CHAPTER 



4 



Physical Aspects 

 of Hearing 



INTRODUCTION 



Physics has established that the thing we call sound travels through 

 matter in a manner similar to the way waves travel. Sinusoidal oscilla- 

 tion of the particles of matter causes one particle to collide with the next, 

 thereby transmitting its energy to the second particle. We therefore 

 speak of sound waves, and ascribe to sound the various properties of 

 waves, such as wavelength, amplitude, intensity, interference, diffrac- 

 tion, etc. The energy in the wave resides in the amplitude of the particle 

 motion and in the number of particles executing the motion. Thus the 

 expression for the intensity of the sound (energy passing through unit 

 area in unit time) has been shown to be proportional to the density of 

 the matter, the square of the sound frequency, and the square of the 

 amplitude. 



These aspects of sound are usually taken up in an introductory course 

 in physics, and will therefore not be dealt with here. We will focus our 

 attention on the following questions. How does our brain get informa- 

 tion about the amplitude, frequency, and intensity of sound waves? To 

 make one of these questions more concrete, how does the frequency 

 information get to the brain? Is the frequency faithfully reproduced in 

 the auditory nerve? Or is some particular nerve fiber sensitive to a par- 

 ticular frequency, so that all that goes to the brain is the information that 

 particular nerve fibers have been fired, meaning that certain frequencies 

 have been received? Such questions clearly involve knowledge of proper- 

 ties of nerves and nerve fibers, so that we shall have to introduce this 

 study of sound with some aspects of nerve physiology. 



The mechanism of transmission of an impulse down a nerve is believed 

 to be associated with an alteration of the electrical state of the nerve 

 membrane. The speeds of these impulses are of the order of 10 m/sec. 

 Once the impulse has passed each point, the recovery processes set in. 

 Upon the length of time required for these recovery processes depends the 

 frequency of impulses that the nerve can transmit. Another factor which 

 affects the frequency of transmission by a single nerve fiber is the time 

 taken to transmit the impulse across a synapse and the recovery time at 



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