166 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 2 



and here it should be recognized how greatly our present knowledge 

 of both hearing and speech is due to the noteworthy investigations 

 of Dr. Harvey Fletcher and his colleagues at the Bell Telephone 

 Laboratories in New York. 



With reference to frequency, the average ear can perceive a range 

 of frequencies from about 20 to 20,000 cycles per second, the upper 

 limit declining with advancing years. In the various practical devel- 

 opments of acoustics, however, attention is largely restricted to the 

 ranges 50 to about 5,000 for speech and 35 to 7,000 for music. 



As regards intensity, it has been shown by Wegel and others, from 

 experiments on pure notes, that there is a certain minimum amplitude 



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Frequency and Pitch of Note. 



FiGDRE 2. — Average area of normal hearing, showing auditory ranges of 

 frequency and intensity 



for each frequency below which the average ear fails to detect the 

 note. Moreover, the ear is much more sensitive to notes of medium 

 pitch than to higher or lower notes. The threshold or lower limit of 

 intensity passes in fact through a minimum at about 2,000 cycles per 

 second as we steadil}'^ change the frequency. 



Similarly there is a maximum amplitude peculiar to each fre- 

 quency, above which the ear no longer functions — hearing is subor- 

 dinated to a tickling sensation or even actual pain. Again the ear 

 shows to advantage in the middle of the range (about 500 cycles per 

 second), and so the upper limit of intensity or threshold of feeling 

 passes through a maximum as we progressively vary the frequency. 



Figure 2 shows the auditory sensation area for the average ear as 

 determined by Fletcher and Wegel. (Fletcher's Speech and Hear- 

 ing.) The boundaries are constituted by the two threshold curves, 



