ABSOLUTE CALIBRATION OF CONDENSER TRANSMITTERS 99 



3. Resonating Tube. — The pressure at the diaphragm end of the 

 tube (see Fig. 4) is computed from a measurement of the air particle 

 velocity at a pressure node. That velocity is obtained by observing 

 the deflection of a Rayleigh disk, R. D., placed in the tube. The 

 sound source R is shown as a moving coil receiver. 



4. Compensation Metlwds. — The pressure in the chamber is de- 

 termined by measuring the force required to prevent motion of a 



=Q= 



MOTOR 



Fig. 3 — Pistonphone method. 



small auxiliary diaphragm Di, Fig. 5. With the sound pressure so 

 determined the corresponding electromotive force of the transmitter is 

 measured. The rest condition of D2 is indicated by absence of sound 

 in an exploring tube communicating with the space back of Di or by 

 absence of frequency variation in a high frequency circuit in which D^. 

 is made one plate of a condenser controlling the oscillation frequency. 



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X' 



Fig. 4 — Resonating tube method. 



4a. Electrodynamic Compensation for Acoustic Pressure. — The com- 

 pensating pressure is provided by sending a current of adjustable 

 frequency, amplitude and phase through D2 placed in a steady magnetic 

 field. 



4&. Electrostatic Compensation for Acoustic Pressure. — The same end 

 is attained with a potential difference of adjustable frequency, ampli- 

 tude and phase applied between D^ and a fixed electrode parallel to it. 



