Learn about the Doppler Effect here.

Echo diagram and instruction here.

How does Sonar work? Find out here.

Does a Duck's quack echo? Find out here.

What causes an echo? Find out here.

Decibel levels of common sounds here.

What do sound waves look like? Click here.

Comparing light and sound waves here.

All the sound information you could want here.

Have you ever heard a sonic boom? NOVA has the science behind it here.