On Transient Noise and its Reduction in Hearing Aids

Doctoral thesis, 1997

The most important property of a hearing aid is to facilitate verbal communication for hearing impaired people. However, tuning of the acoustical parameters in a hearing aid in order to optimize the speech intelligibility may result in a harsh and unpleasant sound. This is so especially the case in listening environments where intermittent relative high-frequency and high-level transient noise (e.g. chinks) are present. In this dissertation, a new approach to minimizing the unpleasantness experienced by hearing aid wearers from such transient noise is presented. The approach relies on the selective identification and reduction of transients carried out so as to minimally affect the intelligibility of contemporaneous speech. Simple and straightforward signal processing has been used, which means that the transient suppression method can be implemented as a part of a digital ear-level hearing aid of today's standard without exceeding the available computational capacity. For real-time evaluation purposes, a two-channel wearable master hearing aid (signal processing unit), based on a general purpose digital signal processor, has been designed. The evaluations conducted in this work, based on a software prototype implementation of the transient suppresser, show that: (1) transients having half the peak level of that of contemporaneous speech can be detected with sufficient precision; (2) a group of moderately cochlear hearing impaired persons showed, in a blind paired comparison test, a clear preference for the transient suppresser over a hearing aid with conventional signal processing; and (3) in a later test with another group of persons having similar hearing impairments, no negative effect was found from the transient suppresser on the ability to understand speech. In a separate questionnaire study, the hearing condition and the use of ear protection among Swedish hunters in relation to sound exposure from gun shots were investigated. The results implies that the relative few shots fired without hearing protectors do affect the hearing of hunters as a group when compared with normative data. It is noted that approximately 50% of the hunters suffer from either a hearing loss and/or from a permanent or temporary tinnitus. Moreover, it was found that the left ear (in right handed shooters) was exposed to 160 - 163 dB peak sound pressure level (SPL), which is about 4 dB higher peak pressure than the right ear is exposed to.