Prediction of Asymptotic Threshold Shift Caused by Hearing Aid Use

This study used a well-verified mathematical model to predict asymptotic temporary threshold shift (ATS) caused by hearing aid use. The model determined the amounts of ATS to be expected if real ear insertion gains (REIGs) recommended by the current National Acoustic Laboratories (NAL) procedure are used. It also determined the consequences of use of excess REIG and of high input levels to hearing aids. If recommended REIGs are used and input levels are normal (average A-weighted input levels of about 61 dB SPL), ATS is unlikely to occur for clients who have typical audiograms with three-frequency average pure-tone thresholds (PTAs) less than 60 dB HL. For people with PTAs greater than 60 dB HL, small amounts of ATS can be expected to occur during hearing aid use, but these amounts of ATS are safe, that is, unlikely to be associated with permanent threshold shift (PTS) for individuals with all PTAs except those greater than about 100 dB HL. If REIGs are 15 dB greater than those recommended, the amounts of ATS will be unsafe for people with PTAs greater than about 80 dB HL. It appears unwise for clients who have this degree of hearing loss to use excess REIG. The use of excess REIG in high ambient levels of sound (average A-weighted input levels of about 75 dB SPL) is likely to cause PTS for hearing aid users with PTAs of about 50 dB HL or greater. Clients who prefer to use excess REIG should therefore avoid high ambient sound levels. The amount of amplification required for people with PTAs greater than about 100 dB HL is likely to cause PTS and is therefore inherently unsafe.

KEY WORDS: hearing aids, temporary threshold shift, safety limits

Submitted on November 1, 1993
Accepted on July 20, 1994

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