The quantity to be measured depends on the application.
Sound (acoustic) pressure is expressed in decibels (dB). The decibel is a dimensionless non-SI logarithmic unit that indicates the ratio of a physical quantity p relative to a specified reference level pref:
Lp = 20 log10
When expressing a quantity in decibels, it is important to state the magnitude of the reference level. In the case of microphones the reference level is 1 V/Pa; for hydrophones it is 1 V/µPa; and for the human ear it is 20 µPa.
Vibrations may be quantified differently depending on the type of instrument used to detect the vibrations. For example, the unit C/m s2 is applied when a capacitive sensitive instrument is used, but other units may also be found.