98 



BELL SYSTEM TECHNICAL JOURNAL 



In the electrical group for pressure calibration are the following 

 methods: 



6. The back electrode (backplate) serving as the driving electrode.^- * 



7. An auxiliary third electrode driving the diaphragm. 



Ail but two of the above methods have been described in detail in 

 the articles to which references have been given so that only brief 

 descriptions of the methods are given in the following paragraphs. 



1. Tliermophone. — The alternating pressure generated in the chamber 

 of which diaphragm D (see Fig. 2) is one wall, is computed from the 

 physical constants of the thermophone T, and of the gas (hydrogen) 

 filling the chamber. A computation similar to that in reference ^ is 

 discussed in Appendix I and II. The difference is in the manner in 



Fig. 2 — Thermophone method. 



which the heat conductivity of the walls is taken into account. Also a 

 slight correction for the yielding of the diaphragm is introduced, which 

 was superfluous with the earlier, less sensitive model. An important 

 advantage of the thermophone method is that it is not necessary to 

 have the heating element parallel to the diaphragm. This makes it 

 applicable to transmitters with curved or corrugated diaphragms. In 

 such cases it is difficult to provide the accurately parallel and narrow 

 spacing between the diaphragm and driving or compensating electrode, 

 required in electrostatic methods. 



2. Pistonphone. — The pressure is generated by means of a recipro- 

 cating motor-driven rigid piston as shown in Fig. 3. The piston 

 amplitude is computed from the dimensions and the angular velocity 

 of the cam driving it. The motor drive makes the method suitable for 

 relatively low frequencies, up to about 200 c.p.s. 



