I have always wondered if sound travels any faster or slower in cold air or hot air?

Answer 1

The previous answerer is incorrect. The speed of sound in air *increases* as the temperature increases. His/her reasoning is also incorrect; the speed of sound in a material is *inversely* proportional to the square root of the density of the material. That means that sound travels *slower* in denser materials (including air). The speed of sound in a materials also depends on the "stiffness" of the material. In this case, the speed *increases* proportionally to the square root of the material stiffness. Denser materials are often "stiffer" than less dense materials, so the stiffness factor often (but not always) wins out.

More generally, the speed of sound in a gas (or mixture of gases) that obeys the ideal gas law is given by:

V = sqrt(g*R*T/M)

where V is the speed of sound

g is a dimensionless constant characteristic of the gas or gas mixture (called the adiabatic constant) g = Cp/Cv, where Cp and Cv are the constant-pressure and constant-volume heat capacities of the gas. For air, g ~= 1.4

R is the universal gas constant (8.314 J/mol K)

T is the absolute temperature (in kelvins)

M is the molecular mass of the gas, in kg/mole

As the temperature increases, the speed of sound increases proportionally to the square root of the absolute temperaure.

A related question is whether sound travels faster in dry air or humid air. Because the molecular mass of water (18 gm/mol) is less than that of dry air (~29 gm/mol), humid air has a lower average molecular mass than dry air, so "M" in the above equation is smaller. As M gets smaller, the speed of sound increases. Sound therefore travels faster in humid air than in dry air.

See sources.

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Note added in response to 3rd answerer, who is also incorrect. I would ask that the 3rd answerer post a link to a reputable source, or cite a textbook that supports his/her contention that the speed of sound increases with increasing density. If they try, they will find that they can't because this is the *opposite* of the actual relationship.

The first answer is not "an idiot". It is a common mistake to think that sound travels faster in denser materials because our experience seems to suggest this. The problem is that it's easier for most of us to assess the density of a material than it is to assess the "stiffness" of a material. As noted above, because denser materials are usually "stronger" in some sense, the sound velocity in those materials is higher, but this is due to the increase in strength, not the increase in density.

Answer 2

Cold air is denser than hot air so it compresses the sound wave, slowing it down. The air molecules in hot air are moving faster so they don't slow the sound wave down as much.

Answer 3

The speed of sound varies as the square root of absolute temperature. Thus, as the temperature of the medium increases, the speed of sound through that medium also increases.

Answer 4

the first guy is right
the second guy is wrong
sound travels faster in denser materials and cold air is more dense

Answer 5

Cold air...it is denser


You're correct hfshaw...I'm an idiot.