# Introduction to Computer Music: Volume One

### 3. How fast does sound travel? | page 2

For comparison, the speed of light in a vacuum is 299,792,458 meters per second or 186,000 miles per second (669,600,000 mph), which is roughly 870,000 times the speed of sound. The difference between the speed of light and the speed of sound is why you see lightening before you hear it (unless you are struck by it, in which case it may be simultaneous!).

Comparing sound in air with sound in other mediums, the speed of sound in helium at 0°C is approximately 972 meters per second (m/s), or around 3 times as fast as in air. Sound travels even faster in liquids and solids than it does in gases because of their greater density (although standard measurement is slightly more complicated, because it is affected by the shape of the material and also the fact that both longitudinal and transverse waves may propagate in solids). Sound in 20° C water travels an average of 1482 m/s, while sound in aluminum travels at 6420 m/s. Modern navies depends heavily on being able to predict the speed of sound in varying water temperature for their SONAR echo-location systems.

If you have ever lived in an apartment with shared walls and a party next door, you are no doubt aware that sound waves can transfer from one medium to another and back again—in this case from air to the rigid wallboard and back to air again. A wide variety of sound isolation products and construction techniques attempt to mitigate this sort of transfer. For further sound-abatement information, see Auralex's Acoustics 101.

The speed with which sound travels changes accordingly with the shift in mediums. As we will detail in Module 13. How the Ear Works, the sound we 'hear' is acoustic energy transferred from gas to solid to liquid and finally to electrochemical signals as it enters and moves through the ear.

Techno-factoid: Snell's law describes the refraction of sound or light as it passes through an interface between two materials of differing sound speed. This is why your tropical fish, when you go to net them, aren't exactly where you think they are as the light passes from air to glass to water. Speaker designers are very aware of this property in sound, as it effects the propagation properties of their speakers.

 It is important to realize that while all sound in the equivalent conditions propagates or travels at the same speed, whether louder or softer, higher or lower, the speed or force of the individual molecules bumping into one another increases with amplitude and/or frequency.

For further study, see Hyperphysics->Speed of Sound

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