A VU meter is often included in audio equipment to display a signal level in Volume Units; the device is sometimes also called volume indicator (VI).
It is intentionally a "slow" measurement, averaging out peaks and troughs of short duration to reflect the perceived loudness of the material. It was originally developed in 1939 by the combined effort of Bell Labs and broadcasters CBS and NBC for measuring and standardizing the levels of telephone lines.
Volume Unit (VU) defined: The reading of the volume indicator shall be 0 VU when it is connected to an AC voltage equal to 1.23 Volts RMS (equal to +4 dBu) at 1000 cycles per second.[1][2]
The typical VU scale is from −20 to +3. The rise and fall times of the meter are both 300 milliseconds, meaning that if a constant sine wave of amplitude 0 VU is applied suddenly, the meter will take 300 milliseconds to reach the 0 on the scale. It behaves as a full-wave averaging instrument, and is not optimal for measuring peak levels.
The behaviour of VU meters is defined in ANSI C16.5-1942, British Standard BS 6840, and IEC 60268-17.
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