How Fast Sound Travels

Author: N.H. Crowhurst

The speed, or propagation velocity, of sound depends on the elasticity of air and on its density. Although we do not normally measure these properties, we can easily find the temperature and barometric pressure of the air, which affect both its density and elasticity. What we would like to know is: "How does the speed of sound change with atmospheric pressure and temperature?"

Experiments and advanced theory provide the following answers:

1. Change in barometric pressure alters the density of air (how much of it there is in a given space) and its compressibility (the more compressed it is, the more it resists further compression). Both change together and neutralize their effect on velocity of sound, so it is not appreciably dependent on barometric pressure.

2. Change in temperature also affects both the density and compressibility of air, but not so as to neutralize as pressure does. A useful formula says that the velocity of sound in air is approximately 1086 feet per second at 0° Centigrade (the freezing point of water), and rises 2 feet per second for each Centigrade degree rise in temperature.

At 20° C (normal room temperature), sound travels at 1086 + (2 X 20) = 1126 feet per second.

Determining the speed of sound

Sound also travels in almost any medium besides air, because everything (except a vacuum) possesses the two necessary properties, density and compressibility. (Even the most "incompressible" substances "give" a little.) For example, water is about 80 times more dense than air, but it also offers an even greater opposition to compression. Owing to its greater density, we should expect sound to travel more slowly in water than it does in air. Water, however, is less compressible, which tends to make sound travel faster. The combined effect makes the speed of sound in water about 4.7 times faster than in air. The following table gives the speed of sound in different substances measured at 0°C: