What does a decibel measure?

Answer 1

Decibels measure a ratio of two values of the same unit.

Answer:

A decibel is measure of loudness of sound although it can be applied to other applications. The base unit is the bel and a tenth of that the decibel. In the real world a decibel of sound is about the weakest sound a person can hear - as an example the rubbing of your finger and thumb together at arms length.

Answer 2

A decibel usually measures power or intensity. Most common application are in Audio levels. A decibel(dB) is actually a ratio of the measured physical quantity to a standard reference value of the same physical quantity. It is expressed in logarithmic scale. As we have in decigram, decimeter similarly a decibel(dB) is also one tenth of a Bel. A 1 Bel (10 dB or decibel) change represents multiplying the intensity or power level by a factor of ten.

It becomes easier to express things in decibel as they are short numbers compared to the real physical measure.