DECIBEL SCALE 



289 



of pain, if maintained over too long a period of time, may cause injury 

 to the aural membrane of the cochlea. 



The experienced loudness sensation depends on the power in the 

 acoustic waves presented to the ear, the frequency of these waves, and 

 the sensibility of the ear. The relation between these determining 

 factors is represented graphically in Fig. VII-19. The lower curve 



20 



100 500 1000 



Frequency in cycles per second 



5000 10,000 



Fig. VII-19. Loudness contours of pure tones as adopted by the American 

 Standards Association, February, 1936. (By courtesy of the American Standards 

 Association, New York.) ■ 



represents the threshold of audibility of a normal ear; the upper curve, 

 the threshold of feeling. The sensation level of a sound presented to 

 the ear is the number of sensation units it is above the threshold level or 

 threshold of audibility. 



Suppose that a 1000-cycle acoustic vibration reaches a normal ear at 

 an intensity level below the threshold of audibility. As the intensity of 

 the acoustic waves is gradually increased, the threshold of audibility 

 is reached. At this point the intensity level is I , and the experienced 

 loudness L = 0. The intensity is progressively increased so as to 

 produce successive equal increments (AL) in loudness. It is found 

 that at first the change in intensity to produce this increase is very 



