12 



Sound and the Ear /I : 3 



exact sound pressure levels of the normal thresholds seem to be rather 

 difficult to determine. The graph in Figure 5 shows the results of 

 several investigations. These emphasize that the threshold depends to 



0.01 I 0. 



30cps 



10 20 kc 



Frequency 



Figure 5. Pure tone thresholds. Note that the laboratory 

 averages with trained, selected personnel are consistently lower 

 than the mass survey averages. Recent studies at The Penn- 

 sylvania State University by Professor J. Corso and his associ- 

 ates gave mass survey values between the two curves shown. 

 Notice that the threshold of feeling is not near the threshold 

 of hearing at either 30 cps or 20 kc. The latter are limits of 

 hearing in the sense that people can no longer distinguish 

 tones outside of these limits. After J. G. R. Licklider, in 

 Handbook of Experimental Psychology, S. S. Stevens, ed. (New 

 York: John Wiley & Sons, Inc., 1951). 



some extent on who measures it. Notice that the ordinate is in decibels ; 

 thus a difference of 20 db means a factor of 10 in the sound pressure. 

 All the curves show the same general shape with a minimum threshold, 

 that is maximum sensitivity, in the frequency range from 1-4 kc. When 

 the tests are conducted under controlled laboratory conditions, with 

 carefully screened young people, the thresholds are lower than those 

 found in mass surveys. 



There exist various types of limits of hearing, none of which are very 

 precise. These limits include a minimum pressure threshold and an 

 upper pressure limit at each frequency, as well as a highest and a lowest 



