Amplitude is the measurement of the displaced air (pressure), and in terms of audio this is perceived as loudness. Amplitude is actually the energy of sound or intensity/power.
The level or intensity of an audio signal (i.e. the loudness) is measured in decibels (dB) using the dB scale. ‘Deci’ being a tenth and ‘Bel’ being the unit.
You may be wondering why we measure in tenths and not in single units. Our ears can hear a vast number of audio levels and it would be a mathematical nightmare to try to use the ‘actual’ numerical representations of audio levels - so we use tenths to make it easy to understand and calculate.
The true definition of dB is far more complicated and relative to ratio, logarithm of the ratio between two power levels etc, but all I want you to understand is that we measure audio level in dB.
The simplest way of explaining dB in terms of both hearing and measuring is to use the following:
Total silence is expressed as 0dB on the decibel scale. So if you had a sound that was ten times more powerful then it would be expressed as 10dB. That should be easy to understand. Where it gets a little confusing is that if you had a sound that was 100 times more powerful than total silence, it would be measured as 20dB. A sound 1000 times more powerful would be 30dB and so on. We basically work on a logarithmic scale when dealing with dB.
However, I will keep it very simple and stick with dB simply being a way to measure loudness/level. If I state that at 30Hz I boosted by 3dB, then that means that the gain knob was moved up by 3dB at that frequency.
I wanted to give a slightly detailed explanation of dB and not just to state it as a measurement but to go any deeper than the above would be both confusing at this juncture and not relevant to the subject matter.
Excerpt taken from the book Art of Drum Layering.