Weak current College】 【free sound field and technical description of anechoic chamber.

Free sound field
From the sound source produces a sound wave in the communication process, the experience is always part of the energy is the interface reflector, scattering or transmission, part of the energy being absorbed into other forms of energy that is reflected in this part of the energy will be issued with the sound source to the acoustic energy superposition, affect the direct measurement of sound waves, which is reflected in this part of the energy of the size and nature of the interface, material and shape, that is, in a different room the size of the wave is not the same.
Therefore, in the examination of audio products, tests and the sound source to the sound volume on some of the often wish to have the interface of reflection sound control to a minimum so that it can be compared with the direct sound and ignore. You can ignore the border of homogeneous isotropic medium sound field we call a free sound field.
Anechoic chamber
In order to meet the needs of scientific research and production, generally sound field with artificial simulation. In the Interior of the boundary on the laying of sound absorption properties of materials, the sound of the sound absorption coefficient of 99% or more, the establishment of an area in the Interior the approximate free sound field, and take good acoustic and seismic isolation devices avoid interference of the external environment, this is the anechoic chamber.
Anechoic chamber performance is good or bad, depends mainly on the low frequency sound absorbing material and structure of sound absorption properties, in, high-frequency main decisions to indoor unit on the reflection of sound waves. General sound absorption properties of sound-absorbing materials is a function of frequency, the lower the frequency, the more sound-absorbing performance. The length of the sound-absorbing wedge determines the cut-off frequency (sound absorption coefficient up to 99% of the lowest frequency), wedge approximately corresponding to the length of the cutoff frequency and wavelength of a quarter.
Sound-absorbing material for high-frequency acoustic performance is better, however, the high-frequency sound waves of short wavelength, anechoic room placement test piece support and network on acoustic scattering is large, in addition, a wedge of cloth as too dense, the acoustic hard penetration, high frequency of reflection are large, interfering sound field so that it cannot meet the law of 1/r.