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 ... Research Article
Research Article  |   April 01, 1993
Temporary Threshold Shift Caused by Hearing Aid Use
 
Author Affiliations & Notes
  • John H. Macrae
    National Acoustic Laboratories Chatswood, New South Wales Australia
  • Contact author: John H. Macrae, PhD, National Acoustic Laboratories, 126 Grevilie Street, Chatswood, NSW 2067, Australia.
Article Information
Hearing Disorders / Hearing Aids, Cochlear Implants & Assistive Technology / Hearing / Research Articles
Research Article   |   April 01, 1993
Temporary Threshold Shift Caused by Hearing Aid Use
Journal of Speech, Language, and Hearing Research, April 1993, Vol. 36, 365-372. doi:10.1044/jshr.3602.365
History: Received May 21, 1992 , Accepted August 25, 1992
 
Journal of Speech, Language, and Hearing Research, April 1993, Vol. 36, 365-372. doi:10.1044/jshr.3602.365
History: Received May 21, 1992; Accepted August 25, 1992

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.

Acknowledgments
I am grateful to Pam Hart, principal of the Garfield Barwick School, for permission to carry out this study at the school and to Julie Decker and David Tomkinson for their assistance in the investigation. I am indebted to Larry Humes for having brought the Modified Power Law to my attention and for raising the question of the appropriate equivalent continuous level for predicting TTS from noise exposure due to hearing aid use. I would also like to thank Larry Humes and David Fabry for reviews that led to improvements in this article.
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