What is a decibel?

Having trouble with the word decibel? Not surprising as the exact definition is more an adventure in advanced mathematics than a number on a dial (or fader). Let's start with a simple definition.

Decibel: A unit of measurement in the volume of sounds.

And... that's probably pretty much good enough for your average musician/person. We'll take this a little further. The word decibel itself is written dB. Notice the capital 'B'. That is because the Bel part is coined after Alexander Graham Bell. A Bel is defined as the logarithm of an electrical, acoustic or other power ratio. A ratio cannot be an individual number. In this case it's a comparison of two values. The 'deci' part implies a tenth. The word decibel therefore roughly translates as 'a tenth of a Bel'. The word itself was created by telephone engineers of the early 1900's as a replacement for the term "transmission unit."

Jumping back to another dictionary definition of decibel we get: a unit used to compare two voltages or currents, equal to 20 times the common logarithm of the ratio of the voltages or currents measured across equal resistances

Great. We've gone back to physics-land when all we really want to know is how far do I turn the dial. Well, any musician knows the answer to that. Turn the dial to 10. Just kidding. Zero dB (0dB) is considered the lowest intensity audible by the human ear, ordinary conversation registers about 60-70 dB, a lawnmower at about 100dB and our friend the amplified guitar comes in at about 120dB. After all that we can come back to a simple, practical definition.

Decibel: A unit for measuring the intensity of sound.

Please don't use the first definition when talking to your sound engineer about signal levels as for them the word 'volume' is a rather elusive concept and will get you lost in ethereal talk, wild hand gestures and possibly hand drawn diagrams.

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