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AUDITORY BIOPHYSICS 



The sensation level of any identified sound is the number of sensation 

 units above its threshold level for audition. The sensation units are not 

 expressed in the decibel notation, but the sensation level is. 



The measurement of sound intensity in the decibel notation, it must 

 be emphasized, presupposes a standard of reference intensity since the 

 decibel scale must always represent the relation of one intensity (power, 

 pressure) to another. This standard level of reference is the threshold 

 of hearing of a 1000-cycle pure tone taken as IO -16 watt/cm 2 comparable 

 to a root-mean-square pressure of 2 X 10 -4 dyne/cm 2 . 



Some typical noises and sounds evaluated in terms of a decibel notation 

 are shown in Table VII-2. 



Loudness 



One of the attributes of the sensation experienced by listening to a 

 source of sound which is changing in amplitude is a change in loudness. 

 The experienced loudness of a pure tone, a subjective phenomenon, may 

 be altered also by changing the frequency of the vibrating source. The 

 experienced loudness is therefore a function of two physical variables, 

 intensity and frequency. 



