NONLINEARITY OF THE EAR 



323 



tone is shifted, due to the presence of noise, is called the masking in 

 decibels at the frequency corresponding to that of the pure tone. 



The masking effect of a pure tone, a narrow band of thermal noise and a 

 wide band of thermal noise is shown in Fig. 13.5. The figures on each of 

 the curves show the intensity level of the masking tone or noise. The 

 ordinates, in each of the charts, show the decibels above the threshold 

 that the various frequencies must be raised in order to be just heard in the 



200 CYCLE TONE 



1000 CYCLE TONE 



600 CYCLES 



NOISE 3000 TO 3260 CYCLES 



10000 100 



Fig. 13.5. Masking audiograms for single frequency tones, narrow bands of thermal noise 

 and wide bands of thermal noise. The curves are labeled in db above zero loudness. (After 

 Fletcher and Munson.) 



presence of the particular masking tone or noise. For example, referring 

 to the 4000 cycle tone having an intensity of 93 db, it is only necessary to 

 raise a 2000 cycle tone 20 db to be heard. On the other hand, a 10,000 

 cycle tone must be raised 66 db to be heard. 



13.8. Nonlinearity of the Ear ^. — When a pure tone of a suitable in- 

 tensity is impressed upon the ear a series of harmonics or overtones of the 

 original frequency are heard. Furthermore, when two loud tones are 

 sounded together, a group of tones is heard consisting of the sums and 

 differences of the two primary tones and their harmonics. These phe- 

 nomena show that the ear is a nonlinear system. 



® Stevens and Davis, " Hearing," p. 184, John Wiley and Sons, New York. 



