TIIP.ORY 0!- I III: liniriJXG TF.LEPIIONE 41 



'File loaded stiffness of the recei\'er diaphrai^m is 



^SrSt+Qt'Sr + Qr'St 



S = '^- y CM) 



^Sr + Qr^ 



I'or the transmitter and receiver used 



(2r=10.3. 

 Let the volume of entrapped air be taken as 10 cc, then 



-y= 1.418 X10\ 



Using these values and the values for Sr and St and the circuit 

 of Fig. 8 with K = }/2 the condition for howling becomes 



//? 3.48 1^° = 27.(3/^+ (o0.3 + .459rj/*-59.9/^-(1.0ir+8o.7)/2+3].<> 

 + (.539r+33)+2r68/^ + 16.7y^-(.0506r+ll.l)/'''-40/2 



+ (.0537r- 233)7+23.2+^1 (32) 



where / is expressed in amperes, T in ohms, / in kilocycles and h in 

 ohms per micron. 



For reverse current or negative / the solution is 



/ = 970 kilocycles i = 24 [Ir 



/i = 30.5 e = 8.7 volts 



T = 290 ohms z = 7.0 microns 



/ = .0407 mils ^^ = 1.9 | 158° microns 



Comparing this to the case where the diaphragms are coupled by a 

 lever having an arm ratio 2.7 it is seen that the air coupling produces 

 a greater e.m.f. in the transmitter and only a slightly increased AC 

 current. The receiver diaphragm in this case, however, has a smaller 

 amplitude than the transmitter diaphragm. At this particular 

 howling frequency the transmitter diaphragm stiffness is only about 

 1/4 that of the receiver diaphragm stiffness which explains this 

 anomalous result. Also, it will be seen that the diaphragms \'ibrate 

 almost oppositely in phase. 



