168 



ANNUAL EEPORT SMITHSONIAN INSTITUTION, 19 3 2 



rapidly than the intensity. Kingsbury's results are set out in Figure 

 3, which shows a number of equal loudness contours superposed on 

 the auditory sensation area. As will be seen, these contours are 

 roughly parallel to each other for medium frequencies above about 

 700 cycles per second. 



An experimental illustration based on such loudness contours is to 

 operate a pure sound at constant intensity and gradually increase the 

 frequency, when the loudness will be heard to pass through a maxi- 

 mum at about 2,000 cycles per second. This is evidenced by the track 

 of the horizontal line in Figure 3 at a level of about 0.1 dyne per 

 square centimeter. 



o" 10.000 



a 1000 



c 



3 





CO 



^Cf 



100 

 10 



I 



01 



001 



OOOI 



OOOOi 



ThrcsKold 

 0^ Fee!inq 



Threshold 

 0^ Hcarincj 



16 



32 

 C. 



128 256 512 1024 2048 4096 8192 16384 Ciicles 



^ «' -^1' „'"! ^iv „v „VI „V1I i „„_ 



C C C C C C C C per sec . 



Frequency and Pit^h of Note. 



64 



c 



Figure 3.- 



" ladder 

 second 



-Auditory sensation area showing equal loudness contours ; also decibel 

 ' and sensation-step " ladder " for a frequency of 1,000 cycles per 



THE MASKING OF SOUNDS 



With reference to the masking of sounds, the masking of one pure 

 note by another is usually measured by the extent to which the 

 threshold of audibility of the masked note is raised. In general a 

 particular note is masked (1) most readily of all by another of ap- 

 proximately the same frequency, (2) more readily by a note of lower 

 frequency than one of higher, at any rate for loudnesses about and 

 above speech level. 



The position is not so simple with loud complex sounds, such as 

 noises, as the masking may be confused by other factors, such as the 

 masking of the individual components and the formation of sub- 

 jective tones. (Fletcher's Speech and Hearing.) 



