1. Differences between air and water

From an acoustic point of view, the fact that the equations are the same should not mask the major differences between water and air. Water has a density around a thousand times greater than air, and a compressibility twenty times lower. In fact, the fluctuation in acoustic density ρ is linked to acoustic pressure p by the relationship :

where c is the celerity of sound.

In air, the reference pressure for the decibel sound level scale is 20 µPa, corresponding to the threshold of hearing. Since the celerity of sound in air is 330 m/s, the density fluctuation corresponding to the reference pressure is 1.8.10 ^-10...