Decibel mathematics may seem strange as numbers don't behave in the ways you expect. That's because the numbers are based on logarithms.

 

This can be confusing as it leads to a lot of strange maths - for instance, 1 + 1 = 4! It also has an interesting property of changing multiplication into addition. For example, when you multiply by 2, you add 3 dB, when you multiply by 4, you add 6 dB. 

 

This also means that each increase of 10 dB represents a tenfold increase. E.g. 70 dB has 10 times more sound energy than 60 dB, and 100 times more sound energy than 50 dB.

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There are a couple of other quirks: zero dB isn't actually zero and negative dBs represent positive (but very small) values. This is because dB represents a ratio compered to a reference value and itself is not an actual value. 

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Below are some simple calculators showing how it works.