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?

Resonance?

magic

>>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

>>4
:3 thank you

He didn't accept the answer D:
He says it's much simpler than diffraction and interference.

Something is causing the sound to return at the same distance as when the lid is closed in a full powerful echo.

>>6
see >>3

>>7
Magic isn't simple at all, magic takes years of schooling!

>>8
that's magikku, not magikyaru!

>>5
IT'S DIFFRACTION GET IN THE TUBE

How does diffraction cause the sound to be reflected?

I still don't understand T_T

>>4
I would think that diffraction would work differently between sound and EM radiation since sound is propagated as a longitudinal wave and light is a transverse wave.

>>13
well yes but sound expands in 3 dimensions, so the refraction comes from the walls of the tube, there is a particular angle at which the  expanding air "bounces" of the walls and into the tube, but as it expands "tridimensionaly" , part of the pressure does bounce back outside of the tube, sure its minimal and has to fight its way back whit the new expanding waves from the sound source but it's still measurable. OP never said the echo was the same intensity in both experiments, just that the calculated speed of sound is the same, which is obvious since  whit the same atmospheric pressure , speed of sound in the air is constant