In May 2014, the VU meter celebrated its 75th birthday. It has served the industry well, and when properly interpreted, it’s still useful.
However, today’s digital recording processes have caused us to take a hard look at the usefulness and inadequacy of both the traditional VU meter and its modern replacement, the LED level meter, as tools for signal-level management.
The classic VU meter, though relatively rare today (primarily because of cost), has long been the most common audio-level indicator. VU stands for Volume Unit, and the VU meter indicates how loud something sounds.
The traditional VU meter is mechanical, analog, and has a standardized (even the color scheme is standard), logarithmic (decibel) scale that runs from –20 to +3, with usable resolution over about a 15 dB range. The zero mark is about two-thirds of the way up the scale. This 0 VU mark is what we mean when we say the meter “reads zero,” not where the pointer rests when the equipment is turned off.
Resolution is very good near the high end of the scale: generally 1 division per dB between 0 and +3. The scale resolution becomes progressively lower (more dBs per scale division) below zero VU.
These days, the mechanical meter’s replacement is usually a column of LEDs, with individual lights serving as the meter scale ticks. On a computer-based digital audio workstation, a meter is often simply drawn on the monitor screen. The Cool Look Factor is lower, but it’s a lot cheaper than using mechanical meters. However, some analog audio devices—notably “boutique-style” preamps and channel strips such as the PreSonus ADL 600, ADL 700, and RC 500—still sport traditional VU meters.
In this article, we’ll review the characteristics of the traditional analog VU meter and also address audio-level measurement and management in the digital domain, the concept of headroom, and how loudness and audio level are related—as well as how they’re not related.
What’s a VU?
The Volume Unit meter was originally designed to help broadcast engineers keep the overall program level consistent between speech and music. There’s a well-defined standard for the mechanical rise-and-fall response characteristics of the pointer (to which few of today’s VU-like meters actually comply).
A standard VU meter responds a little too fast to accurately represent musical loudness but fast enough to show movement between spoken syllables. Unscientific as it may seem, the dynamic response of the VU meter was tailored so that the pointer motion looks good when indicating speech level. It was easy to tell at a glance whether speech or music was going out over the air, whether it was about at the right level, and when something wasn’t working. Engineers learned that brief excursions up to the +3 dB top end of the scale rarely caused distortion in the analog equipment of the day, nor did they sound too loud.
How is Audio Level Measured?
Once sound is converted to electricity, we can represent the audio level by measuring the alternating electrical voltage. A symmetrical signal, such as a sine wave—a single-pitched note with no overtones or distortion—spends equal time on the positive and negative sides of 0 volts.
The numerical average of the positive and negative voltages over many cycles is zero—not a very useful measurement when we want to know how loud a sound that voltage represents.