Glossary definition of 'Decibel'(dB) A way of expressing one number relative to another as a ratio. A Decibel is one tenth of a Bel, although the Bel is almost never seen except in texts such as this. A Bel is a factor of 10 of power so 2 Bels is two factors of 10 or 100 × the power. This turns out to be a rather large unit in practice, so the decibel is used instead. Since the decibel is a power ratio, when working with power values, it is calculated as 10Log10(power1/power2). When working with voltages, since power is proportional to voltage squared, the formula becomes 10Log10(V12/V22) which can be written 20Log10(V1/V2).
It can be seen from this that a decibel is a dimensionless unit - it is just a means of expressing the ratio of two values in the same units. It is particularly useful for representing very large differences, for example, a power ratio of 1 million to 1 becomes 60dB.
For making absolute measurements, the dB can be used with a specified reference. In this way dBm is decibels relative to a level of 1milliwatt. Sound pressure level (dBSPL) is measured relative to a level of 20µPa (twenty micro Pascals) and is calculated as 20Log10(P/P0) where P0 = 20µPa