Compression of water (split from Waveguides)

[dlr]

Originally posted by MJL21193
Here goes:
Velocity of sound in a liquid is governed by density.

Governed, no. Partially influenced, yes. Ignoring all relevant factors does not change the facts. Your argument is wrong on the basic physics of the (off-)topic.

Dave

[MJL21193]

Originally posted by Tom Danley


Here is a thought experiment for you, pretend you are in space, zero G, at 1atm air pressure.

Hi Tom,
First off, there isn't any air in space, but you knew that.

Second, we are not talking about air, we are talking about water. The cool thing about water is that anything that you add to it, that will stay in suspension, will actually decrease density.

The springs analogy is fine, if you understand that the springs go both ways, as one shortens the opposite side lengthens. Molecules are moving and they move faster when excited by the force from a sound wave. Molecular movement denotes a density increase, not the other way around. As molecule move more rapidly, they become further apart, temperature goes up.
Energy into a system will produce heat, not refrigeration.

[MJL21193]

Originally posted by dlr


Governed, no. Partially influenced, yes. Ignoring all relevant factors does not change the facts. Your argument is wrong on the basic physics of the (off-)topic.

Dave

Ok, please explain the other factors that influence sound transmission through a medium, that are not directly related to density.
I'll start holding my breath...now.

[pooge]

Time to use the "ignore" function again. This reminds me of the cartoons where you can't shut off the alarm clock even after hitting it with a hammer.

[dlr]

Originally posted by pooge
Time to use the "ignore" function again. This reminds me of the cartoons where you can't shut off the alarm clock even after hitting it with a hammer.

It is pointless, isn't it? I'll stop contributing to the noise, as that's all it is now.

Dave

[gedlee]

The speed of sound in a medium depends on two things, the density and the stiffness or compressibility. The speed is proportional to the ratio of the stiffness to the density. In water the speed is hundreds of times faster, but the density is also an order of magnitude greater. Thus the stiffness has to be on the order of a thousand times greater for water than air. Very nearly "incompressible", especially compared to air, but compressible non the less.

And pressure is compression. It is the compression of the molecules in an medium that creates the pressure so these two terms are nearly completely equivalent. Can't have one without the other. A pressure wave IS a compression wave.

[SY]

I split all this off into its own thread.

(moderator hat off)

If I can get someone to help me with a drawing or two (I'm a bit tied up today), it might be helpful for pedagogic purposes. I think that John's analogies are not illuminating.

Imagine a fluid that has an average intermolecular distance and particle density normalized to unity at a given temperature and external constant pressure (e.g., the atmosphere). For simplicity, let's imagine it to be very pure, and ignore ionic effects (on the order of 10 e-14).

Now consider a single frequency sound wave in the fluid. There will be a change of average intermolecular distance along the direction of the wave, with the a.i.d. being greater than unity in places where the sine wave has its peaks, and less than unity where the sine wave has its troughs. The average over the fluid is still unity- this is very important to note, the domino thing is inapt, the First Law of Thermodynamics still applies here.

OK, now take a tiny little cube of material from the peaks. Count the molecules. The particle density will be greater than unity. Take an identically sized cube from the troughs. The density will be less than unity. And if for the same volume the particle density differs from unity, the mass differs from unity.

Now most people would say that if two identical volumes from the same fluid show different densities, there's been a compression in one. But if you don't like that definition, here's an alternative one: for a given number of molecules, the resulting volume at the peaks will be smaller than the volume of the same number of molecules at the troughs.

The water suffers no overall compression or expansion, but locally, it indeed expands and contracts by either definition. Don't confuse AC with DC.

[thoriated]

Compressibility has absolutely nothing to do with sound. Zero.

From Wikipedia:

Sound is transmitted through gases, plasma, and liquids as longitudinal waves, also called compression waves.

and from the 'Physics Classroom Tutorial':

If the medium is not rigid as is the case with fluids, the particles will slide past each other. This sliding action which is characteristic of liquids and gases prevents one particle from displacing its neighbor in a direction perpendicular to the energy transport. It is for this reason that only longitudinal waves are observed moving through the bulk of liquids such as our oceans.

So, you're right, and the whole world is wrong? Time to discard your grade school thought experiments by now, I'd think. If you don't forthwith, I'll believe you understand by now that your assertion that sound has nothing to do with material compessibility is totally baseless and wrong but are unwilling to admit it just yet.

[AndrewT]

MJL,
we've talked at length in the past.
We usually agree.
Are you willing to accept my word when I tell you that water is compressible?
And that sound passing through water is a variation in pressure leading to alternate compression and expansion of the medium.

[Eva]

MJL21193 you are plain wrong, but I have to thank you for your obstination and the rest for their patience because this discussion has led me to read good explanations and formulas about sound propagation and I have finally understood a couple of interesting things Including why the air itself produces substantial THD at high SPL and long distances (SPL modulates sound speed!!!)

Yes, in elementary school they teach us that water is not compressible, but that is an over-simplification for kids.