# The speed of sound at different tempertures

1. Homework Statement
does the speed of sound increase as the temperture becomes closer to absolute zero, which would make the substance more and more dense?

2. Homework Equations
dont think there is any

3. The Attempt at a Solution

really confused as, isnt it the more dense a substance is the faster sound travels as particle colisions are faster, but while the temperture decreases the particle dont move as much

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Tom Mattson
Staff Emeritus
Gold Member
Check this page out. There's a simple equation that models the temperature dependence of sound in air (I assume you're talking about air and not some other substance).

http://www.glenbrook.k12.il.us/gbssci/Phys/Class/sound/u11l2c.html [Broken]

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But the disclaimer says:
The above equation relating the speed of a sound wave in air to the temperature provides reasonably accurate speed values for temperatures between 0 and 100 Celsius. The equation itself does not have any theoretical basis; it is simply the result of inspecting temperature-speed data for this temperature range.
And as he's talking about going 273 degrees below 0, it won't hold accuracy.

Tom Mattson
Staff Emeritus
Gold Member
Ah, I read the OP wrong. I was thinking he was going to 0 C.

Look instead at the following page.

http://en.wikipedia.org/wiki/Speed_of_sound

In the section entitled "Speed in ideal gases and in air" they give an expression for $c_{ideal}$, the speed of sound in an ideal gas as a function of temperature (this temperature is in Kelvins). This equation applies down to absolute zero.

From the first website I gave you:

The equation itself does not have any theoretical basis; it is simply the result of inspecting temperature-speed data for this temperature range.
It would seem that this statement is wrong, because the Wiki page does in fact derive it as an approximation of the earlier expression for $c_{ideal}$, which comes from the ideal gas law!