40 BELL SYSTEM TECHNICAL JOURNAL 



Case II — Diaphragms Coupled Together i?y a Small 

 Chamber of Air 



It will be assumed that the air chamber is so small that the phase 

 of the presstire variation is the same on both diaphragms. Let V 

 be the \-olume of air between the diaphragms. Then 



V=Vo-^QRy + QTZ (24) 



where V is the volume of air in the undisturbed state and Qr and Qr 

 are the effective areas of the receix'er and transmitter diaphragms 

 respectively. 



The pressure variation in the chamber (changes considered adiabatic) 

 is given by 



dp= -yYiV=-{(2Hy + QTZ)y^ (25) 



When the steady state is set up this may be considered a vector 

 equation and the variables expressed in rms values. 

 The equations of motion for the diaphragms are 



and 



Solving 





z = ^4^. (21) 



or 



Zi [sT-\-\Qr) 



y= ^ Ti^ — ' ^2') 



yP ' 

 Sr + fQr' 



(29) 



-^SrSt+Qt'Sr+Qr'St 

 yP 



In this case the ratio Ijetween s and y is not fixed, but depends upon 

 St which is a function of the freciuency. 



