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.