DBm Scale

A common dB scale used in audio recording is the dBm scale in which the reference power is taken to be a milliwatt (1 mW) dissipated by a 600 Ohm resistor. (See §F.3 for a primer on resistors, voltage, current, and power.) The signal power is typically averaged over some time period. In audio equipment, exponentially weighted averaging is typically carried out using a one-pole filter having a time-constant near 65.14 ms, which yields a $t_{40}$ (time to reach 99% of the exponential target value) near 300 ms [11].