594 Appendix A 



Thus, sound energy is transmitted readily from water to tissue but very 

 little is transmitted from air to tissue. This difference in characteristic 

 impedance means that only a very small portion of the power incident 

 on the ear can be used in hearing. However, the sound pressure 

 amplitude remains approximately the same in the tissues of the ear as 

 in the incident air. 



It is generally assumed in acoustics that if a generator sends out 

 waves of a given frequency, then only these will be transmitted by the 

 medium and received by the ear. This is a consequence of Equation 6 

 which applies not only to velocity but also to pressure and to excess 

 density. Actually, no real medium propagates sound in the fashion 

 predicted by Equation 6, but the deviations from it are often so small as 

 to be undetectable by human ears. At high amplitudes, and in certain 

 special cases even at low amplitudes, the shape of a propagated wave 

 may change in a fashion that cannot be predicted from Equation 6. 

 An extreme example is the surface waves on the ocean. In this case, 

 the wave velocity c depends on the frequency. As a consequence, 

 waves rise up to a maximum at some places and then seem to disappear; 

 others actually double back and form breakers. In the inner ear, 

 incident waves likewise pile up to give a maximum amplitude at a 

 location which depends on their frequency. 



The waveform is distorted by the production of harmonics in the 

 transmission of waves in air at sound pressure levels in excess of about 

 120 db in the audible range. These effects are referred to as nonlinear; 

 they contradict the assumptions of infinitesimal amplitudes made in 

 traditional acoustics. Similar production of harmonics can be observed 

 in the ear. However, in most cases the harmonic distortion due to 

 nonlinearities is not important in hearing. It is quite possible, however, 

 that they produce some of the important effects when tissue is subjected 

 to ultrasound. 



There are many other acoustic terms used to describe auditory 

 acoustics. It is the author's belief, however, that a familiarity with the 

 words and symbols of Table I is sufficient to understand most journal 

 articles and more advanced texts dealing with the physical aspects of 

 hearing. 



REFERENCES 



1. Hunter, J. L., Acoustics (Englewood Cliffs, N.J: Prentice-Hall, Inc., 1957). 



2. Morse, P. M., Vibrations and Sound 2nd ed. (New York: McGraw-Hill Book 

 Company, Inc., 1948). 



3. Hueter, T. F., and R. H. Bolt, Sonics: Techniques for the Use of Sound and 

 Ultrasound in Engineering and Science (New York: John Wiley & Sons, Inc., 

 1955). 



