Willy Thurbon comes to us from Australia. He has written numerous articles regarding this industry on a technical and social level. Look for more from Willy T. on Roadogz in the near future.

G'day,

This week we are looking at the needles a-flickering and the pretty lights flashing on the mixing desk - the meters.

The Meters allow us to visually monitor the output levels of the Group, Stereo and Auxiliary Send Busses, as well as the Solo Levels. They are designed to operate as ' VU.' - Volume Unit - or 'PPM' - Peak Program Meters, with some desks able to switch their operation between the two types.

VU Meters
The VU meter is amongst the simplest of meter designs and have been used since the very beginnings of the broadcast, recording and live audio industries. VU meters come in the form of the Moving-coil Meter - the traditional 'needle' type of meter - or as a bar-graph of LEDs. [LEDs are the most common, with moving-coil meters now more often seen on 'retro' gear.]

VUs were designed to display an approximation of the RMS voltage level of electrical signals. The 'Root Mean Square' voltage is a technical measure of the average voltage level of electrical signals.

VUs are relatively cheap to manufacture because there is no complex peak-sensing driver circuitry involved. VU meters tend to be used in order to cut costs where there is a requirement for a large number of meters, or where the meter needs only to provide an indication that sound is reaching a particular channel, such as on large mixing desks or multi-track recorders.

VUs give a true reading of a pure sine-wave tones' RMS voltage level. With complex audio signals - like the ones in the real world - this approximation is less accurate & will usually read slightly lower than the true RMS value. However, they still provides a useful tool for most practical live sound tasks.

Because the VU meter measures 'average' levels, sustained sounds will read much higher than a brief percussive one, even when both sounds have the same maximum voltage level. The reading is dependent on both the amplitude and the duration of peaks in the signal. In addition, the standard VU response and fallback times - around 300 milliseconds each - exaggerate this effect, so transients and percussive sounds barely register at all and can cause unexpected overloading and distortion.
[Like cymbals!! They barely register but can easily overload, so be aware!]

The short-term peaks of the programme material can reach up to 8 -14 dBE higher than the RMS value indicated. To be safe, desk electronics are routinely designed so that distortion does not occur until at least '14 dBE above "0" VU'.

As short-term peaks in the programme material do not really effect the average value of the overall sound level, the RMS value closely resembles the average sound levels as perceived by our ears. A VU meter is particularly useful when setting the gain structure at soundcheck, as again like our ears, desks and other equipment tend to recognise average values, and within reason, tend to ignore the peaks.

To compensate for the VUs slow response characteristics, it is common to see an LED that will light up when the signal is approaching the maximum the desk - or gear - will accept without distortion, usually 6 dBE below clipping'. The Overload LED should just flash briefly at the loudest or Peak signals from a source.

Occasionally, you might notice the VU meters on different equipment reacting differently to an identical audio signal - particularly when professional and budget units are used side by side. This is because, though VU meters are supposed to be sensitive to both the positive and negative half-cycles of audio signals, many budget units are sensitive only to one half of the waveform. This can lead to considerable differences between VU readings, as many audio signals are asymmetrical.

Well that's about it for this week, next week we will be looking at PPM meters.

Until then…………Have a good one !!

Willy T.

Email - LIVE_AUDIO_WORKSHOP@Hotmail.com

This is extracted from the book:
"LIVE-AUDIO WORKSHOP: A Hands-on Guide to Rigging and Operating PA Systems." ISBN 0 646 36408 1.
© Giraffe Communications 1998.
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