Input Stimuli for Obtaining Frequency Responses of Automatic Gain Control Hearing Aids

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Abstract
Developing a family of frequency response curves for AGC types of hearing instruments using swept pure tones at varying input levels often produces erroneous results. This problem is caused by exceeding the threshold for activating the AGC circuit at some frequencies but not at other frequencies during the pure-tone sweep, thereby producing a different frequency response from that which would be obtained with a complex input signal such as speech-shaped noise. This measurement artifact may be minimized by ensuring that the threshold for activating the AGC circuit is either always exceeded or never exceeded during the development of a frequency response curve. Three input signals are compared for developing a family of frequency responses for an AGC hearing aid: (1) swept pure tone, (2) swept pure tone with bias tone added, and (3) shaped broad-band noise. The shaped broad-band noise appears to be the input signal of choice.

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