The volume unit (VU) meter, invented in 1939, is an electromechanical device designed to display average audio frequency signal levels. Its measurement scale is logarithmic and typically registers levels from minus 20 dB to plus-3 decibels (dB). The 0 dB (0 VU) reference level represents a 1,000 Hz sine wave signal with a voltage amplitude necessary to dissipate 1mW into a 600 ohm load. The lower range, below 0 VU, is usually marked with a green color to indicate acceptable audio intensity. The upper 3dB range is usually marked with a red color to indicate that some distortion, by clipping of the peaks, is likely.
The internal mechanism is essentially a DC motor with a magnetized armature, with indicating needle attached, suspended by a thin wire, instead of a bearing, to eliminate frictional resistance to motion. A multi-turn coil of very fine magnet wire is driven by an amplified signal such that the needle can easily move in response to the amplitude. The use of such delicate parts provides signal sensitivity but made the vu meters also sensitive to damage from mechanical shock.
A sine wave has a smoothly balanced change in amplitude with respect to time. Most VU meters have a mechanical time response of about 300mS which means they do not average the amplitude changes of wave below 10 Hz (cycles per second) very well. At the reference frequency of 1,000 Hz, the needle will remain steady, showing the average sound level, because the wave changes amplitude much faster than the mechanism can mechanically respond. The meter will respond to complex audio waveforms, like speech and music, though the non-uniform nature of such energy can easily cause the needle to flicker. However, to the recording engineer observing the meter, this additional information is a benefit; not a distraction or error to be avoided or corrected.
VU meters are used in audio recording equipment and have taken many forms from thin, edge designs where there is limited panel space to larger, flat designs which are easier to view. With the invention of Light Emitting Diodes (LEDs) and Integrated Circuits (ICs), VU meters became self-lit, provide a much greater dynamic range and therefore could replace the analog versions. LEDs, and their associated circuitry, can perform additional functions not possible by the electro-mechanical version. For example, the LEDs can be directed to remain lit for detected peaks, thus providing a 'peak-hold' function. However, many users still prefer the analog variety specifically because they are not digital and do display more nuances of the sounds being recorded.
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