On the Influence of the Diffraction of Sound Waves around the Human Head on the Characteristics of Hearing Aids

Abstract

A small hearing aid, hanging in an anechoic room is made to drive an AVC circuit, the output signal of which is conducted to a power amplifier and loudspeaker and can be recorded on a tape. During playback of this tape, with the output of the recorder connected to the power amplifier, the same sound field as existed around the hearing aid is reproduced. If the apparatus is placed on a person in the position where it is to be worn and that person is situated so that the hearing aid is at the same point as during the recording, the output of the hearing aid during playback of the tape will indicate the influence of the diffraction around the human head. Three specimens of hearing aid were measured on different people. The results showed that there exists a large difference between the hearing aids but no fundamental differences between the persons. The curves plotted for males and females showed the same trend, and no correlation was found with the hairdress. We did not succeed in an attempt to replace the human head by a simple model, such as a wooden sphere or a wooden box, the agreement of the diffraction phenomena between model and head being too poor.