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Decibels made easy Decibels crop up in many areas of music and recording but what are they and what do they mean? Ian Waugh offers a quick guide... Of all the measurements in the realm of music and recording the decibel is probably the most confusing. You'll most commonly find it on level meters which measure the input while recording or the output when playing back. You'll find it on dynamics effects such as compressors and limiters, and it's quoted in sound card specs in the signal-to-noise ratio section. Getting louder For our purposes, the decibel (usually abbreviated to dB) is a measure of loudness. It's one tenth of a Bel which was named after Alexander Graham Bell who invented the telephone. The Bel is far too large a measure to be of practical use so the decibel is used instead. The measurement of loudness is complicated slightly because we perceive loudness in a logarithmic way. Our perception of loudness is directly governed by the amplitude or intensity of the sound wave. In order for us to register a sound as being twice as loud as another sound, its amplitude (intensity or sound pressure level) must actually be ten times greater. So to make the sine wave below appear twice as loud to us, we'd have to increase its amplitude by a factor of ten.
Okay, most folks can handle that, especially those who did logs at school. However, for a variety of historical and scientific reasons which we really don't want to go into here - trust me on this - the decibel measuring system is not an absolute one but a relative one. Relative values This means that you can't say that a sound has a loudness of 6dB, for example. What you can say is that one sound is so-many dB louder relative to another. And the way the system works, an increase of 6dB is a doubling of the volume. An increase of another 6dB would double the volume again so 12dB is a four-fold increase in volume. 18dB is an eight-fold increase, and so on. Meter made One other thing - hang on in there, we're nearly done. If you look at a recording meter calibrated in decibels you'll see that 0dB usually occurs at the top and the decibel scale decreases as you move down. For example, a meter might be calibrated like this: 0 - 6 -12 -18 -24 -30 -36 0dB is the reference level or maximum level and you do not want to exceed this level while recording or the audio will distort. As the volume increases, most meters have a yellow and then a red section to warn you that you're getting close to overload. As the volume level rises it goes from, say, -24dB to -18dB and then to -12dB, each 6dB increase doubling the perceived volume level. Signal-to-noise ratio Here's another example of the use of the decibel. If you look at the specs of a sound card you may see a signal-to-noise ratio (SNR). Let's say it's 90dB. Sometimes it may have a minus sign in front as in -90dB but it represents the same thing. The SNR is a comparison between the sound (which we want to hear) and the noise (which we don't). The sound is taken to be 0dB which is the reference level, and the spec says the noise is 90dB lower than the sound. However loud the sound may be, the noise floor is 15 (90/6) times lower. In a nutshell That's essentially the decibel. In a nutshell. If you remember that most volume-measuring equipment uses a decibel scale and that an increase of 6dB is a doubling of the loudness, you won't go far wrong. What's more, you'll know more that most music people about decibels - and that includes many recording engineers! But the bottom line is that you don't have to worry about it as long as you keep your eyes on the meters and your ears on the mix.
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