AUDITORY LOCALIZATION 325 



quency control is varied, the result is frequency modulation. If the 

 position of the loud speaker is varied with respect to the observation point, 

 the result is phase modulation. 



The vibrato is used as an artistic embellishment by singers. It is an 

 example of frequency modulation. The average rate of the vibrato is 

 seven cycles per second. 



When two tones of nearly the same frequency are sounded together, they 

 produce beats at a rate equal to the frequency difference between them. 

 In the case of very slow beats the intensity seems to rise and fall contin- 

 uously. Faster beats appear as intermittent impulses. 



13.11. Auditory Localization i^' i^- ^^ — The human hearing mechanism 

 can localize sounds with great accuracy. This property is due to two 

 effects, namely: the difference in intensity and the difference in phase 

 between the sound at the two ears. The difference in phase between the 

 sounds at the two ears is due to the difference in time arrival at the two 

 ears. The difference in intensity at the two ears is due to diffraction. 

 The pressure at the two ears may be obtained from Fig. 1.5. by assuming 

 the head to be a rigid sphere. This assumption has been verified experi- 

 mentally. 



The binaural phase effect is confined principally to the lower frequencies, 

 namely: below 1000 cycles. The binaural phase effect has been utilized 

 for the location of airplanes, submarines, etc. 



If attachments can be made to the ears which will virtually separate 

 them further (for example, spread the microphones on the dummy of 

 Fig. 12.18), then a small rotation of the apparatus will mean a larger dif- 

 ference in phase at the receivers than at the unaided ear. Of course, there 

 may be ambiguity as to whether the sound comes from in front or behind, 

 but this does not vitiate the method. 



The difference in intensity at the two ears due to diffraction is very small 

 below 1000 cycles. However, at the higher frequencies the difference in 

 int-ensity may be 20 to 30 db. 



The human hearing mechanism is also a directional collecting system. 

 Using the system of Fig. 12.18, the reverberation in a room appears to be 

 normal. However, if only one microphone is used, the apparent reverber- 

 ation will be increased. This indicates that the human hearing mechanism 

 is very directional and discriminates against reverberation and other 

 undesirable sounds. For this reason it has been found necessary to use 



" Stewart, G. W., Phys. Rev., Vol. 15, No. 5, p. 425, 1920. 



^^ Stevens and Davis, " Hearing," p. 167, John Wiley & Sons, New York. 



i« Steinberg and Snow, Bell Syst. Tech. Jour., Vol. 13, No. 2, p. 245, 1934. 



