380 BELL SYSTEM TECHNICAL JOURNAL 



Reference Tone 



A plane or spherical sound wave having only a single frequency of 

 1,000 cycles per second shall be used as the reference for loudness 

 comparisons. 



Note: One practical way to obtain a plane or spherical wave is to 

 use a small source, and to have the head of the observer at least one 

 meter distant from the source, with the external conditions such that 

 refected waves are negligible as compared with the original wave at 

 the head of the observer. 



Loudness Level 

 The loudness level of any sound shall be the intensity level of the 

 equally loud reference tone at the position where the listener's head is 

 to be placed. 



Manner of Listening to the Sound 



In observing the loudness of the reference sound, the observer shall 

 face the source, which should be small, and listen with both ears at a 

 position so that the distance from the source to a line joining the two 

 ears is one meter. 



The value of the intensity level of the equally loud reference sound 

 depends upon the manner of listening to the unknown sound and also 

 to the standard of reference. The manner of listening to the unknown 

 sound may be considered as part of the characteristics of that sound. 

 The manner of listening to the reference sound is as specified above. 



Loudness has been briefly defined as the magnitude of an auditory 

 sensation, and more will be said about this later, but it will be seen 

 from the above definitions that the loudness level of any sound is ob- 

 tained by adjusting the intensity level of the reference tone until it 

 sounds equally loud as judged by a typical listener. The only way of 

 determining a typical listener is to use a number of observers who have 

 normal hearing to make the judgment tests. The typical listener, as 

 used in this sense, would then give the same results as the average 

 obtained by a large number of such observers. 



A pure tone having a frequency of 1000 cycles per second was chosen 

 for the reference tone for the following reasons: (1) it is simple to 

 define, (2) it is sometimes used as a standard of reference for pitch, 

 (3) its use makes the mathematical formulae more simple, (4) its range 

 of auditory intensities (from the threshold of hearing to the threshold 

 of feeling) is as large and usually larger than for any other type of 

 sound, and (5) its frequency is in the mid-range of audible frequencies. 



There has been considerable discussion concerning the choice of the 



