VU (Volume Unit) meters are used extensively to monitor signal levels
in audio recording-studio equipment
[41].F.5A 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.,
[11].