A VU (volume unit) meter is an audio metering device. It is designed to visually measure the "loudness" of an audio signal.
The VU meter was developed in the late 1930s to help standardise transmissions over telephone lines. It went on to become a standard metering tool throughout the audio industry.
VU meters measure average sound levels and are designed to represent the way human ears perceive volume.
The rise time of a VU meter (the time it takes to register the level of a sound) and the fall time (the time it takes to return to a lower reading) are both 300 milliseconds.
The optimum audio level for a VU meter is generally around 0VU, often referred to as "0dB". Technically speaking, 0VU is equal to +4 dBm, or 1.228 volts RMS across a 600 ohm load.
VU meters work well with continuous sounds but poorly with fast transient sounds.
See also: Peak Program Meter (PPM)