What is the unit for measuring the amplitude of a sound?

Answer 1

That would be the 'Bel', a unit of sound amplitude named after Alexander Graham Bell, the inventor who studied sound in order to help deaf people.

Unfortunately, a Bel was a very large amount of sound - close to the loudness of a jet taking off. As a result, much later, a tenth of a Bel came into common use. The latin for one tenth of anything is, "deci", hence deci-Bel, or deciBel. It is correct to spell it with the capital 'B'. That is why the abbreviation for deciBel is "dB", not "db".

+++

Adding to that, the deciBel is not a linear unit nor necessarily of sound. It is used for example in measuring amplifier gains. it is a logarithmic ratio of the measured signals' amplitude relative to the reference level signal for the system being measured.

For airborne sound the 0dB reference level is the minimum pressure detectable by the healthy human ear, a staggeringly tiny 20µPa (micro-Pascals) in linear terms. That translates as one-fifty-thousand-millionth of standard atmospheric pressure at mean sea-level.

The ear's amplitude response is roughly logarithmic, and the maximum endurable (though painful and harmful) SPL is one million times that reference, so 20Pa, or 120dB re 20µPa to give the unit's proper full name. (log to base-10 of 1 million is 6 so 20X6 =120.)

Marine sonar uses 1µPa as the reference or 0dB pressure, i.e. 26dB below the air acoustics reference level.

The calculation for that is 20log(1/20), again, the logarithm base is 10,

Incidentally, the B is often in lower-case when written in the full word, rightly or not, but the correct abbreviation is still dB.

Answer 2

The unit for measuring the amplitude of a sound is decibels (dB). Amplitude in sound refers to the strength or intensity of the sound wave, with higher amplitudes corresponding to louder sounds.