320 



SPEECH, MUSIC AND HEARING 



3000 and 4000 cycles. The sensitivity of the ear decreases above and below 

 this frequency. The response frequency characteristics of ears are useful 

 and of fundamental importance in the design of reproducing systems. For 

 example, the threshold of hearing at 60 cycles is 48 db higher than that at 

 1000 cycles. 



In general, sound is reproduced at a level lower than that of the original 

 sound. To compensate for the difference in frequency balance, due to the 

 lower reproduction level, an acoustically compensated volume control is 

 used to increase the relative low frequency response as the level is reduced. 

 See Sec. 12.2 J. 



These characteristics must be considered in the measurement of noise. 

 The response frequency characteristic of the noise meter is adjusted to 

 correspond to the ear characteristics. See Sec. 11.7. 



30 40 50 60 70 80 



LOUDNESS LEVEL IN PHONS 



Fig. 13.2. Loudness versus loudness level. (After Fletcher and Munson.) 



13.5. Loudness of a Sound. — Loudness of a sound is the magnitude 

 of the auditory sensation produced by the sound. The units on the scale 

 of loudness should agree with common experience in the estimates made 

 upon sensation magnitude. A true loudness scale must be constructed so 



