ABSOLUTE CALIBIUTION OF CONDENSER TRANSMITTERS 97 



mitter. That ratio [i?(/) = e/p'], as a function of frequency, gives the 

 caHbration. Where and how is the acoustic pressure to be measured? 

 This can be done in any one of a number of ways, all of which in 

 general lead to different calibrations. The two calibrations most 

 useful and amenable to measurement are when the pressure is uniform 

 over the diaphragm and measured at the diaphragm and when the 

 pressure is the pressure in a progressive plane wave, undistorted by the 

 transmitter or any other obstacles; when the electromotive force is 

 measured the distortion of the sound field must be due to the trans- 

 mitter alone. 



It is convenient to designate the former as "constant pressure" or 

 "pressure" calibration, the latter as the "constant field" or "field" 

 calibration. In general the field calibration will depend on the angle 

 of wave incidence. Incidence normal to the diaphragm gives the 

 "normal field" calibration. Where no confusion can arise, "field" 

 calibration will be used to imply normal incidence. The pressure and 

 field calibrations tend to coincide when the transmitter dimensions are 

 small compared to the sound wave-length and when there are no 

 appreciable impedances between the diaphragm and the sound field in 

 front of it. Neither condition obtains for the No. 394-Type Trans- 

 mitter, except at very low frequencies. 



Which of the two calibrations — "pressure" or "field" — is more 

 significant depends on the particular use made of the transmitter. 

 Thus in the receiver testing machine, where the sound is substantially 

 uniform throughout a small chamber closed by the transmitter 

 diaphragm and by the receiver under test, the pressure calibration is 

 important. When the transmitter is used to pick up sound in the 

 open air at a distance from the source, the field calibration applies. 

 For other cases, neither calibration is directly applicable, this being 

 discussed at the end of the paper. 



Constant Pressure Calibrations 



For the several methods available for constant pressure calibration, 

 the pressure may be applied either acoustically or electrically. In the 

 acoustical group are the following methods : 



1. Thermophone.2 



2. Pistonphone.^' 2 



3. Resonating tube.^ 



4. Compensation methods. 



a. Electrodynamic compensation for acoustic pressure.* 



b. Electrostatic compensation for acoustic pressure.^ 



5. Membranephone. 



