A Volume Unit (VU) meter or Standard Volume Indicator (SVI) is a device displaying a representation of the signal level in audio equipment. The ASA standardized it in 1942 ( C16-5 - 1942) for use in telephone installation and radio broadcast stations. Consumer audio equipment often features VU meters, both for utilitarian purposes (e.g. in recording equipment) and for aesthetics (in playback devices).
The original VU meter is a passive electromechanical device, namely a 200µA DC d'Arsonval movement ammeter fed from a full wave copper-oxide rectifier mounted within the meter case. The mass of the needle causes a relatively slow response, which in effect integrates the signal, with a rise time of 300ms. 0VU is equal to +4 [dBm], or 1.228 volts RMS across a 600 ohm load. 0VU is often referred to as "0dB". The meter was designed not to measure the signal, but to let users aim the signal level to a target level of 0 VU (sometimes labelled 100%), so it is not important that the device is non-linear and imprecise for low levels. In effect, the scale ranges from −20 VU to +3 VU, with −3 VU right in the middle. Purely electronic devices may emulate the response of the needle; they are VU-meters inasmuch as they respect the standard.
The VU-meter (intentionally) "slows" measurement, averaging out peaks and troughs of short duration, and reflects more the perceived loudness of the material than the more modern and initially more expensive PPM meters. For this reason many audio practitioners prefer it to its alternatives, though the meter indication does not reflect some of the key features of the signal, most notably its peak level, which in many cases, must not pass a defined limit.
In the broadcast industry, loudness monitoring was standardized only recently.
Since a VU meter does not reflect the signal peaks for complex wave forms and for varying program material, it might be easy to overload a recording device or broadcast system. To protect against this, a lead or margin of safety may be inserted in the meter circuit. This consists, in effect, in an amplifier making the VU meter commonly 6 or 8 dB more sensitive.
Standard characteristics 
Reference level 
Volume Unit (VU) defined: The reading of the volume indicator shall be 0 VU when it is connected to an AC voltage equal to 1.228 Volts RMS across a 600 ohm resistance (equal to +4 [dBm]) at 1000 cycles per second.
Rise time 
The rise time, defined as the time it takes for the needle to reach 99% of the distance to 0 VU when the VU-meter is submitted to a signal that steps from 0 to a level that reads 0 VU, is 300 ms.
The overshoot must be within 1 to 1.5%.
The fall time is the same as the rise time, 300ms.
Frequency response 
The level specification is meant at 1000Hz. The reading should not depart from the reading at 1000Hz by more than 0.2dB from 35 Hz to 10kHz or more than 0.5dB between 25Hz and 16kHz.
Note that the specification mentions only sinusoid waveforms. Given the electromechanical principle of the meter, the deviation of the needle is actually approximately proportional to the average of the part of the signal with more than approximately 0.4V instantaneously because of the two copper-oxide rectifiers always in series, which transfer function curve has a knee around 0.2V. Signals generally do not have a sinusoidal waveform by far, even if they all fall within the VU-meter bandpass. The reading is not an indication of the power of the signal, which is proportional to the average of the square of the voltage, nor the root-mean-square (RMS) value. As a conventional VU reading, however, it served its purpose as an indication a) of the overall level and dynamics of the signal and b) of the proximity to the maximum admitted level, to the operators of recording and broadcasting equipment. Maintenance staff could also use it as a measurement apparatus, to check for losses in transmissions and level alignment, provided that they used exclusively sine waves as test signals.
The VU meter and its attenuator should present a 7500 Ohm impedance to the circuit it is applied to, measured with a sinusoid signal that sets the indicator to 0dB.
Other level meters 
The consumer audio industry has produced a number of volume indicators that often do not comply with the standard.
See also 
- Audio equipment
- Crest factor
- Loudness monitoring
- Mixing console
- Nominal level
- Peak meter
- Peak programme meter
- Programme level
- Rane Pro Audio Reference definition for VU meter
- Q&A on the Standard Volume Indicator by John McNight
- A New Standard Volume Indicator and Reference Level by Chinn, Gannett & Morris
- The Bewildering Wilderness - "Navigating the complicated and frustrating world of audio standards"
- Audio Level Meters
- VU meter video