4 Sound and the Ear /I : I 



influenced by the sense of hearing. Many of our forms of entertainment 

 — concerts, theatre, movies, radio, and even television — depend upon 

 our sense of hearing. 



Hearing can be studied from many different points of view. Physi- 

 cists have learned how sound waves are generated and how they are 

 transmitted. Anatomists have probed into the structure of the ear on a 

 gross level and also on a microscopic level. They have traced the path- 

 ways by which auditory nerve impulses travel from the ear to the brain. 

 Psychologists, physiologists, and physicists all have studied the thresholds 

 of sensitivity of the hearing system and the way in which we understand 

 speech. Most of these groups, and especially biophysicists, have been 

 interested in the manner in which the hearing organ operates, how 

 sounds are analyzed, how they are converted into nervous impulses and 

 then separated according to pitch, quality, and loudness. In this 

 chapter and in Chapter 6, "Neural Mechanisms of Hearing," an attempt 

 has been made to synthesize all of these different avenues of approach, 

 while emphasizing those parts of each which have the greatest interest 

 to the biophysicist. 



The first careful study of the ear and attempt to relate its structure 

 to hearing was carried out by Helmholtz. Before that period, various 

 theories of hearing existed, but few have had more significance than one 

 which has survived in our colloquial speech. This was the idea that the 

 ears were connected to a common hollow region within the head where 

 the sound was somehow stored. If, so this theory went, we were not 

 careful, the sound would go in one ear (through the storage chamber) 

 and out the other. 



Since the middle of the last century, hearing has been the subject of 

 many scientific investigations. The nature of these studies was radically 

 altered around 1930 by the introduction of electronic techniques. 

 These techniques have completely changed the study of hearing ; they 

 have dramatically influenced the interpretation of all phases of hearing 

 from pure acoustics to the final analyses of sounds within the brain. So 

 complete is the dependence on electronic techniques today, that it is 

 hard to remember that Helmholtz and Lord Rayleigh could do acoustic 

 experiments successfully without electronic instrumentation. 



Hearing is the response to mechanical, vibratory stimuli. Not all 

 such stimuli evoke the sensations of hearing. The sound must be loud 

 enough to be heard and also be of a suitable pitch. The latter condition 

 is physically equivalent to saying the vibration must be within the 

 audible frequency range. Vibrations outside of this frequency range 

 may be detected by human sensory systems other than hearing. At 

 frequencies too low to be heard, vibrations are perceived through the 

 sense of touch ; much greater amplitudes are needed for touch than are 



