Speed of Sound

Sound is a pressure wave which travels through a medium. The speed at which it travels depends upon the medium. For example, with air, sound travels as each air particle hits its neighbor. The speed of sound, therefore, relates to the speed at which the pressure wave travels as each particle hits its neighbor. Because air is not particularly dense, sound travels relatively slowly as compared with water or the steel of train track rails.

Because sound is traveling through a medium, the properties of the medium may affect the speed at which sound travels. The most important factors are the internal properties and the elastic properties. Sound travels about three times faster in helium because the helium molecules are much lighter than air molecules and have less inertia and, therefore, react much faster to the sound pressure wave. Sound also travels much much faster in steel, because the steel is inelastic and, therefore, propagates the pressure wave much faster.

The speed of sound in air depends on several conditions:  temperature, pressure, humidity and CO₂ content. Pressure affects the mass density or inertia of air, while temperature affects the particle interaction or elastic property of air.

An approximate formula for the speed of sound through air in standard atmospheric conditions is:

Speed of Sound = 331.45 m/s + 0.6 m/s * Deg C

This corresponds to approximately 1100 feet per second, or 750 miles per hour.

Refraction of Sound Waves

As we can see from the formula above, the speed of sound differs noticeably with temperature. This has some very interesting effects where sound seems to "bend," both in air and in water. (This property is prevalent in light refraction.) Have you ever noticed that you don't hear the freeway as loudly across a lake during the day as you do at night? At night, the water temperature is warmer than the air, causing sounds to be forced downward, toward the surface, rather than dissipating.

This effect is essentially the same as what happens in a mirage. In that case, the light waves are bent upwards, reflecting the sky. When we see the reflection, we automatically assume it to be a reflection off of water.

Speed of Sound in Water

Because water is a much less elastic fluid than air, it transmits sound about four times faster or 1400 meters per second. It is interesting to note that sound travels at different speeds at different depths, the slowest being at about 3,000 feet below sea level. This difference is due to changing temperatures and pressures. As you go down, the water turns colder, making sounds travel slower. At some point, the temperature stabilizes, letting the pressure component take over, which slowly increases the speed of sound with increased depth.

This speed differential leads to an interesting focusing property of water. The change in speed of sound with depth creates a "focused" sound channel at this 3000 foot level, making it possible for sounds to travel around the world. Research has been done to locate downed airplanes or other events based on the location of the source of the noise. You can read more about this here.

You can calculate the speed of sound in water for a given temperature, salinity, and pressure here.

Distance to a Lightening Storm

You can approximate your distance from a lightning storm if you measure the time between when you see the flash and when you hear the thunder. Sound travels approximately 1/5 of a mile per second or 1 mile in 5 seconds. That converts to about 1/3 of a kilometer per second. So if you see the lightning 3 seconds before you hear it, the strike is approximately 0.6 miles, or 1 kilometer away.