Let’s review interference and nodes for a minute…

How waves interact with each other depends on whether the waves are in phase or not. If they are in-step (in phase) with each other, then it’s easy to add up to double the displacement (constructive interference). If they are completely out of step, then they cancel each other out (destructive interference). We’ve seen this for transverse waves, but what about compression waves, like for sound waves?

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Destructive interference happens when a compression and rarefaction section meet and the next effect is that there’s no push or pull on the particles. The waves don’t destroy each other (as the name implies), but rather they cancel out the effect of each other when they interact with each other, which results in no sound at all. Which is kind of odd to think about: two sound waves interacting to make no sound at all. This happens at the nodes, where there’s no particle displacement.

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Music is a wonderful combination of sound waves that are at different frequencies that are pleasing to hear.

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Beats refers to the pattern that two similar waves make when they interfere with each other. A beat pattern is one that varies in volume (amplitude) as the waves constructively and destructively interfere with each other. The beat frequency is the wave that forms when you hear the volume go from high to low volume.

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The Doppler Effect describes how moving sound waves can shift frequencies for either the observer, the source, or both, depending on the motion of each. You’ll find Doppler shifts with waver, sound, and light waves. For sound waves, if the source is moving toward the observer, the observer will hear higher pitch sounds. And if the source moves away from the observer, then they will hear lower pitch sounds. The frequency at the source didn’t change, only what the observer perceives is difference because there’s motion between them. It’s a shift in the apparent frequency.

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Sound can change according to the speed at which it travels. Another word for sound speed is pitch. When the sound speed slows, the pitch lowers. With clarinet reeds, it’s high. Guitar strings can do both, as they are adjustable. If you look carefully, you can actually see the low pitch strings vibrate back and forth, but the high pitch strings move so quickly it’s hard to see. But you can detect the effects of both with your ears.

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Click here to go to next lesson on Behavior or Waves at the Boundary.

When waves go from air to water, they must pass through a boundary between the two, and depending on the properties of two mediums, the wave will do one (or more) of four possible behaviors: reflect, diffract, transmit transmit through, and/or refract.

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