102 



BELL SYSTEM TECHNICAL JOURNAL 



In this case, however, that does not give the force acting on the dia- 

 phragm unless the air impedance between the diaphragm and back- 

 plate is negligible in comparison with that of the diaphragm itself. 

 Hence the method does not apply to the No. 394-Type Transmitter. 

 The same consideration as to non-uniformity of the driving force over 

 the area of the diaphragm which was mentioned in connection with 

 Method 46, applies to this case. 



7. Auxiliary Third Electrode Driving the Diaphragm. — Here an 

 auxiliary electrode M and a circular metal screen furnishes the electro- 

 static drive (see Fig. 8). It has nearly the same diameter as D and is 

 parallel to it. The gap between M and D is about thirty times greater 



'-<2r 



TO HIGH FREQUENCY 

 OSCILLATOR 



HIGH 



FREQUENCY 



CHOKE 



^hKSH 



Fig. 7 — Electrostatic method — Back electrode serving as driving electrode. 



than between D and the backplate. Hence the electric force on D is 



uniform over the surface of D, and its absolute value can be computed 



with some accuracy. The calculation is given in Appendix IV. Care 



must be taken to avoid acoustic loading of Z) in a manner that would 



materially change its impedance. With this possibility guarded 



against, this method admits of an absolute transmitter calibration 



from 20 to 20,000 c.p.s. A comparison of a calibration so obtained 



wdth that given by a thermophone for the same transmitter,* is shown 



in Fig. 9. The two are quite independent. The discrepancy between 



the two up to about 6,000 c.p.s. is regarded as being within limits of 



experimental error. The acoustic load imposed on the diaphragm by 



* This particular instrument happened to be about 4 db less efficient than tlie 

 aAcrage No. ,S94-T}pe Transmitter. 



