ABSOLUTE CALIBRATION OF CONDENSER TRANSMITTERS 101 



face of the diaphragm. This must be done for each instrument to be 

 caUbrated. 



5. Membranephone. — In principle this method is similar to the 

 pistonphone. An acoustically driven membrane M (see Fig. 6) 

 replaces the motor-driven piston. From the volume displacement, 

 AV, of M the pressure on the transmitter diaphragm D is computed. 

 The value of A V is given by a measurement of the alternating variation 

 in capacitance between M and an auxiliary perforated electrode G. 

 The range of the method is from the lowest frequencies up to those at 

 which the linear dimensions of the chamber become comparable with 

 the sound wave-length (X). As with the thermophone, that upper 

 limit can be extended through the use of hydrogen instead of air. 



The computation of A F is given in Appendix III. It will be noted 

 that the computation is independent of the mode in which the mem- 

 brane vibrates. However, for frequencies above the first resonance of 



TO AMPLIFIER- 

 RECTIFIER 



Fig. 6 — Membranephone method. 



the membrane the requirement as to smallness of chamber dimensions 

 relative to X, becomes much more stringent than in the thermophone 

 case. 



Methods Employing Electrical Drive. — Since the driving forces in this 

 group are electric the pressure on the diaphragm is affected by the 

 acoustic load on the front face of the diaphragm. To obtain the true 

 pressure calibration that acoustic load must be known. Practically 

 this is taken care of by making that load sufficiently small, rather than 

 accurately determining its value. 



6. The Back Electrode Serving as the Driving Electrode. — The alter- 

 nating potential difference, Vi sin w/, is impressed in series with the 

 steady potential Fo, see Fig. 7. This gives a driving force component 

 aFoFi sin wt. The corresponding alternating variation in the trans- 

 mitter capacitance is determined by having that capacitance control 

 the frequency of a high frequency oscillator circuit. Absolute values 

 are obtained by means of a static pressure calibration as in Method 4. 



