The Humble Decibel

I’ve been thinking about decibels a lot of late.
A decibel is, by definition, a 10th of a bel (a little used unit of measurement named in honour of Alexander Graham Bell), and the problem with these dB’s is that EVERYONE wants a piece of them!

You see, we in the audio industry use them (generally) to express Sound Pressure Level (SPL), whereas in the electronics industry it’s used to express power ratios. In the world of optics the decibel is all about optical power and loss, but if you’re into video then you’ll know the decibel represents both video voltages and digitised light levels.

If all of this wasn’t enough to confuse even the most robust of technical minds, within the audio field there are several different flavours of dB including dBV, dBu, dB SIL, dB SWL, dBm, dBFS and dBTP.

But, I hear you asking, the loudness of a sound should be easy to measure, right? So why do we have so many different ways of expressing loudness?
Well, in answer to the first question, yes and no…
Yes, sound pressure (SPL) can easily be measured. It’s the deviation between normal or ambient air pressure when a sound is present. This, unfortunately, is only half the story though.

Any of us who have ever heard a compressor or a limiter in action will testify to the fact that a sound can be made to appear louder (without actually being louder) by virtue of the thickening effect these units lend to the source. And then the complexity of the source signal can also affect our perception of loudness.

For these and other reasons, many different scales and standards have been developed over the years.

A cursory glance around my own studio reveals thirteen VU meters on various pieces of equipment. The VU or Volume Unit scale runs, in decibels, from -20dB to +3dB and, if you’re using any kind of outboard equipment such as mic Pre-amps etc., it’s quite likely you’ll have one or more of these.
The thing about VU meters is that they do not display sound pressure levels but rather a smoothed out display of the signal which reflects ‘perceived levels’. Here’s a couple of my own VU meters.

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The VU meter was developed in 1939 jointly by Bell Laboratories (dear old Alexander again), NBC and CBS. Originally intended to measure and standardise the varying levels of signal in telephone lines, it was quickly adopted by the recording industry.

There is, however, an inherent problem metering using VU’s, especially so within the modern digital studio. The problem with VU’s is that they just don’t move quickly enough to show transient bursts of high energy signals. And in the digital studio if you exceed the theoretical maximum headroom you won’t get warm luscious gentle (and I must say, rather attractive) distortion as in the analogue domain. No, you get gut-wrenching, ears-are-being-torn-off, screaming heebeegeebee distortion – yuck!

Even before the digital age it was decided VU’s simply were not really all that useful in audio level-sensitive applications like radio broadcasting. The engineers needed to be sure if the sound was over modulating and so the PPM was developed.

Peak Programme Meters act much faster than VU meters. And boy, those needles can really travel! But, because looking at really fast moving needles can be very tiring, they are attenuated to slow down when moving left – when the signal is not so strong. This means they only really show the peaks and do not really represent the overall rise and fall of the material. To further complicate matters, they use a completely different dB scale than VU meters – doh! And worse, there are geographical differences between scales too. I do some work for the BBC, so I have a pair of PPM meters as the Beeb’s level guidelines are in “BBC PPM”. Here are my BBC meters.

20130912-174431.jpgOh, I also have a Dorrough Meter. Now this beast allows you to see both the peaks and the “Persistence Range” (volume units) at the same time! You get dancing lights around the peaks allowing you to check for ‘overs’ plus you get a standard bargraph meter with VU habits. The combination of the two a allows you to see at a glance just how much dynamic energy is in a recording – the closer the two signals, peak and VU are, the more compressed the signal.

20130912-174437.jpgAlongside all of this I still need to mention dBFS. If you use a DAW you’ll be familiar with this scale. It’s the scale generally used within DAW’s and, apart from 0dBFS has only negative values. Anything negative is under the maximum, anything positive simply clips.

Thing is, although all of these methods of metering use the decibel, they all use different scales and, more confusingly, they sometimes use differing scales within the same metering protocol. A quick look at the table below will show you a small sample of the varied ways of measuring the sound that we hear. Also notice that even scales of similar types have different alignment or test points.

20130912-202154.jpgWe all seem to be agreed that we love the decibel, but isn’t it high time we agreed on a single way that we can measure it?

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