296 AUDITORY BIOPHYSICS 



masking effect the person wrongly increases the intensity and often 

 raises the frequency level of the shouted conversation. To make the 

 best of the situation the voice should be pitched below the predominant 

 frequencies of the masking noise because low tones are not effectively 

 masked by the high-frequency content of the general background or 

 masking noise. 



In general, it has been found that if an observer is surrounded by a con- 

 stant source of noise the noise does not mask all frequencies the same 

 amount. Low-frequency tones mask high-frequency tones far more 

 effectively than high tones mask low ones. 



Masking is also very pronounced when two tones lie close together in 

 frequency. If their frequencies are only slightly different, the masking 

 is so pronounced that only beats are heard. 



Pitch 



The attribute of a sustained simple harmonic vibration of changing 

 frequency incident on the eardrum is experienced as a change in pitch. 



The words frequency and pitch are not interchangeable, because 

 pitch is not uniquely determined by the frequency of the stimulus. The 

 human auditory mechanism possesses a frequency characteristic which 

 is a function of the magnitude of the driving force. 



That the pitch of a pure tone depends on its intensity can be verified 

 by asking an observer to reproduce vocally the pitch of a tuning fork, 

 middle C, when the fork is held at various distances from the ear. It 

 will be found that when the fork is held close to the ear the pitch of the 

 voice reproducing the tone is slightly lower than when the fork is held at 

 some distance from the ear. Apparently, the observer hears the louder 

 tone as lower in pitch. 



Pure tones of low frequency decrease in pitch as the intensity increases, 

 whereas high-frequency tones increase in pitch with increase in inten- 

 sity. Experimental results (Stevens [1935]) show that the frequencies 

 at which the perceived change in pitch with change in intensity is least 

 lie in the band to which the ear is most sensitive, i.e., around 2000 cycles. 



According to Bekesy [1936], an observer experiences his first sensation 

 of pitch at a frequency of excitation of 18 cycles. At this frequency one 

 suddenly passes from the perception of a succession of impulses to a 

 single fused sensation which may be designated as a tone. This may be 

 called the fusion frequency of pitch perception. The upper limit of pitch 

 perception cannot be determined with great precision on account of the 

 marked variation with age of the individual ear. The upper limit is 

 usually set at 20,000 cycles. 



