# Glossary definition of 'Decibel'

_{10}(power1/power2). When working with voltages, since power is proportional to voltage squared, the formula becomes 10Log

_{10}(V1

^{2}/V2

^{2}) which can be written 20Log

_{10}(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 20Log

_{10}(P/P

_{0}) where P

_{0}= 20µPa