What is a VU meter?
The VU meter shows an averaged signal level and gives an impression of perceived loudness. The origins of the mentioned VU meter go back to 1939. Its original name was SVI or Standard Volume Indicator. A collaborative project by CBS, NBC and Bell Labs in America developed this meter. What’s more, since the meter scale was calibrated in ‘volume units’, that’s the name that stuck! The SVI/VU meter is amongst the simplest of all audio meter designs. It essentially behaves as a simple averaging voltmeter, with a moderate attack (or ‘integration’) time of about 300ms. The needle fall-back time is roughly the same. Moreover, the full meter specification is enshrined in the IEC 60268-17 (1990) standard.
VU display
Both the amplitude and duration of the applied signal influenced the VU display. With a steady sine-wave signal applied to the input, a VU meter gives an accurate reading of the RMS (root-mean-square, or average) signal voltage. However, with more complex musical or speech signals the meter will typically under-read, and a sustained sound will produce a significantly higher indication than a brief transient signal, even if both have the same peak voltage. In theory, a VU meter should respond to both the positive and negative halves of the input audio signal, but the cheapest implementations sometimes only measure one half of the waveform, and so can provide different readings with asymmetrical signals compared to full VU meters.
Consistent audio production work
One common type of the VU meter has a moving needle on a graduated scale. Usually, the top of the scale is calibrated in (Decibels) dB, and the lower portion scale is calibrated in percentages. In audio engineering, a value of 0 dB is valued as 100 % of the volume, or the loudest you want the signal to go. The VU meter is important for consistent audio production work. On the other hand, how loud something sounds is very subjective. What is loud to one announcer may not be to another, especially if the set the same monitor set differently. In short, the VU meter basically gives an electronic reading of volume that is not subjective.
VU accuracy
People often argue about the accuracy of VU meters cin two areas. First, VU meters have trouble detecting transient-sudden, sharp, or short increases of the sound signal. Most VU meters indicate an average volume level and ignore these occasional sound bursts. Second, VU meters tend to overreact to the low frequency (bass) portion of the sound. In other words, if a lot of bass frequencies saturate the bass, the VU meter will probably show the higher value on the display.
In the digital world
However, in our modern digital world, the VU meter really doesn’t perform adequately. The current state of the art is enshrined in the new ITU-R BS 1770 standard. Consequently, the broadcast sector worldwide has adopted it very rapidly. This is an excellent metering system that provides a new and very accurate Loudness Meter scaled in LUFS — which does a much better job than the VU — along with an oversampled True Peak Meter scaled in dBTP, which does a much better job than the PPM.
Additional resources & source texts
https://books.google.me/books?id=AUU2pEkfg4MC&printsec=frontcover&hl=sr&source=gbs_ge_summary_r&cad=0#v=onepage&q&f=false
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