The intensity of a pure sine wave is defined as the square of its amplitude. It is normalised in such a way that a pure sine wave with the maximal amplitude has an intensity of exactly 1. Decibels (abbreviated as dB) are a logarithmic unit that better reflects the fact that our perception of sound intensity is not linear. A difference of 10dB corresponds to a ratio of 10 in intensity. It is normalised in such a way that a sine wave with maximal intensity 1 corresponds to 0dB and a sine wave with intensity 0:1 corresponds to –10dB. This is why intensity values displayed in dB will always be negative.
For physical
rather than digital
sounds, the
reference is usually chosen such that 0dB corresponds roughly to the
threshold of human hearing, so you will often see sound intensity
measurements with positive dB values. Since a given physical
sound can end up at any intensity in the computer, depending on the
setting of both the recording and the reproduction equipment, there is
no convenient way of converting between the two. However,
differences in dB values have the same meaning in both cases.
