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Author Topic: Help me with this one... - Speed of sound and density of medium  (Read 11477 times)

hamza

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Help me with this one... - Speed of sound and density of medium
« on: 05/06/2007 17:06:50 »
I know that the velocity of sound "v" is inversely proportional to square root of the density of a medium. Than why is it that speed of sound is greater in more denser mediums.. Should'nt it be less in more denser mediums as, more the density less the velocity?? But speed of sound is more in water and solids which are more dense than air.

Moderator: Changed the title to be more descriptive so that people can see at a glance what the question is.
« Last Edit: 05/06/2007 17:50:44 by another_someone »

DoctorBeaver

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Re: Help me with this one... - Speed of sound and density of medium
« Reply #1 on: 05/06/2007 17:23:20 »
I'm not a physicist, but I would imagine it's to do with how close the atoms are to each other. The closer they are, the easier & faster it is to transmit a force from one to another. The closer the atoms are, the denser the material. Therefore, it follows that any form of energy would travel faster through denser materials.

*waits for a qualified person to tell me I'm wrong as usual*

daveshorts

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Help me with this one... - Speed of sound and density of medium
« Reply #2 on: 05/06/2007 18:20:40 »
The speed of sound
v √ K/ρ

the square root of the bulk modulus (stiffness)/ density

More dense materials do have a lower speed of sound IF the stiffness is constant - hence why the speed of vibreations through jelly is so slow. However most dense materials that you are thinking of (eg rock or water) are also very stiff which more than makes up for the density. Diamond is the stiffest material known and hence has a speed of sound of 12km/second

DoctorBeaver

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Help me with this one... - Speed of sound and density of medium
« Reply #3 on: 05/06/2007 18:31:20 »
So was I right?

daveshorts

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Help me with this one... - Speed of sound and density of medium
« Reply #4 on: 05/06/2007 21:11:55 »
Yes increased density does slow down waves, stiffness just makes it slightly more complex.

DoctorBeaver

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Help me with this one... - Speed of sound and density of medium
« Reply #5 on: 05/06/2007 22:52:01 »

lightarrow

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Help me with this one... - Speed of sound and density of medium
« Reply #6 on: 06/06/2007 12:20:09 »
It has more to do with the strength of interatomic bonds, than with atomic distances.

DoctorBeaver

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Help me with this one... - Speed of sound and density of medium
« Reply #7 on: 06/06/2007 12:29:43 »
It has more to do with the strength of interatomic bonds, than with atomic distances.

Does that mean it's to do with the passing of particles rather than molecules or atoms banging against each other?

lightarrow

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Help me with this one... - Speed of sound and density of medium
« Reply #8 on: 06/06/2007 21:29:11 »
Does that mean it's to do with the passing of particles rather than molecules or atoms banging against each other?

I haven't understood what you mean.

However:
Make this experiment. Connect two iron balls 100 grams each with a very weak spring ~ 30cm long, then move one of the balls for 1 cm and stop it, and measure after how much time the other ball will move = how much time it takes to make exactly 1 cm.

Let's say it takes 2 seconds. Now repeat the experiment with a stiffer spring, made of the same material as the first one: you'll notice the time is reduced, for example it's became 0.5 seconds.

So the speed at which the force propagates from a ball to the other increases with the spring's stiffness, that is, with the strength of the "bond" between the balls.

This is because F = ma: greater force --> greater acceleration --> greater speed in the same interval of time = the same displacement in less time.

(Notice that in this case the speed of sound inside the spring's metal is not relevant since it is so fast with respect the ball's movement that the propagation of force inside the metal could be considered instantaneous).

If then you repeat the experiment varying the ball's mass, you'll notice that the speed of propagation is reduced (this to explain how the material's density come in the formula daveshorts wrote)
« Last Edit: 06/06/2007 21:33:27 by lightarrow »

DoctorBeaver

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Help me with this one... - Speed of sound and density of medium
« Reply #9 on: 07/06/2007 11:35:40 »
Does that mean it's to do with the passing of particles rather than molecules or atoms banging against each other?

I haven't understood what you mean.

Neither have I

I think what I meant was that interatomic bonds are caused by a force & forces work by the passing of particles. I was thinking originally thinking that maybe the atoms were banging against each other but after I'd posted that reply it occurred to me that the atoms' electron shells would probably repel one another; and that would be caused by the electromagnetic force which, of course, is caused by the passing of particles.

So just forget I said it. Or better still, blame an imposter who just happened to get my password so he could post a really dumb reply here

lightarrow

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Help me with this one... - Speed of sound and density of medium
« Reply #10 on: 07/06/2007 19:38:53 »
Does that mean it's to do with the passing of particles rather than molecules or atoms banging against each other?

I haven't understood what you mean.
Neither have I
I think what I meant was that interatomic bonds are caused by a force & forces work by the passing of particles. I was thinking originally thinking that maybe the atoms were banging against each other but after I'd posted that reply it occurred to me that the atoms' electron shells would probably repel one another; and that would be caused by the electromagnetic force which, of course, is caused by the passing of particles.
So just forget I said it. Or better still, blame an imposter who just happened to get my password so he could post a really dumb reply here
Ok, I've understood what you mean. I don't know QFT (quantum field theory) but it should relate the strength of a force between two particles, e.g. 2 electrons, to the number of virtual particles exchanged between the electrons, or something like that.
So, the answer should be "yes" (I presume).

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Help me with this one... - Speed of sound and density of medium
« Reply #10 on: 07/06/2007 19:38:53 »