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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Reflection is when the wave hits the end and comes back the way it came, bouncing off the boundary. We’ve already looked into fixed end (where some of the energy was transmitted to the door handle and some was reflected back) and free end (where some of the energy was bounced back) reflections on a string.

The amount of energy reflected back depends on how similar the two mediums are. If they’re nearly the same, then there’s very little reflection at all because a lot of energy will be transmitted to the new medium. If they’re really different, then a lot of energy will get bounced back. Echoes are reflections, because the cave wall is so different (smooth, solid and hard) from the air surrounding it.

Designers of concert halls use materials and textures that don’t reflect and reverberate and echo sound waves. Acoustic tiles and fiberglass are often used to absorb the sound waves (reduce reflection and increase transmission). Echos take more than 0.1 seconds after the source makes the sound to reach your ear. If it takes less on 0.1 seconds, then it’s called a reverberation because of the way your brain interprets the sound (whether it’s a delayed first sound or a new sound).

Sound waves bend depending on the medium they encounter. Diffraction is one form of this bending (refraction is the other). Sound waves diffract around large obstacles, like doorways and pillars, so you can hear just fine behind a column at a concert, or hear a conversation in the next room.

Animals use this principle to communicate with each other. Owls hoot at low frequencies since lower frequency (longer wavelengths) sounds travel further than higher frequency sounds. Bats use high frequency ultrasonic waves (echolocation) to detect objects in the air, because if they used lower frequencies, the waves would diffract around their prey and they’d never eat dinner.

Click here to go to next lesson on Resonance and Standing Waves.

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