Why does sound travel slower in air than most other materials?

Answer 1

Short and simple:

Sound is a compression (or pressure) wave.

There are two things that affect the speed of sound, the density of a material and its compressibility. Specifically, the speed of sound is proportional to the square root of the ratio.

v=sqrt(B/rho)

v=speed of sound.

B= bulk modulus. (There are different names for this material property such as coefficient of stiffness).

rho= mass density.

A greater density of a material tends to slow the velocity of sound but a greater stiffness tends to increase the velocity of sound.

The bulk modulus of steel is million times greater than air while the density of steel is merely ten thousand times greater.

Bulk modulus wins out and the velocity of sound in steel is more than ten times the velocity in air.

This relationship causes the speed of sound to be greater in most materials, but there are exceptions.

More Details:

The reader should be warned that there is much more to sound in materials than one see in the simple compression waves of air. Further, materials like steel come in various compositions and the micro structure depends on the processes that make it, so there is no such thing as a single number fore the speed of sound in steel.

Here are a few typical numbers for bulk modulus (stiffness) and density.

Air(adiabatic)

B= 1.42×10^5 Pa rho= 1.22 kg/m^3

Water

B=2.2×10^9 Pa rho= 1,000 kg/m^3

Steel

B= 160 x10^9 Pa rho=7,860 kg/m^3

v_air = 340 m/s.

v_water = 1482 m/s

v_steel= 4500 m/s

Caveat: As stated above, these numbers should be taken as approximate since various complications such as temperature and composition have not been explained.