LOUDNESS OF A SOUND 



321 



that when the units are doubled the sensation will be doubled and when 

 the scale is trebled the sensation will be trebled, etc. Units on the scale 

 are called loudness units, abbreviated L. U. The loudness level of the 

 reference tone, expressed in phons, is the intensity level of the reference 

 tone (1000 cycles) in decibels. The loudness level of any other sound is 

 determined by adjusting the reference tone until it sounds equally loud. 



RELATIVE PRESSURE 

 100 1000 



10000 



100000 



lOOOOOr 



10000- 



3 100 



40 60 80 



INTENSITY LEVEL IN OB 



100 



Fig. 13.3. The relation between the intensity level and the loudness of pure tones of the 

 frequencies indicated. db = 0.000204 dyne per square centimeter. (After Fletcher and 

 Munson.) 



The loudness level of the measured sound is the loudness level of the refer- 

 ence sound expressed in phons. A scale ^ showing the relation between 

 loudness level, in phons, and the loudness, in loudness units, is shown in 

 Fig. 13.2. 



The loudness of pure tones of various frequencies is shown in Fig. 13.3. 

 For tones between 800 and 2000 cycles the loudness is the same for the same 

 pressure. The difference is small up to 8000 cycles. For higher frequen- 

 cies than this the loudness decreases as the frequency increases. Further, 



2 Fletcher and Munson, Jour. Acous. Soc. Amer., Vol. 9, No. 1, p. 1, 1937. 



