VU (Volume Unit) meters are used extensively to monitor signal levels in audio recording-studio equipment. A VU meter reads 0 (0 VU) at +4 dBu, where the dBu scale is given by , and where denotes the root-mean-square (rms) voltage of the signal--not its peak voltage amplitude, and not its mean square (average power) as in the dBm scale.
The rms voltage reference used in the dBu scale can be calculated as . Thus, when the measured voltage is driving a resistance of Ohms, the dBu and dBm scales are identical. When the load resistance is other than Ohms, then the current associated with the measured voltage is different, hence so is the power, and the dBm and dBu scales diverge. Thus, the dBu scale ``only cares about voltage'', regardless of the impedance it drives, but it coincides with the dBm scale when the dBm reference impedance ( ) is used. The dBu scale is common in practice because electronic audio equipment nearly always uses voltage transfer from one circuit module to the next, i.e., low-impedance outputs drive high-impedance inputs, and signals are represented by voltage alone, neglecting ``loading effects''.
In addition to VU meters, which measure rms voltage level, there is usually also a peak meter, for displaying sudden voltage transients that could overload the audio equipment. Maximum peak values are usually also latched and displayed until cleared, so that any past overload is indicated.
Since modern digital systems can easily measure and display signal levels more accurately psychoacoustically, VU meters are generally used today mostly for their ``vintage look and feel'', including original looking meter displays with needles that swing in a circular arc with ms, etc. For more info on VU meters, see, e.g., .