door 65.0 62 .6 58"8 61.4 58'5 55 .7 51 .7 58-1 54 .9 63.0

Background noise level

Both the background noise level in the chamber and the outdoor level were measured in four different periods, i.e. 09:00-10:00am, 15:00-16:00pro, 19:00-21:00 pm, and midnight. The data obtained is listed in Table 1 and indicates that the background noise level in the chamber was always lower than the design target (20dB(A) and PNC-15).

Sound insulation

The sound transmission loss of walls and doors were measured with octave band white noise as sound source, the results being shown in Table 2. In addition, an impulse sound source 1"5 m away from the outer wall with SPL of 128 dB{A) was used to examine the sound insulation of the walls; under these conditions the sound pressure level in the chamber remained unchanged thus indicating excellent insulation.

Vibration isolation

The anechoic chamber rests on vibration isolators made from springs and rubber cushions to isolate chamber from external vibration, especially

T A B L E 2 Transmission Loss of Doors and Walls {dB)

Frequent3" 63 125 250 500 lk 2k 4k 8k (H-}

Doors 64-6 73"9 91"l 86-I 87-8 87-0 89-6 81-2 Walls - - 79-4 9l-0 90"0 91.0 88.0 92.0 - -

Acoustic design o f an anechoic chamber 147

impulsive shocks. The 50 isolators were evenly compressed by the anechoic chamber, the difference in static compression value between them was only 1-0 mm.

The natural frequency of springfo t can be obtained from:

fot = 5/Xcm (2)

wherefo t is the natural frequency of a double-series spring and Xcm is the static compression value (cm).

The actual compression value of a double-series spring Xcm is 4"63 cm, so its natural frequency is 2"32 Hz.

The natural frequency of rubber, fo2, is

fo2 = ( 5 / X c m ) × Ea/E~ (3)

where fo2 is the natural frequency of rubber; Ea and E~ are respectively dynamic and static moduli. The compression value of the rubber cushions of each isolator is 0-4cm, giving a natural frequency of 8"3 Hz according to eqn (3).

A vibration test was carried out to examine the actual effect of the isolators, with a hammer, only 1.5m away from the outer wall, as the vibrating source which gave very low shock frequency (around 2"5 Hz) and rather strong impulse. The test data, shown in Table 3, indicate that during the shock the noise level inside the chamber was lower than the design level.

Characteristics of the sound field

The deviation from free field is an important parameter for determining the measuring radius in the anechoic room. The variation of the sound pressure level in the chamber is shown in Fig. 4 as a function of distance from a source. The result was obtained by using the particular measuring method recommended in ISO 3745. With the source located in the center of the

TABLE 3 The Results of the Vibration Test

Octave centre frequency Weight network

31"5 63 125 250 A C Lin

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On the isolator 12.0 13.0 5-0 5.0 13-0 17.0 28-0 In the chamber 14.0 12-0 4.0 4.0 13.0 15.0 26.0

Noise level (dB) In the chamber 41-0 30"0 13.7 10 17-6 49.0 57-0

148 Xiang Duanqi, Wang Zheng, Chen Jinjing

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represent the standard inverse-square curve.

Acoustic design of an anechoic chamber 149

chamber, the microphone was moved at a speed of 20mms -1. The measurements were carried out in the O-A and O-B directions respectively (see Fig. 4).

The measurement results reveal that:

(a) In both directions the decay curves are smooth and steady. The low frequency cut-off is 50 Hz, with maximum deviation between the measured curve and the standard inverse-square curve equal to or less than l '0dB in the frequency range 50-16000 Hz when the radius of the free field is less than 2-5 m.

(b) The free field range where the maximum deviation is equal to or less than 1-0dB is 6-7m in the O-A direction and 5-6m in the O-B direction.

CONCLUSIONS

The measurements indicate that all the acoustic design criteria for the anechoic chamber have been reached, some are better. The project has shown the following:

(1) To condition the air in the chamber, it is not only necessary but also possible to install a ventilation system. The critical problem is the fitting of attenuation in order to prevent the ingress of outdoor noise.

(2) Being very low in natural frequency, the vibration isolators consisting of double-series springs and rubber cushions perform extremely well as vibration isolators.

(3) The problem of a door in the anechoic chamber which is too heavy to move easily can be solved by dividing the door into two parts.

(4) The low-frequency absorption coefficient is the most important index, which is usually expressed by the low-frequency cut-offfo, above which the absorption coefficient is greater than 0-99. Generally speaking, the low-frequency cut-off of the wedges should be the same as that of the anechoic chamber, but it has been found that in the existing anechoic chambers in China, the low-frequency cut-off is usually higher than that of the chambers. Taking the anechoic chamber discussed in this paper as an example, the low-frequency cut-off of wedges is 63 Hz, while that of the chamber is 50 Hz.