The intensity of sounds are measured in units?

Answer 1

Sound intensity or acoustic intensity (I) is defined as the sound power Pac per unit area A. The usual context is the noise measurement of sound intensity in the air at a listener's location as a sound energy quantity - measured in W/m².

Our eardrums and microphone diaphragms are moved by sound pressure deviations (p), which is a sound field quantity (measured in pascals Pa). So we measure the sound with a SPL meter (Sound Pressure Level) in decibels.

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There's a gap in that explanation. Sound pressure levels are normally quoted in dB but linearly they are in µPa (microPascals) because the pressure difference across a given sound wave is so tiny. 1µPa = 1 / 100 000 000 000 Bar (1 Bar is the standard atmospheric pressure at mean sea-leve)l.

The faintest whisper the healthy human ear can hear is at only 20µPa, but the ear's working pressure range is 1 million µPa (ie 1Pa). So to simplify the calculations, cut out large numbers in the sums and also to cope with the ear's logarithmic response, we use Decibels.

A decibel is not a linear unit like a volt or a metre. It is a logarithmic ratio, and is referred to a base level which here is that 20µPa. Since sound power = 20log-to-base-10(measured sound / reference pressure), that reference 20µPa is therefore at 0dB since the fraction is 20/20 = 1 and the log of 1 = 0.

We cannot leave those poor decibels hanging in fresh air so the full name of the unit is "0dB re 20µPa" - though the "re" etc is normally omitted if the context is clear.

That means the tiniest sound's pressure level our ears can detect (assuming in full health and depending on frequency as the ear's frequency response is not very linear) is a vanishingly small one-five-thousand-millionth of sea-level atmospheric pressure.

And the maximum , which is above harmful volume for our ears? 120dB re 20µPa (Log-base-10 of 1 000 000 is 6, so 6 X 20 =120. Long exposure to above about 85dB is harmful.

In marine sonar the reference level is 1µPa, so 26dB below the airborne reference based on our human hearing. In that case its marine and air-acoustics levels in dB are respectively 0dB re 1µPa and -26dB re 20µPa.