Any equipment used in audio applications has at least one and often several VU panel meters. These monitor the audio signal for "loudness" with sensitivity beyond that of the human ear.
VU Meter Background
Bell Labs and the CBS and NBC radio networks developed the first VU (volume unit) panel meter in 1939 for the purpose of monitoring the audio transmission of telephone and radio signals to ensure audio was being broadcast at an appropriate level.
Most people know what a VU meter looks like even though they might not realize it. These gauges with their needles that bounce in response to sound are ubiquitous symbols of audio recording, particularly music, used everywhere from professional studios to handheld tape recorders.
By looking at VU meters during a recording, a person can be sure the recording volume is high enough to create a good reproduction without excessive background noise but not so high it will lead to distortion and poor audio quality. If there are multiple audio sources the meters can help the user balance the signals so the final recording sounds best.
Digital and Analog Meters
The original VU panel meters were analog gauges with needles that moved in response to changing audio strength. The gauges were marked with a red zone that indicated volume that was too high and often had an LED that lit up if the needle ever "pegged" meaning it hit the top of the range.
Today some audio equipment has switched to digital VU meters. These use rows of LEDs that light up as the audio signal changes, often with the peak level staying lit up so a listener can judge the overall range of the audio signal. Digital meters have the advantage that they don't suffer from inertia; the mechanical effort to move the needle creates a small delay in traditional analog panel meters. However this difference is not large enough to be important for most applications and analog meters are still popular choices and used in a variety of electronics.
Present Day VU Meters
One limitation of VU panel meters is they measure an average of the sound signal so a single loud, percussive sound will often not register on these meters and yet such an audio event can create significant distortion in a recording. More sophisticated tools such as true peak-reading meters are available but VU meters are still in widespread use. They are significantly less expensive than the more advanced meters and most applications don't require the additional functionality.
Modern audio equipment, particularly equipment directed at the consumer rather than the professional market, use inexpensive VU meters to offer the important audio monitoring features but still keep the price of the equipment low. Although more advances will come in the future, it is likely the basic VU meter will be around for a long time to come.
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