A sound wave is different from a light wave in that a sound is a mechanical wave, which requires particle interaction in order to exist. Light waves can travel in the vacuum of space, and we’ll talk more about this in our next section when we get to light.

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Sound waves are longitudinal waves, meaning that the particles vibration in the same direction as the wave moves in. If a wave is moving left to right, then the particles are also vibrating from left to right.

As the particles move back and forth, they creates small differences in pressure. For example, if we slow the vibrating fork WAY down, we see that when it moves to the right, it pushes on the air around it and moves those particles to the right, causing the particles to be compressed a little. As the fork vibrates back to the left, it opens up the space and lowers the pressure of the air, causing the particles to move to the left now, and this back and forth motion sets up the wave.

Sound waves have compressions (higher density areas, or higher pressure) and rarefactions (lower density areas, or lower pressure) since they are longitudinal waves. This is useful because when we measure the wavelength, we usually measure from one rarefaction to another, or one compression to another.Sound waves are longitudinal pressure waves, because they are a pattern of higher and lower pressure areas moving through the air (or other medium) in the same direction that they are traveling in.

Click here to go to next lesson on Seeing Sound Waves.

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