# Density and Speed of Sound

*Most recent answer: 10/24/2015*

- Abhishek (age 15)

Australia

It's true that the, other things being equal, the speed of sound goes down as the mass density goes up. The problem is that other things usually aren't equal. A little ripple in the density of a piece of steel creates much bigger forces than a similar percentage ripple in the density of some air. So those bigger forces speed up the sound. Another way of saying that is that the sound speed goes up for bigger bulk modulus. (That's the compressive longitudinal sound. Transverse sound depends on the shear modulus.) For ideal gases, the two factors just cancel and the speed of sound doesn't depend on the density. In typical solids the bulk modulus goes up by a bigger factor than the density does, compared to gases, so solids typically have faster sound waves.

Mike W.

*(published on 10/24/2015)*