The charging voltage applied to the capacitor/diaphragm of a condenser (capacitor) microphone.
"Back in the late 40's our father, then a single man worked in a Philadelphia music store. He made a voice over demo record single side on a 78. Dad died in 1962 and the record was tucked away for decades until I found it recently quite by accident. A friend had turntable for 78 speed and i found Dad was quite the tenor, but as expected the ..."