The purpose of this paper is to discuss the acoustic properties of the external canal of the ear and of the middle ear with special reference to the fitting of hearing aids and the design of individual ear molds.
If the receiver of a hearing aid is connected with an individual ear mold and inserted into the ear canal, it can be observed that the high frequency partial tones of the speech sound seem rather weak. This results in greater difficulty in distinguishing consonants like s, th, f and similar sounds. When the hearing aid is connected with a flat ear piece, the mentioned tones are perceived more clearly than with the first arrangement. It is also observed that patients suffering from perceptive hearing loss hear better with a flat ear piece than with a so-called midget receiver connected to an individual ear mold. The final choice is the latter
GROSSMAN FM. ACOUSTIC SOUND FILTRATION AND HEARING AIDS. Arch Otolaryngol. 1943;38(2):101–112. doi:10.1001/archotol.1943.00670040112001
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