. The Bell System technical journal . he following methods: 1. 2. Pistonphone.^2 3. Resonating tube.^ 4. Compensation methods. a. Electrodynamic compensation for acoustic pressure.* b. Electrostatic compensation for acoustic pressure.^ 5. Membranephone. 98 BELL SYSTEM TECHNICAL JOURNAL In the electrical group for pressure calibration are the followingmethods: 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 inthe articles to which references
. The Bell System technical journal . he following methods: 1. 2. Pistonphone.^2 3. Resonating tube.^ 4. Compensation methods. a. Electrodynamic compensation for acoustic pressure.* b. Electrostatic compensation for acoustic pressure.^ 5. Membranephone. 98 BELL SYSTEM TECHNICAL JOURNAL In the electrical group for pressure calibration are the followingmethods: 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 inthe articles to which references have been given so that only briefdescriptions of the methods are given in the following paragraphs. 1. Tliermophone.—The alternating pressure generated in the chamberof which diaphragm D (see Fig. 2) is one wall, is computed from thephysical constants of the thermophone T, and of the gas (hydrogen)filling the chamber. A computation similar to that in reference ^ isdiscussed 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 aslight correction for the yielding of the diaphragm is introduced, whichwas superfluous with the earlier, less sensitive model. An importantadvantage of the thermophone method is that it is not necessary tohave the heating element parallel to the diaphragm. This makes itapplicable to transmitters with curved or corrugated diaphragms. Insuch cases it is difficult to provide the accurately parallel and narrowspacing 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 pistonamplitude is computed from the dimensions and the angular velocityof the cam driving it. The motor drive makes the method suitable forrelatively low frequencies, up to about 200 ABSOLUTE CALIBRATION OF CONDENSER T
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