In water, sound waves are typically measured by their pressure using a device called a hydrophone. When
discussing ambient noise, the implicit assumption is that sound pressure fluctuations are being described,
although it is not clear whether a particular marine organism is able to sense particle velocity or
acceleration. Sound pressure is measured in Pascals (Pa) in the international system of units (SI), although
it is expressed in bars by the geophysical community (1 Pa = 10-5 bar). Since mammalian hearing and
sound production cover a large range of pressure values, sound pressure level (SPL) is usually measured on
a logarithmic scale called the decibel (dB), and compared against a 1 µPa reference (Po) for underwater
sound as follows: