200 AUDITORY BIOPHYSICS 



where p is the instantaneous-pressure change and v the instantaneous- 

 velocity change of an air particle. At the midpoint of the simple har- 

 monic displacement of the particle, where the maximum velocity (^o) and 

 the pressure (po) changes take place, 



Vo = pv Q V 



Acoustic Intensity in Terms of Acoustic Pressure 



Since 



/ = 2ir 2 n 2 A 2 P V 



and 



t»o = 2irnA 



Tin 



I = — — watts/cm 2 

 2p V 



The intensity may be expressed in effective or root mean square value 



of the pressure variation where po = v2p e . 



In air under conditions where neither the density nor the velocity of 

 propagation of the wave changes, the intensity is directly proportional 

 to the square of the particle pressure. 



If the minimum audible intensity is 10 -16 watt /cm 2 at 1000 cycles, p = 

 0.00121 gram/cm 3 , and V = 343 meters/sec at 20° C and 760 mm air 

 pressure, the pressure associated with the threshold of minimum audi- 

 bility is „ 2 



io- 9 = ^4 



2 X 121 X 10~ 5 X 34,300 



po = 2.9 X 10~ 4 dyne/cm 2 



In the range of acoustic frequencies from 1000 to 6000 cycles Geffcken 

 [1934] has observed that human cars can detect pressure changes even 

 as low as 5 X 10 -5 dyne/cm 2 , thus justifying the above calculations. 



Auditory Mechanism 



The next problem to consider is the way the auditory mechanism 

 responds to the changes in pressure set up in the object space. For the 

 purpose of this discussion the structure of the ear may be divided into 

 three parts. 



I. The external ear is that portion of the acoustic mechanism upon 

 which the sound waves impinge. It consists of the auricle (pinna), whose 

 foundation is a framework of elastic cartilage, and the external acoustic 



