How sound moves

Sound waves are compression waves. This means that the disturbance of the particles is in the same direction as the propagation of the wave. Imagine bunching up several loops of a stretched out slinky and then releasing them. As the wave moves from one end of the slinky to another, the loops of the slinky are seen to move along the length of the slinky as well, not at right angles as is the case for transverse waves. This is an excellent model of a compression wave, which is also known as a longitudinal wave.

Sound travels through a medium from the source to the observer, thus if there is no medium, you can't hear anything. To convince yourself of this, see if you can get access to a bell jar. Put a continuous source of sound in it, like a ringing bell and slowly pump out the air. The sound will be fainter and fainter until eventually there will be no sound reaching your ear at all. This occurs when there is no air in the jar, and thus no medium for the sound to propagate through.

Like all waves, sound travels at different speeds depending on the medium it is traveling through. The exact velocity for the speed of the propagation of sound in a given medium can be found by using the equation

This equation makes sense because the speed of a compression wave is dependent on how far from their neutral positions the molecules of a medium can be moved (its elasticity) and how close together these molecules are (its density). As you can see, the greater the elasticity and the lesser the density of the medium, the faster the sound travels through it. The reason that sound travels faster in air (see table below) is that the rising temperature decreases the density of the air without really affecting the elasticity of the medium. While temperature affects the speed of sound in ideal gases, pressure, interestingly enough, does not. This is because a change in pressure affects both the elasticity and the density of a gas in the same proportion, so the velocity of sound remains unchanged.

Refer to the following table for the speed of sound in various media.

 Medium Speed of sound (m/s) Air (0º C) 331 Air (20º C) 343 Helium 965 Hydrogen 1284 Water (0º C) 1402 Water (20º C) 1482 Seawater 1522 Aluminum 6420 Steel 5941 Granite 6000

Not only must a sound-producing body be in contact with some medium that transmits the sound, but it must also be a vibrating body. Remember that a sound wave is a compression wave. This name gives a hint to how exactly sound is transmitted. When sound waves travel, they produce alternating condensations and rarefactions. Although sound is a longitudinal wave, a condensation is equivalent to a crest in a transverse wave, and is an area of greater than normal pressure. A rarefaction is equivalent to a trough in a transverse wave and is an area of smaller than normal pressure. Picture a tuning fork. As the tines vibrate, they bend back and forth. As they bend outwards, the air on either side of the fork is compressed, increasing its pressure and causing it to push against the air next to it which only has normal pressure. As they bend back inwards, they create more space for the air, causing an area of lower pressure to form. This motion sets up a wave of alternating lower and higher pressure, which is propagated outward. In this manner, compression waves like sound travel. See the illustration below.

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