Given its appearance in one form or another in all but the cheapest audio gear produced in the last 70 years or so, you’d be forgiven for thinking that the ubiquitous VU meter is just one of those electronic add-ons that’s more a result of marketing than engineering. After all, the seemingly arbitrary scale and the vague “volume units” label makes it seem like something a manufacturer would slap on a device just to make it look good. And while that no doubt happens, it turns out that the concept of a VU meter and its execution has some serious engineering behind that belies the really simple question it seeks to answer: How loud is this audio signal?
Miles of Cable
Unsurprisingly, the modern VU meter can trace its roots back to the twin formative technologies of the 20th century: telephone and radio. For the first time in history, the human voice was projecting further than the distance the loudest person could shout, and doing so by means of electrical signals. Finding a way to quantify that signal and turn it into a value that represented the perceived volume of the original sound was crucial to design a system that could faithfully transmit it.
Given the nature of their network, the early telephone pioneers’ efforts at sound level metering were based on line losses over a “standard mile” of cable. Meters calibrated to this standard made it easy for them to adjust their vacuum tube repeaters to compensate for the speech power loss over a known length of wire.
As radio became commercialized and more widespread, the correlation between sound levels and loop length began to make less sense. In the 1920s, radio and telephone engineers began to converge on a better solution. The transmission unit (TU) was used to measure the power ratio between two different sound sources. It’s a logarithmic measure, and as such better reflects how the human ear perceives sounds. The TU measurement also had the advantage of being usable at any frequency since it doesn’t factor in the inductance and capacitance of a miles-long loop of wire.
Many TU meters were marketed over the 1920s and 1930s to sound engineers, whose ranks swelled when the film industry introduced “talkies.” There was no real standardization, however, and it was becoming increasingly hard to compare sound levels between industries, and often between different pieces of sound equipment. In the late 1930s broadcasters, motion picture companies, and the telephone industry all got together to hammer out a standard that could be used for all audio signal measurement needs. They dubbed the new measurement “Volume Units,” and the VU meter was born.
Follow the Bouncing Needle
Given the technology of the time, the definition of what exactly constitutes a volume unit was based on the characteristics of the moving coil meter. The standard device was a microammeter with a 200 μA full-scale deflection. Because the incoming signal is actually an alternating current, the meter was specified to have an internal full-wave bridge rectifier. Zero on the meter scale was defined as where the needle points when a 1-mW pure sine wave audio signal at 1000 Hz is placed across a 600 Ω load. This point ends up about two-thirds of the way across the scale.
The scale to the left of the zero point is generally in black, with a value of -20 dB all the way at the bottom of the scale. Past the 0 dB point, the scale is usually shown as a solid red region that ranges up to 3 dB at the top. Some VU meters include a second scale in arbitrary units where the 0-dB point is shown as 100%.
The original spec for the VU meter recognized the fact that it takes a finite amount of time for the needle to deflect from rest to 0 dB. It limits the time to get to within 99% of 0 dB to 300 milliseconds and allows for a little overshoot — about 1%. The lag that an electromechanical meter introduces might seem like a bug, but it’s really a feature because it imitates the way the human ear perceives sound. Think about that 1000 Hz tone that’s used as a standard for the VU meter spec. If you have an audio oscillator set to 1 kHz and turn it on really loud, it won’t be too long before everyone within earshot tells you to turn it off. But if you take that same tone through the same amp at the same volume, but only play it for half a second, it won’t seem nearly as loud. The VU meter perfectly reflects that — the continuous tone will bring the needle to 0 dB quickly and leave it there, but a single pip of the same tone will barely move the needle.
Of course in the 80 years or so that VU meters have been around, the moving coil meter upon which it’s based has become somewhat passe. Alternate meters have come into use, from fully electronic LED bar graph displays to meters that just exist as software. These meters still need to mimic the lag of a physical meter to be useful as a VU meter, and perform so-so either through software or via circuitry that mimics the ballistic nature of a mechanical meter.
The VU meter is not the only game in town when it comes to measuring audio levels, of course. The peak-program meter (PPM) is common internationally and as the name suggests, measure the audio peaks rather than the average signal like a VU meter. Pro audio engineers and audiophiles debate the relative merits of the two metering methods with an intensity only pro audio engineers and audiophiles can understand, but it’s clear that both meters have their place.