2G6 



AUDITORY BIOPHYSICS 



produced. This very large deflection is detected as a tickle in the middle 

 ear due to one of the ossicles making contact with the tissue in the 

 cavity. This sensation is the threshold of feeling and is experienced just 

 before the painful feelings, due to dangerously loud noises which may 

 cause injury to the aural membrane in the cochlea (Fig. VII-6). 



To visualize what this means we may consider a gunner standing in the 

 midst of heavy ordnance gunfire. He will be subjected to an amplitude 

 of air pressure equal to about 0.25 mm. The air amplitude of nearby 



100 1000 10,000 



Frequency in cycles per second , 



Fia. VII-6. Intensity thresholds of the average human ear at various frequencies. 



thunder is estimated at 0.1 mm. For comparison, some measurements 

 by Shaw [1905] show that a telephone vibrating at 256 cycles will pro- 

 duce an audible tone when its double amplitude is 7 X 10~ 6 mm. At 

 comfortable loudness it attains a total displacement of 0.0005 mm, and 

 at excessive uncomfortable loudness a value of 0.01 mm. 



The displacement of the tympanic membrane at the threshold of 

 audibility of a 1000-cycle tone is, as previously shown, 8 X 10 -10 cm. 

 At 2000 cycles Wilska [1935] found the tympanic membrane to move only 

 10 -9 cm, and at 10,000 cycles 5 X 10 -9 cm. Of special interest is the 

 fact that at the threshold of hearing (Fig. VII-6) the movements of the 

 eardrum are of atomic dimensions. 



If the response to amplitude changes is so microscopic, is it possible 

 for a human ear to be aware of the Brownian movement of the individual 

 air molecules that produce fluctuations in pressure on the eardrum? An 

 approximate answer is found in some work by Sivian and White [1933], 

 who calculated that the mean pressure change caused by thermal noises 

 at ordinary temperatures are just below the threshold of audibility. It 



