314 



TABLE 309.— SENSITIVITY OF THE EAR (FIG. 2) 



The minimum effective sound pressure of a specified signal that is capable of evoking an 

 auditory sensation is called the threshold of audibility for that signal. The characteristics 

 of the signal, the manner in which it is presented to the listener, and the point at which 

 the sound pressure is measured must be specified. Two classes of ear-sensitivity deter- 

 minations are shown in figure 2. M.A.P. is just-audible sound pressure measured at the 

 observer's ear drum. M.A.F. is the sound pressure level that is just audible to an observer 

 in an acoustical field free of reflecting surfaces (the sound-pressure level is measured after 

 the observer's head is withdrawn from the field) ; the observer faces the source of sound 

 and listens binaurally. These curves were derived by Sivian and White from measurements 

 on young adult observers all having very good hearing. 10 " The average person cannot detect 

 pressures as low as those given. He will have a threshold curve displaced upward on the 

 chart. (See Table 309 A for data on hearing losses.) 



"50 100 500 1000 5000 10000 



FREQUENCY IN CYCLES PER SECOND 



Fig. 2. — The variation of two classes of ear sensitivity. Curve 1, Monaural M.A.P. The 

 ordinate for curve 1 is 20 logio p/po where p = M.A.P. at ear drum (dyne/cm 2 ) and p = 

 2X10"* (dyne/cm 2 ). Curve 2, Binaural M.A.F. Observer facing source. (0 db = 10"" 

 watts/cm 2 ). 



The term "differential sensitivity of frequency and intensity" refers to the- smallest 

 changes in frequency and intensity, respectively, that can be perceived by an observer with 

 normal hearing. The values depend to some extent on the method of presentation of the 

 test stimuli. For pure tones above 500 cps having levels greater than 40 db above threshold, 

 the measurements of Shower and Biddulph indicate that the smallest perceptible difference 

 in frequency has the approximate constant value of 0.3 percent. For levels greater than 

 40 db above threshold and for frequencies between 200 and 7000 cps, the measurements of 

 Riesz and others indicate that the smallest perceptible difference in intensity varies from 

 one-quarter to three-quarters of a decibel. 



The range of frequency perceived by the average ear varies considerably ; however, the 

 figures of 20-20,000 cycles are frequently quoted as covering the range heard by the average 

 of a group of young adults having no hearing impairments 



106 Sivian, L. J., and White, S. D., Journ. Acoust. Soc. Amer., vol. 4, p. 228, 1933. 



SMITHSONIAN PHYSICAL TABLES 



