THE LOUDNESS FUNCTION 293 



Loudness may be denned as that attribute of the experienced auditory- 

 sensation in terms of which sounds may be ordered on a scale from 

 " very faint " to " very loud." Using such a scale, a listener experi- 

 ences great difficulty in deciding exactly how much louder one sound is 

 than another. Fletcher and Munson [1933] suggested a method of 

 obtaining a rational loudness scale on the idea that the experienced 

 loudness " is cut in half " when only one ear is used for listening to a 

 sound. 



In order to establish a base for a loudness scale a standard simple 

 harmonic source, having a frequency of 1000 cycles, excited at an inten- 

 sity level of 40 db above threshold, and listened to with both (normal) 

 ears, was chosen, and to it was assigned an arbitrary basic loudness of 

 1000 phons. That the loudness range above this level is sufficiently 

 extensive may be judged from the fact that the loudness experienced from 

 the 40-db source is equal to about 3 of 1 per cent of the maximum loud- 

 ness the human ear can withstand. 



Scale of Loudness 



An observer is asked to listen to the 1000-cycle, 40-db-above-threshold 

 reference tone with the aid of earphones, using one ear. He is then 

 asked to use earphones over both ears and adjust the intensity level of 

 the reference tone until the loudness is the same as that experienced with 

 one ear. It will be found, for instance, that an intensity level of 33 db, 

 when heard with both ears, matches the loudness experienced by the 

 observer when listening to the 40-db intensity level when one ear was 

 used. 



If listening with one ear cuts the loudness in half, then it follows that 

 an intensity level of 33 db is subjectively experienced as one half as loud 

 as an intensity level of 40 db. Consequently, if a loudness unit of 1000 

 phons is assigned to an intensity of 40 db, then 33 db must be assigned to 

 a loudness level of 500 phons. In a similar way it is found that an 

 intensity level of 49 db is experienced as a loudness of 2000 phons and a 

 73-db intensity level as a loudness of 10,000 phons. Therefore the 

 standard sound is judged ten times louder at the 73-db than at the 

 40-db level. Comparable data are then developed similarly for other 

 loudness levels. 



The Loudness Function 



The above data are used to construct a graph representing the loud- 

 ness function. In Fig. VII-20 are shown results of how the loudness 

 of the 1000-cycle reference tone changes with its intensity level. In a 



