Ceasium Posted January 13, 2013 Share Posted January 13, 2013 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 Link to comment Share on other sites More sharing options...
swansont Posted January 13, 2013 Share Posted January 13, 2013 Do you notice a trend, given that air is mostly N2 and O2? (mass between 28 and 32 amu) Link to comment Share on other sites More sharing options...
alpha2cen Posted January 14, 2013 Share Posted January 14, 2013 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? 1 Link to comment Share on other sites More sharing options...
Ceasium Posted January 14, 2013 Author Share Posted January 14, 2013 (edited) 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 Ekin of sound is the same in every gass? Ekin= 1/2 * m * v2 . Edit : - Thanks Alpha & Swansont Edited January 14, 2013 by Ceasium Link to comment Share on other sites More sharing options...
alpha2cen Posted January 14, 2013 Share Posted January 14, 2013 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. Link to comment Share on other sites More sharing options...
overtone Posted January 14, 2013 Share Posted January 14, 2013 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. Link to comment Share on other sites More sharing options...
alpha2cen Posted January 15, 2013 Share Posted January 15, 2013 (edited) This is the concept diagram. Edited January 15, 2013 by alpha2cen Link to comment Share on other sites More sharing options...
Ceasium Posted January 16, 2013 Author Share Posted January 16, 2013 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? Link to comment Share on other sites More sharing options...
alpha2cen Posted January 18, 2013 Share Posted January 18, 2013 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. Link to comment Share on other sites More sharing options...
alpha2cen Posted January 18, 2013 Share Posted January 18, 2013 This is more exact Figure about the speed of sound through a liquid. Link to comment Share on other sites More sharing options...
Ceasium Posted January 21, 2013 Author Share Posted January 21, 2013 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? Link to comment Share on other sites More sharing options...
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