As such, dB levels are always reported as “dB re 1 μPa” for a sound in water or “dB re 20 μPa” for a sound in the air. In water, a different reference is used: 1 μPa. For instance, in air, measurements are referenced to 20 micropascals (μPa). Decibel levels indicate how loud a sound is relative to some reference (represented in units of amplitude called micropascals, or μPa). The loudness of a sound is measured in decibels (dB). Substituting the value of a micropascal for the rms pressure in the plane wave intensity expression, we find that a plane wave pressure of 1 μ Pa corresponds to an intensity of 0.67 × 10 −22 W/cm 2 (i.e., 0 dB re 1 μ Pa). In seawater, ρ c is 1.5 × 10 5 g cm −2 s −1 so that a plane wave of rms pressure 1 dyne/cm 2 has an intensity of 0.67 × 10 −12 W/cm 2. The average intensity, I, of a plane wave with rms pressure p in a medium of density ρ and sound speed c is I = p 2/ ρ c. Pressure ( p) ratios are expressed in dB re 1 μ Pa by taking 20 log p 1/ p 2 where it is understood that the reference originates from the intensity of a plane wave of pressure equal to 1 μ Pa. Therefore, taking 1 μ Pa as I 2, a sound wave having an intensity, of, say, one million times that of a plane wave of rms pressure 1 μ Pa has a level of 10 log(10 6/1) ≡ 60 dB re 1 μ Pa. The presently accepted reference intensity is based on a reference pressure of one micropascal (μ Pa): the intensity of a plane wave having an rms pressurex equal to 10 −5 dynes per square centimeter. Absolute intensities can therefore be expressed by using a reference intensity.
Two intensities, I 1 and I 2, have a ratio, I 1/ I 2, in decibels of 10 log I 1/ I 2 dB.
The decibel (dB) is the dominant unit in underwater acoustics and denotes a ratio of intensities (not pressures) expressed in terms of a logarithmic (base 10) scale. Kuperman, in Encyclopedia of Physical Science and Technology (Third Edition), 2003 VIII Appendix: Units
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