Output Limitation/Compression

Abstract

The primary objective of an adequate hearing aid fitting is to achieve amplification of acoustic energy at all frequencies in the audible range of human hearing. Most important is the ability to present speech information at a comfortable loudness level without exceeding the user's loudness discomfort when input signals are increased at the microphone of the hearing aid. This objective is easily and naturally accomplished for normal hearing individuals in whom the dynamic range of hearing covers an area from the threshold of hearing to the threshold of discomfort and pain of more than 125 to 130 dB SPL. The input to the ear in most environments, from quite to very loud, basically covers the same range. Normal hearing individuals are able to protect against a discomfort producing sound level by removing themselves from the loud sound or by providing some sort of input limitations such as covering the ears. This objective is not as easily accomplished when dealing with most types of sensorineural hearing impairment wherein the dynamic range of residual hearing is far smaller than the range for normal environmental sounds. The dynamic range of hearing is that range of intensities with which the ear can deal effectively; from the weakest to the loudest sound or from threshold of hearing to threshold of loudness discomfort. As mentioned earlier, this range is very large for normal hearing individuals (approximately 125-130 dB). In the presence of hearing impairment, the dynamic range of hearing is often decreased dramatically. For example, an individual with a threshold of hearing at 50 dB and a threshold of discomfort at 110 dB will only be able to deal effectively with the range of 50 to 110 dB or a dynamic range of 60 dB. Over the years, output limiting in hearing aids has been used to protect the user against loudness discomfort or pain when input signals reach critical levels. From an engineering view, this has been accomplished by means of compression or peak clipping circuits. The peak clipping circuits are fairly simple to employ in that the objective is to protect the user against excessive loudness. However, compression amplification is probably the most difficult and confusing system to investigate and understand in the appropriate fitting of hearing losses with amplification. Conventional, linear amplifiers function on a 1:1 ratio relating input/output function. That is, as the input signal is increased, the output