volume unit or VU

photo of face of VU meter

©Istockphoto.com/Ed Dickie

A unit used for the scale of meters (“VU meters,” but in standards referred to as “volume indicators”) used to monitor audio power levels in recording and broadcast studios, so that the signal can be weakened or amplified to bring it within the optimal range for recording or broadcast. Abbreviated, VU. The term “volume unit” is rarely heard; readings are usually described as, for example, “minus 4 vee you”.

The VU meter was developed in the late 1930s as a joint project of Bell Labs, CBS and NBC. Its response was intended to approximate the response of the human ear to complex, changing waveforms, and its readings lie somewhere between the peak and average values. The specifications cover the meter's ballistics (how fast and far the needle moves when various signals are applied) as well as frequency response. In May 1939, a reference power level of 1 milliwatt into 600 ohms was adopted, and readings on the VU scale are roughly log to the base 10 of the power ratio referred to that reference level. For a pure 1000-Hz sine wave, 0 VU = 0 dBm, but the value of the “volume unit” for real world audio signals is embodied in the unique characteristics of the meter itself.  Further, in practice, engineers found it necessary to add an isolating resistor, which made 0 VU = +4 dBm (again, for a pure sine wave).

Many inexpensive meters, though marked in VU, do not meet the standard. In Europe, the VU meter has generally been scorned in favor of peak reading meters. A VU meter is not suitable for monitoring during digital recording; peak-reading meters are used instead.

H. A. Chinn, D. K. Gannett, and R. M. Morris.
A new standard volume indicator and reference level.
Proceedings of the Institute of Radio Engineers, volume 28, pages 1-17 (January 1940).

Page 15: “3. Dynamic Characteristics
“If a 100-cycle voltage of such amplitude as to give a steady reading of 100 on the voltage scale is suddenly applied, the pointer should reach 99 in 0.3 second and should then overswing the 100 point by at least 1.0 and not more than 1.5 per cent.
...
“5. Calibration
“The reading of the volume indicator (complete assembly as shown schematically in Fig. 18) shall be 0 vu when it is connected to a 600-ohm resistance in which is flowing 1 milliwatt of sine-wave power at 1000 cycles per second, or n vu when the calibrating power is n decibels above 1 milliwatt.”

standards

Acoustical terminology.
American Standards Association Z 24.1 (1951).

IEEE Standard 151-1965 (reaffirmed 1971)
Standard Definitions of Terms for Audio and Electroacoustics.

ANSI C16.5-1954.

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