Noise room (noise room) is to simulate the free field space indoors, so the sound absorption coefficient of each surface in the room is required to be above 99%. It is impossible to design a sound absorber which can achieve 100% of the half frequency band (20 Hz-20Hz-20000Hz). Because 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.
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