# Speed of sound in substances

**this is not a homework question**

I need to know in which substance does sound travel fastest in. I know that sound travels faster when the medium is denser, therefore solids, however I would like to go more in depth. I've heard that it is iridium or hydrogen solid, but I can't get a definite answer. I was wondering if anybody knew for sure?

much appreciated!

mgb_phys
Homework Helper
In solids speed of sound = sqrt ( E / 3*density*( 1-2v) )

E is Young's modulus (the stiffness)
v is Possion ratio (a measure of how much the material expands sideways when you press it)

At a guess the fastest speed of sound would probably be in diamond - it's hard = stiff

In solids speed of sound = sqrt ( E / 3*density*( 1-2v) )

E is Young's modulus (the stiffness)
v is Possion ratio (a measure of how much the material expands sideways when you press it)

At a guess the fastest speed of sound would probably be in diamond - it's hard = stiff

Speed of sound should be same as the propagation speed of a mechanical vibration in the medium, right?

would sound travel faster in an Einstein-Bose condensate.

f95toli
Gold Member
Speed of sound should be same as the propagation speed of a mechanical vibration in the medium, right?

Yes, remember that sound IS just a mechanical vibration (which happens to be in the audible range) so from a physical point of view there is no difference between the two.

hey I'm in eighth grade, can you give me some examples? How would I use the formulas you gave me to find out the answer? Is it a natural or manmade substance?

Redbelly98
Staff Emeritus
Homework Helper
At a guess the fastest speed of sound would probably be in diamond - it's hard = stiff

Diamond also has a low density (3.5 g/cc), which helps.

elasticity increases the speed of sound. so metals will give highest speed of sound in them. not only this the temperature, pressure, density, and molecular wt. also play role.

Does the frequency of the sound matter?

Redbelly98
Staff Emeritus
Homework Helper
Does the frequency of the sound matter?

In the human range of audible sound, no.

However, what I remember from my solid state physics (taken 25-30 years ago), at frequencies where the sound wavelength is comparable to the distance between adjacent atoms the speed of sound does change.

I don't remember at what frequencies this typically happens, or whether the speed of sound becomes faster or slower ... perhaps somebody else knows?

To get a ballpark estimate, figure a speed-of-sound of 5000 m/s in a "typical" solid, and a wavelength of 0.2 nm:

f = v/λ = 5x103 / 2x10-10 Hz ~ 2x1013 Hz​

I.e., at frequencies corresponding to I.R. radiation we'd expect changes in the speed of sound.

FYI, I found this listing for speed of sound in some different materials. Berylium is 12.9 km/s, slightly higher than diamond (12 km/s)
http://hypertextbook.com/physics/waves/sound/ [Broken]

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