412 BELL SYSTEM TECHNICAL JOURNAL 



we are not yet completely clear as to the relationship between sensation 

 and stimulus, although Fletcher and Munson ^ have developed 

 formulae whereby the loudness level of a steady sound can in most cases 

 be computed from the intensity level of its components. For single 

 pure tones the relationship between sensation and stimulus has been 

 extensively explored with the results which are indicated in the audi- 

 tory chart shown in Fig. 9, as given by Fletcher and Munson. The 

 various curves give the intensity level of pure tones of equal loudness. 

 This chart gives some idea of the complexity of the relationship between 

 loudness and stimulus. The threshold of audibility of course varies 

 widely with frequency and the relationship between sensation level 

 and intensity level is not the same at the various frequencies and levels; 

 for instance, at a loudness level of 40 db above threshold, a change of 5 

 db in the stimulus at 100 cycles produces the same change in sensation 

 as a change of 10 db at 1,000 cycles. Fechner's law does not hold 

 strictly over a wide range of intensities at any of the audible frequen- 

 cies. The difficulty of devising an instrument which would have 

 similar characteristics is apparent. "Sound level meters" have, 

 however, been designed which give a reading which is approximately 

 proportional to the subjective intensity of the sound. These meters 

 are generally so designed that they have a frequency characteristic 

 corresponding to the auditory curve at about the level of the noise 

 being measured. They have proved themselves extremely useful 

 although from our knowledge of hearing phenomena we might expect 

 large variations in the actual loudness of sounds of different character, 

 even if a noise meter of the above type should show them to be equal. 



5'' Jour. Acous. Soc. Am. 5, 82 (1933). 



