Hearing impairment is undoubtedly a common disease. Around 1.06 per 1000 children are born with a significant, permanent hearing impairment (40 dB or greater increase in threshold in their better hearing ear), and by the age of nine years, this number has risen to around 1.65 per 1000 (Fortnum et al., 2001). The prevalence of hearing impairment continues to increase with each decade of life, until 40% of the 71-80 years age group and 80% of the 80+ age group have a hearing loss of 35 dB or more (Davis, 1989; Davis and Moorjani, 2002). In total, approximately 20% of all adults over 18 in the UK suffer some form of hearing impairment (25 dB or greater hearing loss in at least one ear), and the proportions for other countries are very similar (Davis and Moorjani, 2002). The increase at various impairment levels is illustrated in Figure 33.1. However, thresholds are a crude reflection of the impairment, because it is not just the amplitude but the clarity of hearing that is affected. Our ability to distinguish speech sounds, to focus on specific sound sources such as one speaker in a noisy room, and to localize sounds in the environment all require accurate frequency and temporal discrimination, features that are disproportionately affected by hearing impairment. Much hearing loss with age affects sensitivity to high frequencies first, although some types of hearing loss have a more even effect across the frequency spectrum. Figure 33.3 illustrates typical hearing levels in age-related progressive hearing loss (Rosenhall, 2002).
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