The Rochelle salt crystal, diaphragm-type microphone chosen for use on the General Radio Type 759-B Sound-Level Meter is a low-cost device, which fulfills all of these requirements satisfactorily so long as the microphone is connected directly to the input terminals of the sound-level meter, and so long as moderate variations of temperature and humidity are encountered.
However, when it becomes necessary to make measurements with the microphone separated from the sound-level meter by a long cable or when high temperatures and humidity are encountered, the Rochelle salt crystal microphone becomes a less satisfactory pickup. While the output voltage of the microphone changes by about .02 db per degree F., its capacitance varies considerably as the temperature changes so that loss added by a long cable is very markedly a function of temperature.
Figure 1 shows the variation in capacitance with temperature for a typical Rochelle salt crystal microphone. At the top of Figure 2 the dashed curve shows the change in output of the microphone with temperature, and the solid curve shows the change in meter reading of the Type 759-B Sound-Level Meter due to temperature changes at the micro phone with the microphone mounted on the sound-level meter. The two lower curves show the large changes in meter readings that occur when 25-foot and 100-foot cables respectively are used between the microphone and the sound-level meter. The maximum safe temperature at which the Rochelle salt unit can be used is about 1 15° F., since it is permanently damaged at temperatures above 135° F. Although the unit is sealed, extensive use at a relative humidity below 30% or above 85% should be avoided.
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