# Fastest speed of sound

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TeethWhitener
Gold Member
Don't know for certain, but diamond's speed of sound is pretty fast (12 km/s). Graphene and carbon nanotubes probably also have a high speed of sound in-plane/along the tube's axis. What you're looking for is a large bulk modulus (a measure of stiffness) and a low density, so your best bet is to look for light, strong materials.

jim mcnamara
Mentor
Typical values for P-wave velocity in earthquakes are in the range 5 to 8 km/s. The precise speed varies according to the region of the Earth's interior, from less than 6 km/s in the Earth's crust to 13 km/s through the core.
https://en.wikipedia.org/wiki/P-wave P-waves from earthquakes are very low frequency sound waves.

Sound waves are in fact compression waves: http://www.physicsclassroom.com/class/sound/Lesson-1/Sound-is-a-Pressure-Wave

See Birch's law : https://en.wikipedia.org/wiki/Birch's_law which holds for materials not under enormous pressure. Which seems to be somewhat at odds to what @TeethWhitener indicated.

davenn
Gold Member
2019 Award
See Birch's law : https://en.wikipedia.org/wiki/Birch's_law which holds for materials not under enormous pressure. Which seems to be somewhat at odds to what @TeethWhitener indicated.
well not entirely, as in the earth, higher density will generally be directly proportional higher pressure, so no problem there
It's just that you can have high density materials that don't need to be under high pressure, eg a block of lead

TeethWhitener
Gold Member
https://en.wikipedia.org/wiki/P-wave P-waves from earthquakes are very low frequency sound waves.

Sound waves are in fact compression waves: http://www.physicsclassroom.com/class/sound/Lesson-1/Sound-is-a-Pressure-Wave

See Birch's law : https://en.wikipedia.org/wiki/Birch's_law which holds for materials not under enormous pressure. Which seems to be somewhat at odds to what @TeethWhitener indicated.
The velocity of sound has a very simple functional form:
$$c=\sqrt{\frac{K}{\rho}}$$
Where c is velocity of sound, K is bulk modulus, and $\rho$ is density.
EDIT: The discrepancy between Birch's law and the Newton-Laplace law (above) really bugged me, so I did some digging, thinking maybe Birch's law was the first few terms of a Taylor series approximating Newton-Laplace. The only thing I could really find were some "Birch Diagrams" from old geology papers, where the empirical linear velocity-density relationship from Birch's law is given by two points per material.
Example: http://onlinelibrary.wiley.com/doi/10.1029/JB078i029p06926/epdf
So here's where I'm stuck (and maybe we need a geologist to sort this out): are they really drawing a general law from two points of data? Because in that case, of course you get a linear relationship. So what gives? Is there some assumption in Newton-Laplace that doesn't apply here? Inhomogeneity, shear, dispersion, etc.? I dunno.

Last edited:
jim mcnamara