The frequency response of an electret microphone is essentially flat, but there are some changes in the flat response due to design and some accidents. The electret microphone used in hearing aids intentionally introduces low frequency cuts. Low frequency cuts make the hearing aid less sensitive to the intensity of the low frequency sounds that often surround us. These may not be easy to detect for people with normal hearing, but if the microphone can't cut these sounds, it will cause the hearing aid to be overloaded.
Completing the low-frequency cut is simple: the small path between the front and back of the diaphragm allows the low-frequency sound to enter the sides of the diaphragm almost simultaneously, thereby reducing the effects of the diaphragm as it moves. The more the path is opened, the more the cut is made, and the frequency range in which the cut occurs is larger. The passage also balances the static pressure between the front and back of the diaphragm, similar to the eustachian tube function of the ear. Traditional hearing aids often use microphones with different amounts of low frequency reduction to help achieve the desired gain-frequency response.
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