VU meters measure the volume (intensity) of analog audio signals. They display signal levels in volume units (VU), a measure of average volume level. By definition, a volume indicator (VI) reading of 0 VU describes a 600-ohm resistance in which 1 milliwatt (mW) of sine-wave power flows at 1000 cycles per second (CPS). Most VU meters have a scale from -20 to +3, and rise and fall times of 300 milliseconds (ms). If a constant sine-wave of 0 VU is applied, the meter will achieve 0 VU in 300 ms. Readings on the VU scale are approximately log to the base 10 of the power ratio referenced to that level. For a pure 100-Hz sine wave, 0 VU equals 0 dBM or 0.775-V root mean square (RMS). In practice, however, VU meters contain an isolating resistor so that 0 VU equals +4 dBM for a pure sine wave.