Since sound travels as longitudinal waves, sound waves should only be able to propagate in a medium through compressions and rarefactions. However, water, as a liquid, is generally treated as an incompressible fluid. Since compression is essential to sound propagation, how do phenomena such as whale calls and underwater speakers work?
[Physics] If water is incompressible, how can sound propagate underwater
acousticsfluid dynamicspressurewaves
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Best Answer
Water is compressible (nothing can be completely incompressible). Treating water as incompressible is just a (usually very good) approximation. Therefore, longitudinal waves are possible.
Wikipedia reports the bulk modulus to be about $2.2\ \mathrm{GPa}$. This puts the speed of sound in water at about $$v=\sqrt{\frac{\beta}{\rho}}=\sqrt{\frac{2.2\ \mathrm{GPa}}{1000\ \mathrm{kg/m^3}}}\approx1500\ \mathrm{m/s}$$