# speed of sound vs. density

Discussion in 'MCAT Study Question Q&A' started by km1865, Jul 25, 2011.

1. ### km1865 7+ Year Member

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I was a bit confused on the dependence of the speed of sound on density. I know that the speed is usually indirectly related to the density, thats only if the materials have the same elastic properties though (i.e. you cant compare density of steel to rubber when comparing speed of sound, b/c the elastic properties are a greater determining factor in that case). but if you have two metals ,then the speed of sound is effected by density, the more dense metal would transmit sound slower because higher density= more mass in a given amount of volume, and it takes more kinetic energy to make the more massive molecules vibrate than the slow ones.. that all makes sense.

Now, the confusing part is when we're talking about density of AIR and its effect on speed of sound. The above relationship seems to reverse...in that the denser substance has more mass packed into a given amount of volume.. so the molecules are closer together and stronger, so sound would travel faster through the stronger bonds? This contradicts what i mentioned about the metals above.. Can anyone clarify this please? Also, should I just be thinking of speed of sound in air relative to temperature and not think in terms of density (only in air).. i.e. the higher temperature, warmer air has more kinetic energy to make the molecules vibrate so sound travels faster.. ?

Thanks!

2. ### darkhope 10+ Year Member

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I must have remembered something wrong, somehow I thought the higher the density, the faster the speed, for example sound travels faster through solid than through air. I thought it was because there was more medium. This is opposed to say electromagnetic waves

3. ### muhali3 7+ Year Member

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I thought v = sqrt.(Bulk modulus/density)

which is analogous to v = sqrt.(tension/mass per unit length) in a string.

I think more molecules per unit volume in the air (higher density) would result in more waves being generated and more energy would be dissipated in each individual wave with less wave propagation overall.

4. ### mitchlucker 2+ Year Member

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As temperature increases, speed of sound increases.

You are right about the second part.

Sound travels faster in a solid than in air.

In denser air, sound travels slower than in normal air.

This is because of the elastic properties of the particles in the air.

More particles = more energy dissipated like muhali said.

5. ### paul411 ANES 7+ Year Member

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Another quick note: more humidity in the air = lower density of the air = higher speed of sound

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6. ### Meredith92 5+ Year Member

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Wouldn't more humidity mean more density cuz there are
More water molecules?

7. ### MedPR Removed

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Less dense. Density = mass/volume. Volume is the same. H2O (18) is less massive than O2 (32).

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8. ### Meredith92 5+ Year Member

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