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Re: [Phys-L] The resonance phenomenon in an open ended cylindrical tube



Another way to view the standing wave is in terms of the pressure, not the displacement. At the air-water interface there is a pressure anti-node indicating no phase reversal of the pressure.

At the risk being rebuffed, I would suggest that the "information" carried by the sound wave is in the pressure depiction. Consider two sources (speakers) which are distance L apart and driven by a common source. Place a detector/microphone directly between the speakers at L/2. Ignore reflected waves; consider only direct waves. Would you detect no sound or maximum sound? Many students say none because they are thinking displacement waves and the displacements cancel. The experiment (as best as I can do it in the intro lab) says maximum indicating to me that the pressure is the better thing to consider for a sound wave. This conclusion is supported by a minimum when I move the microphone to a pressure node/displacement antinode.

-> -----Original Message-----
-> From: Phys-l [mailto:phys-l-bounces@www.phys-l.org] On Behalf Of
-> Savinainen Antti
-> Sent: Monday, January 12, 2015 12:10 PM
-> To: phys-l@phys-l.org
-> Subject: [Phys-L] The resonance phenomenon in an open ended cylindrical
-> tube
->
-> Hi,
->
-> I suppose the following is a standard demonstration. An open ended
-> cylindrical tube is partially filled with water. When a certain frequency is
-> applied, using a tuning fork or otherwise, a clear resonance is heard. The
-> end correction could be invoked to get a better result in determining the
-> speed of sound. Nothing special here (although an analysis of the end
-> correction takes quite a lot of physics, well beyond HS).
->
-> However, a very good student recently asked me: how come there is a
-> node at the air-water interface? A sound wave is reflected from the water
-> surface, and, the incoming and reflected waves interfere with each other to
-> produce a node. The trouble is, she noted, that sound propagates faster in
-> water than in air. This means that there should be no phase change in
-> reflection. Hence, the incoming and reflected wave should not be out of
-> phase by half lambda to create a destructive interference (node).
->
-> How would you answer the question? Is there a mistake in her reasoning?
->
-> Regards,
->
-> Antti Savinainen
-> Finland
->
->
-> --
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