Acoustics: The scientific study of sound, using things like slinkies and underwater bells.

Active sonar: Sonar that can both create and detect sound waves. What more could you ask for?

Amplitude: The maximum magnitude (r.e. height) of a sine or cosine curve from the midpoint of the wave. That's half of the height a wave has when you measure trough to crest. Don't confuse it with Amperes, which measure electricity, or amplifiers, which make electric guitars more exciting.

Compressions: When you push together molecules in a mechanical system. Sometimes the molecules are okay with that, but other times they just aren't.

Decibel: A unit used to show you the relative difference in power or intensity, usually between two types of sounds. Any time you go up a level, you're actually multiplying the intensity by ten, making differences that look not so big actually very big.

Doppler effect: If you're standing in one place and you hear a sound moving towards you, that sound is going to rise in pitch as it moves towards you and lower in pitch once it starts moving away from you. Unless it hits you; then we have no idea what'll happen.

Echolocation: The process of finding—locating, you could say—an object by making sound waves and using the echoes to figure out where things were reflected and when.

Medwin's formula: The formula you can use to estimate how quickly sound can move (especially in water) base on a material's temperature (T), salinity (S), and depth (D).

Passive radar: A radar that doesn't make any sound waves; it just takes in the sounds that happen to reach it.

Piezoelectricity: the ability of some materials to turn pressure into electricity. Quartz is like the over-achieving slacker of the rock community: the more pressure you put on it, the more energy it's going to put out.

Rarefactions: When molecules in a system resist compressions, you'll get some rarefactions, which means the molecules pull back apart after being shoved together. If only our dominoes could do that after we knocked them over.

Transducer: An object that changes the input energy to a completely different output energy. It's like how you take in food and turn it into essay-writing, math-solving, and TV-watching energy.