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I have been thinking about underwater sounds (i.e. sounds produced and recorded underwater), what exactly the water does, and how to emulate this effect. Imagine a sound played underwater (e.g. from speakers or from a dolphin) and being recorded underwater by a hydrophone. Imagine that it is in a very large basin (e.g. the sea) which doesn't produce any sound of it's own (this would not be very realistic for the sea I admit). What I would like to know is the frequency response of this medium.

Intuitively, I would say that water has a low-pass filtering effect: as the viscosity of water is much higher than in air, I think that high frequencies are attenuated more than low frequencies. But that doesn't feel quite right: when I think about when you're underwater, it feels like high frequencies are very pronounced, and sounds have a bit of a 'ring' to them.

In other words, I can't quite figure it out on a theoretical basis. I have tried to find underwater impulse responses and proper underwater recordings, but I can't find a satisfying recording. Let alone a frequency response for water. Can someone help me in this?

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Several things to consider: The speed of sound is much higher in water than in air. Just how much depends on a lot of factors. On http://www.waterencyclopedia.com/Re-St/Sound-Transmission-in-the-Ocean.html it says:

The state properties of water (temperature and pressure) and the degree of salinity also affect the speed of sound. The propagation of sound waves in sea water can be directly affected by suspensions of particulate matter that can scatter, absorb, or reflect the waves. Laboratory experiments demonstrate that distilled water—water from which salts and other suspended particles have been removed—provides a medium in which the speed of sound exceeds the speed of sound in ocean water. The difference in the speed of transmission is significant—speed in distilled water may be 20 to 30 times that of speeds found in ocean water.

So the type, and quality of water can make a big difference - note particularly the bit on 'scatter, absorb, or reflect', and thosw are likely frequency dependent to boot.

Next, while you can buy underwater speakers, you generally need to massage the waveform so you hear it 'properly' underwater (i.e. as if you were in air). My kids' swim team has such a rig, which is much better than trying to listen to Iron Butterfly through an uncorrected audio system in college...

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