Re: How does temp. and pressure affect the speed of sound in water?

The speed of a sound wave through any liquid will be affected by the 
pressure and temperature, but usually not very strongly.

As with a gas, increased velocities of the molecules increases the speed 
of sound, so an increase in temperature (at constant density) would 
increase the sound speed.

But in dense fluids, another factor is important that is not as important 
for gases, and that is how crowded together the molecules are.  The higher 
the density, the faster the sound can be propagated as the molecules bump 
into each other more.  Therefore, increasing the pressure on water (at 
constant temperature) will increase the sound speed because it increases 
the density (but not very much, because water is not very compressible).

One interesting thing is what happens if you increase the temperature of 
water at constant pressure.  The first effect (increased velocities) 
causes a higher speed, but also the liquid expands when heated, so the 
density gets lower which lowers the sound speed.  So you have two 
competing effects, and you can't know without measuring it which 
effect "wins" at a given condition.  It turns out that, for water at 
atmospheric pressure, the speed of sound goes through a maximum around 75 
degrees C because of the competition of these two effects.

Scientists and engineers typically get values of various water properties, 
including sound speed, from books of "Steam Tables" and also software such 
as that described at: http://www.nist.gov/srd/nist10.htm

You can get values for sound speed and other properties for water and 
several other important fluids over the Web by going to: http://webbook.nist.gov/
entering, and going to "Thermophysical Properties of Fluid Systems"

Allan Harvey
"Don't blame the government for what I say, or vice-versa."