AURAL HARMONICS AS CONDITIONED BY INTENSITY 301 



phonics produced in the cochleas of cats' and guinea pigs' ears. These 

 experiments indicate that the pressure disturbances which cause the 

 eardrum and ossicle chain to move and impress these motions on the 

 cochlear fluids generate a fluctuating electrical potential in the cochlea, 

 simulating the mechanical wave disturbance in intensity and frequency. 



The cochlear potentials can be detected by observing the change in 

 potential of a pair of electrodes, one of which is placed on the round 

 window and the other in contact with some other part of the animal. 

 These cochlear potentials are amplified so that their amplitudes may be 

 measured and their frequencies determined by means of a vacuum-tube 

 wave analyzer, tuned to respond to any component of the generated 

 electrical potential wave form. The cochlear potential has been found 

 to be a true measure of the sensation level of the stimulus. 



Figure VII-23 shows the re- 

 sults of an analysis of the coch- 

 lear microphonics obtained by 

 Stevens and Newman [1936] from 

 a cat's ear when stimulated by a 

 pure tone of 1000 cycles at suc- 

 cessive higher intensities. Of 

 special interest is the fact that, 

 as the intensity of the stimulus 

 increases, the fundamental (first 

 harmonic) and the third and fifth 

 harmonics maintain their relative 

 response magnitudes with respect 

 to each other throughout the 

 range of intensity, but the second 

 and fourth harmonics decrease 

 after passing through a maximum 

 of response at the higher intensi- 

 ties of stimulation. 



The fundamental was found to 

 possess the greatest magnitude 



at all stimulus intensities. It increases almost linearly until a maximum 

 saturation value is reached, where an increase in stimulus produces no 

 further increase in response. 



When the potential generated in the cochlea was analyzed at a sensa- 

 tion level of 60 db, it was found that the second harmonic had a magni- 

 tude about 20 db smaller than the fundamental, the third harmonic 

 possessed a magnitude as much as 35 db below the fundamental, and the 

 fourth and fifth harmonics were negligibly small. 



3- 



O CO 



— *-• 



-Oa>10 

 J2 E 



.2-S20 



-Sr 30 



tz'o 

 ■^E40 



c/> 3 



§,§50 



to to 



£E60 



10 20 30 40 50 60 70 80 90 100 110 

 Stimulus in decibels above threshold 



Fig. VII-23. Analysis of the cochlear 

 microphonics obtained from a cat's ear. 

 Stimulating tone 1000 cycles. Horizontal 

 axis represents intensity of stimulus above 

 average human threshold. The upper 

 curve is the response to the fundamental, 

 and the lower curves are higher harmonics 

 which appear as the intensity increases. 

 (Stevens and Newman [1936].) (By cour- 

 tesy of Stevens and Davis, Hearing, John 

 Wiley & Sons, New York.) 



