TBR Phy VI # 4

[zoner]

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The answer says that the speed of sound increase if the air temperature increases while its saying that hot air is less dense than cold air. I understand that hot air is less dense, but I thought sound travels faster in solid and liquid because they are more dense.

 

[UIUCstudent]

You can't compare the speed of sound on two different phases (like gas vs liquid ) solely on density. Yes liquid is more dense than gas, but liquids have more restoring force. The better the restoring force the faster the speed of sound in that medium.

Now if the phases are the same (gas vs gas) you compare the density. The less dense gas has the faster speed of sound.

So in general, different phases (compare restoring force), same phase (compare density).

Take a look at this awesome summary ilovemcat made comparing light and sound waves if you're still confused.

http://forums.studentdoctor.net/showthread.php?p=10622826

 

[zoner]

You can't compare the speed of sound on two different phases (like gas vs liquid ) solely on density. Yes liquid is more dense than gas, but liquids have more restoring force. The better the restoring force the faster the speed of sound in that medium.

Now if the phases are the same (gas vs gas) you compare the density. The less dense gas has the faster speed of sound.

So in general, different phases (compare restoring force), same phase (compare density).

Take a look at this awesome summary ilovemcat made comparing light and sound waves if you're still confused.

http://forums.studentdoctor.net/showthread.php?p=10622826

Thanks

but that page still doesn't explain why the sound travels faster in less dense gas and why we can't compare gas, liquid and solid? They are different mostly due to their density. If a space had 10 % gas of some gas particles and the same space had 90 % filled with the same gas particles the sound wave would travel faster in the more dense, 90% filled space due to higher density. Higher density means more particles in the space and more atoms to move around to cause higher restoring forces.

The logic that I am having problem with is that if the sound does indeed travel faster in less dense gas due to higher temp, it would be traveling fastest in almost vacuum which is least dense, which we all know that it doesn't.

Whats the dealio? am i in a coma?😕

however, I am aware of that equation v=root of Temp

 

[jphwki82]

The answer says that the speed of sound increase if the air temperature increases while its saying that hot air is less dense than cold air. I understand that hot air is less dense, but I thought sound travels faster in solid and liquid because they are more dense.[/QUOTE


Things that affect the speed of sound:

1) Media
2) Temperature
3) Relationship between density and pressure
4) Viscosity (No so much a factor for water and air)

For speed of sound in a fluid, it will depend on its compressibility and inertia. So its Bulk Modulus and density will play a role in determining its speed in a liquid media.

v = speed of sound in liquid
B = bulk modulus
d = density

v = sq. root (B/d)

For solids, the speed of sound will depend on the elastic property of the medium, Young's modulus (Y), and the inertial property of the medium, density (d).

v = sq. root (Y/d)

For gases, the speed of sound will depend on temperature of the medium, T.

v = 331m/s x sq. root (T/ 273 K)

where 331 m/s is the speed of sound in air at 0 degrees C.