Characteristics of anechoic chamber

For people who enter the anechoic room for the first time, they are really unaccustomed, which is completely different from the external auditory experience. There was no response to the high-five, but the sound of clothes rubbing when walking was unprecedented. It seems that you can hear your own blood flowing quietly here without moving. In this extremely quiet environment, people who come into contact with the anechoic room for the first time will immediately feel lonely and slightly afraid.

The characteristics of anechoic chamber are mainly reflected in the following points:

1. Free field space

Free field means that there is no reflector or reflecting surface when sound waves propagate in infinite space. The main function of anechoic chamber is to provide a free field space or a semi-free field space for acoustic testing.

The radius of free field is an index to measure the size of free field. The free field radius of a well-designed anechoic chamber should be the distance from the center point to the wedge1.0 m.

2. Background noise

Another function of anechoic room is to provide a low background noise environment to meet the requirements of test environment. 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.

3. Cut-off frequency

In the design of anechoic chamber, the lowest frequency with a sharp wedge sound absorption coefficient of 0.99 is usually called cut-off frequency. When the sound absorption system of the wall can ensure the sound absorption coefficient of 99%, the anechoic room can meet the free field condition above the cutoff frequency. The measured values below the cutoff frequency can be corrected according to the standards of ISO 3746 and ISO 3747. For example, in a laboratory with10×10m, the low-frequency cutoff frequency can reach 50Hz if each side is paved with 1m long sound-absorbing wedges.

According to the 50Hz national standard ISO 3745 &；; Ldquo acoustics. Determination of sound power level of sound source. Accurate methods of anechoic chamber and semi-anechoic chamber. Rdquo and national standard GB 6882-86 & ldquo；; Determination of sound power of acoustic noise sources: Test the free field identification method of anechoic chamber specified in Precision Methods of Anechoic Chamber and Semi-anechoic Chamber. That is, single-frequency signals with octave center frequency interval of 1/3 are used below 125 Hz and above 4KHz, and single-frequency signals with octave center frequency interval of11are used between 125Hz and 4 kHz to measure the difference between the measured sound pressure level and the theoretical sound pressure level.

On this basis, according to the maximum allowable difference specified in ISO3745 and GB6882-86, the allowable measurement radius and the free sound field gap meeting the standard can be determined. The maximum allowable difference between the measured sound level and the theoretical sound level specified in ISO3745 and GB6882. Acoustical performance parameters of anechoic room (semi-anechoic room) are ISO3745 and GB6882-86 cut-off frequency 125Hz (which can be extended to 50Hz as required). The external and internal dimensions of this low noise 16 dB(A) can be achieved by special design.