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So you want to understand decibels?

Although sound engineers talk in terms of decibels all the time, few actually understand them. But that doesn't matter as long as you use them correctly. Many non-sound engineers don't...

The whole of sound engineering runs on decibels. It's almost every other word in a sound engineering conversation, so if you don't understand decibels, how can you cope?

One answer is that most sound engineers don't actually understand decibels, they just use them.

Decibels are used to compare two sound or signal levels. There are three possibilities...

  • The level you are interested in compared to a standard reference level
  • One level compared to another level
  • The level now, compared to the level that you have just changed it from, or intend to change it to.

There is no such thing as a sound level of 100 dB. That's because there is nothing to compare to. For real sound traveling in air, there is a standard reference level known as 0 dB SPL (sound pressure level). This corresponds to a precisely measured sound pressure, and subjectively to the sound of a falling autumn leaf, more or less.

So to say that a certain sound is 100 dB SPL is to say that it is 100 dB louder than the standard reference level of 0 dB SPL. Without including 'SPL' the whole thing is meaningless.

The same applies to electrical signals traveling through wires, except that the reference level is 0 dBu, which equals 0.775 volts - a good healthy voltage for a mixing console to get to work on. So a signal that is -40 dBu is 40 dB lower in level than 0 dBu.

But to use decibels for comparison purposes you could say, for instance, that you wanted the vocal 6 dB higher in level. This is a small but signficant change in perceived level. 3 dB is hardly noticeable to non-sound engineers. 1 dB is about the least that would be worth bothering with for a sound engineer. 20 dB is a big change.

It is important to get used to talking in terms of decibels. If you know what a change in level of a certain number of decibels sounds like, then you are talking sound engineering. You don't actually have to understand it (although it would be nice!).

By David Mellor Sunday April 3, 2005