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Semi-anechoic chamber is an extremely important experimental place in ac oustic experiment and noise test. Its function is to
provide a low-noise testing environment in a semi-free field space. Main functions of semi-anechoic chamber.
●Provide acoustic semi-free field environment.
●Low noise measurement environment
Half-field space
The main function of the semi-anechoic chamber is to provide a semi-free field space for acoustic testing. Half-free field means
that when sound waves propagate in infinite space, there is no reflector (suface) except the ground.
The semi-free field space in nature means that the free space has a total reflection surface, and the sound wave can only be at
180? Azimuth free propagation. Common semi-free space such as large open-air parking lot on cement floor, outdoor basketball
court, apron, etc.
Sound wave propagation in semi-free field space has a specific physical definition as follows:
●The sound pressure of a point sound source decreases with distance, which is the inverse square law of sound energy;
●Sound pressure level is equal to sound intensity level at room temperature. This is the theoretical basis of measuring sound
power in anechoic chamber.
background noise
Another function of the semi- anechoic chamber is to provide a low background noise environment to meet the requirements of
the test environment. According to. IS03745-2003 ac oustics-determination of sound power levels of noise sources using sound
pressure. precision methods for anechoic and hemi- anechoic rooms、
ANSI S12 55-2006 Acoustics - Determination of sound
power levels of noise sources using sound pressure - Precision methods for anechoic and hemi-anech Oic rooms and GB6882
"Measurement of Sound Power Level of Acoustic Noise Sources Precision Method of Anechoic Room and Semi- anechoic
Room", in the test frequency range, the sound pressure level of background noise is at least 6dB lower than that of the measured
sound source, preferably 12dB lower.
cut-off frequency
Semi-anechoic room is to simulate a semi-free field space indoors, so the sound absorption coefficient of each surface in the
room is required to be above 9%. It is impossible to design a sound absorber which can achieve 100% of the half frequency
band (20 Hz-20Hz 20000Hz). Bec ause common materials can easily absorb high-frequency sound waves, while low-frequency
absorption is related to the thickness of the material (the length of the wedge). Cut-off frequency means that above this
frequency, the sound absorption system of the wall can guarantee 99% sound absorption coefficient. In practice, the semi-free
sound field space refers to the sound wave above the cut-off frequency. The lower the cut-off frequency, the greater the length
requirement of the wedge. Generally speaking, the length of the wedge is suitable for the 1/4 wavelength theory.
Above the cut-off frequency, the semi-anechoic chamber meets the free field condition, but below the cut-off frequency, the semi-
anechoic chamber cannot meet the free field condition, so it is necessary to correct the free field according to GB 6882 when
measuring.
Evaluation method of semi-anechoic chamber
In practical engineering, the free space of sound field is the key basis to identify the soundproof chamber. ISO 3745.. ANSI
S12. 55 and GB 6882 provide identific ation methods to identify the free space of semi-anechoic chamber.
In addition, the bac kground noise is another important index of the semi- anechoic chamber. The measured background noise is
the background noise of the instrument plus the background noise of the anechoic chamber.
I. Acoustic performance parameters of semi-anechoic chamber
1. The size and main technical indicators of the semi-anechoic chamber
※Semi-anechoic chamber design standards ISO 3745.. ANSI S12 55 and GB 6882
※Cut-off frequency is 125Hz (it can be extended to 50Hz according to requirements)
※Background noise is 16dB(A) (special design can reach 11dB(A).
※Extemal dimensions
※Intermal dimensions are designed according to requirements and standards.
※Available space
