Temporary threshold shift caused by hearing aid use.

Temporary threshold shift (TTS) over a wide range of frequencies was found after 4 hours of hearing aid use by a 15-year-old student with severe sensorineural hearing loss who was using real-ear insertion gains 10 to 20 dB greater than those recommended by the current National Acoustic Laboratories (NAL) procedure for selecting the gain and frequency response of hearing aids. Measurements were made of her noise exposure during hearing aid use with a noise dosimeter. The real-ear insertion response and input-output function of her hearing aid were measured with a real-ear gain analyzer and were used to calculate in-ear noise levels from the noise levels measured by the dosimeter. The amount of TTS could be predicted from the in-ear noise levels and the student's hearing levels (HLs) by means of a mathematical model consisting of the Modified Power Law (MPL) of Humes and Jesteadt (1991) combined with equations for predicting TTS in listeners with normal hearing published by Mills, Gilbert, and Adkins (1979). The mean of the instantaneous A-weighted in-ear noise levels proved to be the appropriate equivalent continuous level (ECL) for use in the predictions. The MPL was also used to determine safety limits for TTS due to hearing aid use. The observed TTS exceeded the safety limits at all frequencies up to and including 2000 Hz. It was therefore considered desirable for the girl to use less gain at frequencies from 500 to 1500 Hz.