Why Does Covering One End of a Pipe Change the Pitch of the Sound Produced?

[john-b]

Homework Statement

[/B]
When a stream of air is directed over the end of a 40 cm long piece of plastic pipe, open at both ends, a sound is produced.
(a) Explain why this occurs
(b) If the bottom end of the pipe is covered, what will happen to the pitch of the sound? Explain

Homework Equations

Ln = (n x λ)/2
F1 = v/(2 x L1)
Ln = (2n-1) x (λ/4)
F1 = v/(4 x L1)

The Attempt at a Solution

For a), my best guess is that the stream created vibrations in the air molecules which caused the plastic pipe to resonate at its natural frequency and that created a sound

For b), I recreated this question by blowing over a straw and noticed the pitch decreases when the bottom is covered, and I know that a decrease in pitch means a decrease in frequency, but I do not know why this happens.

A nudge in the right direction would be appreciated. Thank you.

 

[.Scott]

You are creating a standing wave in the straw. Opening or closing the end of the straw changes how the wave reflects of that endpoint.

 

[john-b]

I see. Is the standing wave created by vibrations in the pipe from the moving air?

 

[haruspex]

I see. Is the standing wave created by vibrations in the pipe from the moving air?

A standing wave arises when a wave is reflected back to the source with little attenuation.
Any change in medium tends to create a reflection, and that includes the end of a pipe, whether open or closed.
At a closed end, the air is unable to vibrate along the pipe, so the phase of the reflection is so as to cancel the movement of air at that point.
At an open end, the air pressure cannot change, so in that case the phase of the reflection is so as to cancel the pressure variation there.
Google standing waves for more information and some useful diagrams.