Meter Madness: What Your Level Meters Tell You And What They Don’t

The Classic VU Meter
Today, a VU (or pseudo-VU) meter is most often used for little more than to indicate an impending overload. We don’t watch a recorder’s VU meter to tell how loud our recording is, but rather, to ensure that we don’t exceed the available headroom—the 10 dB or so between 0 VU and the point where THD reaches 1%.

However, the VU meter is only a good headroom indicator if you’re working with program material that’s fairly consistent in level and doesn’t require a generous allowance for surprise peaks.

Take a close look at the VU meter scale. While the meter scale has a total range of 23 dB, fully half (the top half) of the scale represents only 5 dB.

This is good resolution for measuring steady tones when calibrating the recorder or setting levels within a system but pretty wasteful when working with a recorder that’s capable of handling a dynamic range between 65 dB (analog tape) and better than 90 dB (garden-variety digital [QQQ 16/44.1?]).

There’s no usable resolution below -10 VU. If we assume that we have at least 10 dB of headroom above 0 VU, a VU meter is really only informative within about a 13 dB range.

The classic VU meter scale

Why such a compressed scale? Practicality. Perceived loudness is a logarithmic function. It takes more than twice the signal voltage for something to sound twice as loud. A linear scale that represents loudness just wouldn’t look right. Remember that one of the design criteria for the VU meter was that the pointer response looked good for speech.

It’s no surprise that modern, highly compressed music will shoot the meter pointer well up scale, and it’ll stay right there until the fadeout. On uncompressed material with a wide dynamic range, there’s plenty of audible material down below the -20 mark on the VU meter, but an inexperienced engineer who trusts the meter rather than his ears can be misled into thinking that anything that barely moves the meter is too soft.

Today, many meters, both mechanical and LED or LCD ladder-style, have scales that look like a VU meter but don’t meet the VU standards. These are useful for establishing steady-state calibration levels when setting up a system, but they don’t accurately represent loudness or headroom.

LED ladder meters are often found on digital equipment but until you dig into the inner workings, you usually don’t know whether the meter indicates an analog voltage or the amplitude of a digital sample. In either case, the garden-variety meter doesn’t provide the same dynamic response of a real VU meter. It can show you average and sometimes peak level but it won’t tell you much about apparent loudness.

Zero VU is an arbitrary voltage level. It’s whatever is “normal” at the point in the circuit where it’s measuring. A meter that indicates input or output level is generally calibrated according to one of several industry “sort-of” standards. The most common is that 0 VU represents a voltage level of +4 dBu, about 1.23 volts RMS.

Most modern mixers are calibrated to this convention: When the output meter reads 0 VU, the device is putting out +4 dBu RMS. But this isn’t always the case. For many years, Mackie believed this was too confusing so they calibrated their meters so that 0 VU = 0 dBu. The “semi-pro” recording gear popular throughout the 1980s was generally calibrated for 0 VU = –10 dBV (10 dB below 1 volt, about 0.32 volts). Professional recorders are usually calibrated so that their meters read 0 for an input level of +4 dBu. In the broadcast world, “line level” is often +8 dBu, so that’s where their VU meters are calibrated.

Are you beginning to see what the “madness” is in the title of this article? Wait! There’s more!