Sound >_>

We're doing an experiment in physics (12th grade physics ;_;) to measure the speed of sound.  We get a long cardboard tube, plug off one end, and set a microphone at the other.  Then we clap and see how long it takes for the sound to reach the microphone, travel through the tube, and have the echo to reach the microphone again.  We did this and divided the change in time between when the sound was detected and when it was returned by the length of cardboard tube to find velocity.

When the cap on one side is removed, however, the sound still echoes back; and it comes back in the same amount of time.  What causes it to do so?

>>1
It's called diffraction. As the sound waves reach the end of the tube, they are distorted as they cross the tube boundary, and are partly reflected back.

If the tube is small enough (relative to the wavelength of the sound), the sound waves won't be able to exit the tube at all; they'll be completely reflected, even though the end of the tube is open. This is the principle by which a screen on an ordinary microwave oven works. Metal reflects electromagnetic waves; those small holes on the door are small enough that microwaves get completely reflected as they try to pass through, while visible light can pass through the holes almost entirely undistorted.

http://en.wikipedia.org/wiki/Diffraction