PHYSICAL MEASUREMENTS OF AUDITION 



157 



This standard is a pure tone having a vibration frequency of 700 

 cycles per second. Its absolute loudness is defined as the change in 

 loudness measured on the scale defined above, from the loudness 

 value corresponding to the threshold pressure for normal ears which for 

 700 cycles is exactly 0.001 dyne. This frequency was arbitrarily 

 chosen as a standard for measuring loudness because of this particular 

 value of its threshold pressure, and because it is close to the frequency 



*,R.M. 5. Pressure Art. Units 



a: 

 5: 



400 800 wo ISOO 2000 2400 



, R. M. 5. Pressure Arb Units Frequency 



400 



100 



1200 1600 2000 2400 



Frequency 



6G 



40 



5s 



20* 40 60 80 



Units above threshold Sound A 



Fig. 6 



at which the loudest tones used in conversational speech occur. 

 By this definition, the loudness of a tone of frequency 700, for which 

 p is the pressure variation, expressed as a root mean square value, 



L =60+20 log p 



and the loudness of any other sound, pure or complex, is defined as 

 being equal to that of a tone of frequency 700, seeming equally loud. 

 Such a definition implies that experimental measurements can be 



