Since the 1940s, the principle of gradual transition has been applied to making porous (or fibrous) materials into conical or wedge-shaped sound absorbers, which are collectively called sound-absorbing wedges. When the sound wave is incident from the tip, the acoustic impedance of the material can be well matched with the acoustic impedance of the air due to the gradual characteristics of the sound absorption layer, so that the sound wave is transmitted into the sound absorber and absorbed efficiently. Up to now, high-quality anechoic chambers at home and abroad have adopted wedge-shaped structures as sound absorbers.
The sound absorption characteristics of sound-absorbing wedge are related to the length of wedge, filling material and cavity depth. Under the same material, the longer the length of the wedge, the better the low frequency sound absorption performance of the wedge. Adjusting the cavity depth can also effectively improve the low-frequency sound absorption characteristics of wedge-shaped structures.
Whether the performance of anechoic room meets the requirements of use is generally tested by verifying the free field, that is, the sound pressure generated by one of the sound sources should be inversely proportional to the distance from the sound source. Measuring the deviation between the sound field and the ideal free field is the main index to measure the performance of anechoic room. In general acoustic test, this deviation is required to be less than1db; ; For microphone calibration, the deviation should not exceed 0. 1dB near the calibration distance.
Anechoic chamber should not only meet the requirements of free field, but also have low background noise. Therefore, some vibration isolation measures should be taken between the anechoic chamber and the foundation.