Glenis R. Long - long@rayleigh.physics.purdue.edu
Dept. of Audiology and Speech Sciences
Carrick L. Talmadge and Arnold Tubis
Physics Department
Purdue University
West Lafayette, IN 47907
Popular version of paper 5aPP3
Presented Friday morning, 17 May 1996
Acoustical Society of America, Indianapolis, IN
Embargoed until 17 May 1996
Otoacoustic emissions (discovered by Kemp in 1978) are narrowband acoustic signals generated by the inner ear of normal hearing individuals, either in the absence of acoustic stimulation (spontaneous emissions) or in response to acoustic stimulation (evoked emissions). These emissions can be detected by analyzing the signals obtained by placing a tiny microphone at the entrance to the ear canal, a procedure both simple and noninvasive. The mechanisms generating these emissions appear to be the same nonlinear feedback processes which responsible for the remarkable ability of the normally hearing individual to detect and analyze low level sounds. Most hearing impairment begins with a weakening or loss of this nonlinear feedback. Consequently, otoacoustic emissions potentially provide a useful clinical tool for the evaluation of inner ear function and diagnosis of hearing impairment.
All different types of otoacoustic emissions in humans share one characteristic with an aspect of hearing sensitivity that is often ignored. The specific ability of an individual to detect the presence of a pure tone as a function of frequency is unique to each individual. Thresholds, the lowest audible levels, for frequencies as little as 75 Hz apart can vary up to 15 dB. The ripple behavior of the threshold as a function of frequency is known as threshold microstructure. The ear's ability to generate different types of otoacoustic emissions shows similar fluctuations with frequency (fine structure). The spacings of the threshold microstructure and the otoacoustic emissions fine structure are similar. We will present evidence from theoretical studies and computer simulations of cochlear mechanics that the generation of otoacoustic emissions and their frequency spacing characteristics stem from wave interference which arise from a very small amount (less that 0.1%) of roughness of features of the basilar membrane, in combination with a sharp frequency response.
The combination of experimental research on otoacoustic emissions combined with theoretical models of cochlear function leads to deeper insight into the mechanisms governing normal and impaired hearing. A better understanding of auditory processing in man has immediate implications in the design of noise control systems, hi-fi systems, telephone and communication systems, hearing protheses, etc.
Recent Related Publications
Malcolm Brown, "Ear's own sounds may underlie its precision" New York Times. June 9. 1992.
B. Engdahl and D. T. Kemp, "The effect of noise exposure on the details of the distortion product emissions in humans", J. Acoust. Soc. Am. 99:1573-1588, 1996.
P. Kummer, T. Janssen and W. Arnold,"Suppression tuning characteristics of the 2f1-f2 distortion product otoacoustic emission in humans", J. Acoust. Soc. Am. 98:197-210, 1995
C. Talmadge, G. R Long and A. Tubis, "New off-line method for detecting spontaneous otoacoustic emissions in human subjects," Hear. Res. 71:170-182, 1993.
C. Talmadge and A. Tubis, "On Modeling the Connection between Spontaneous and Evoked Otoacoustic emissions", In Biophysics of Hair Cell Sensory Systems, H. Duifhuis, J. W. Horst, P. van Dijk and S. M. van Netten (Eds), World Scientific, Singapore, 1993, pp. 25 - 32.
G. Zweig and C. A. Shera, "The origins of periodicity in the spectrum of evoked otoacoustic emissions," J. Acoust. Soc. Am. 98:2018-2047 (1995).