base and sound-level meter without cable correction. Figure 3 is an elementary schematic diagram of the system. As is indicated, no external grid-leak is used to determine the bias for the preamplifier. The insulating material supporting the back plate structure of the microphone is a glass-bonded mica with a leakage resistance of more than 107 megohms and is specially treated to remove and seal out moisture. So long as the leakage resistance is maintained at this high value, the grid bias is established by the voltage gradient within the electron cloud surrounding the cathode. Under these conditions it has been found that the bias is stable and the noise level over the 20-kilocycle band at the output of the preamplifier is approximately 20 a«volts or low enough so that sound levels of 40 db (re 0.0002 ¿ubar) can be meas ured with the system. The output of a 50-db microphone is 20 /¿volts for a sound-pressure level of 30 db (re 0.0002
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