Why does sound travel at a different speed through different gasses?

[Ceasium]

Hey,

 

I am currently studying for my physics exam, but this has nothing to do with my homework. Whilst learning I was wondering why sound travels at different speeds through different gasses.

 

At 273 K all the gasses have + - the same amount of molecules per m³. So in my 'logic' thinking the molecules would pass on the waves at the same speed, because of the same molecule density. But apparently this is not the case.

 

T= 273K

 

Speed of sound through:

 

Helium: 0.965 * 10³ m*s^-1

CO₂: 0.259 * 10³ m*s^-1

Air : 0.332 * 10³ m*s^-1

 

I could not get a nice answer with google, so could someone properly explain to me why sound does this?

 

Thanks

[swansont]

Do you notice a trend, given that air is mostly N2 and O2? (mass between 28 and 32 amu)

[alpha2cen]

Hey,

 

I am currently studying for my physics exam, but this has nothing to do with my homework. Whilst learning I was wondering why sound travels at different speeds through different gasses.

 

At 273 K all the gasses have + - the same amount of molecules per m³. So in my 'logic' thinking the molecules would pass on the waves at the same speed, because of the same molecule density. But apparently this is not the case.

 

T= 273K

 

Speed of sound through:

 

Helium: 0.965 * 10³ m*s^-1

CO₂: 0.259 * 10³ m*s^-1

Air : 0.332 * 10³ m*s^-1

 

I could not get a nice answer with google, so could someone properly explain to me why sound does this?

 

Thanks

Is it a difference of the molecular kinetic energy when a gas density is varied?

[Ceasium]

The speed seems to decrease exponentially when the gas becomes heavier per m³. So I can conclude that this has to do with inertia -The heavier the gas the more 'slowly' the molecules bump into each other, and the sound travels more slowly. Is this the right conclusion?

 

Can I state as well that the E_kin of sound is the same in every gass?

 

E_kin= 1/2 * m * v² .

 

Edit : - Thanks Alpha & Swansont

Edited January 14, 2013 by Ceasium

[alpha2cen]

When the gas density varies, the kinetic energy is changed differently.

At the high temperature the kinetic energy is largely changed with the gas density variation.

Low molecular interacting gas or low molecular weight gas also have the large kinetic energy change with the gas density veriation.

[overtone]

Momentum is conserved (basic principle) - a sound wave is a transferal of momentum, and in a gas (to reasonable approximation) there are no inter-molecular bonds to carry any of it - the only transfer is massXvelocity at successive impacts. So lighter gas molecules have to move faster or collide more often to transfer a given amount of momentum.

[alpha2cen]

This is the concept diagram.

Edited January 15, 2013 by alpha2cen

[Ceasium]

Thanks for all the info, I now understand how the energy transfer works with gas molecules .

 

How does sound transfer work on solids? They have no way in which they can move around like in a gas or liquid.

 

Also does it work in the same way as in a liquid?

[alpha2cen]

Also does it work in the same way as in a liquid?

This is a model of the speed of sound moving through the liquid. Liquid is more complex than gas. Therefore, in the molecular explanation, we first think about the liquid structure.

[alpha2cen]

This is more exact Figure about the speed of sound through a liquid.

 

[Ceasium]

Thanks for the diagrams alpha, they were very helpful for me

Now math made me think about sound. I've learned about Mollweide's formula's a couple of days ago. Imagine yourself having a setup like this: Two Identical speakers, distance x apart, playing the same tone in HZ*s^-1 so that there are some lines in the room where the sound is amplified, and some lines where the strenght of the sound is decreased. If you know the ladba, temperature frequency etc. could you calculate how much db the sound is increased or decreased? And if so could anyone show me an example of how this is done, so that I can do it for myself in the future?