From: Andreski, Martha (andreskim@dlshs.org)
Date: Sun May 18 2003 - 20:53:49 PDT
Subject: RE: Variations in Frequency Date: Sun, 18 May 2003 20:53:49 -0700 Message-ID: <AE9D0B330DD5BD4189660D1020C70B560A9397@dls-exchange.dlshs.org> From: "Andreski, Martha" <andreskim@dlshs.org>
You are exactly correct - that you'll hear additional resonances each 1/2 wavelength. We achieved 3 resonances with a high frequency tuning fork and a contraption I made from cutting in half an 8' section (maybe 4" diameter) PVC pipe, capping and gluing one end. Fill that up with water - well you can't fill it all the way but you'll figure that out - and insert another 4' length of PVC - this time a narrow (ie 3/4") pipe. That's the one you raise out of the water. Advice - use good duct tape to affix the larger pipe semi-permanently to a lab bench - tape near the top and near the bottom or you'll have a large volume of water to mop up. Students have to stand on the lab bench to do this lab - adding to the fun. The tricky part is viewing the water level - it's dark in that tube and as the narrower tube is raised higher and higher the level is lower and lower. I've toyed with using a piezo buzzer "conductivity meter" with graduations that is inserted just until the buzzer sounds but this will annoy ev
eryone trying to hear the resonances. Perhaps a simple LED would be better. Oh, for those of you suggesting to use a flashlight or insert a stick - I've tried that but I'll take other suggestions!
Marti Andreski, currently "femme de la chem" but soon returning to Physics!
De La Salle HS
andreskim@dlshs.org
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Subject: Pinhole Digest #1219 - 05/18/03
Pinhole Digest #1219 - Sunday, May 18, 2003
variations on frequency
by "Steve Miller" <nanodog2@hotmail.com>
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Subject: variations on frequency
From: "Steve Miller" <nanodog2@hotmail.com>
Date: Sat, 17 May 2003 11:01:32 -0700
In Hewitt's lab book there is a great lab on finding the speed of sound by
using a tuning fork that is held just above a 50 cm piece of pvc that is in
water. They use a one liter graduated cylinder. You hit the tuning fork and
raise the pipe until a distinct ringing is heard. This length is 1/4 the
wavelength. That length, plus the frequency of the tuning fork, allows you
to find the speed of sound.
The 1/4 length is where the air column resonates. I presume that iff you had
a longer piece of pvc, you would find resonance again at 1/2 the wave
length.
I have been playing around with this. This is my best thinking so far. I
would really appreciate any corrections or advice. Is the thinking
correct???!???
I have been trying this with 9th graders. Use the same set up. Use different
length PVC pipes. Blow over the end to make a tone. The tone occurs at any
length. By raising the tube, the tone goes down. However I presume that
sooner or later you will come to a point where the air column is truly
resonating.
By playing around with this, it seems that when you reach a point of
resonance you hear a much more resonant tone - and - you can really feel the
graduated cylinder (and the water) vibrating. This does not occur so
dramatically at other lengths.
So my questions: Are these steps correct? Can you really find a resonance
point this way?
Assuming you can, you could use the simple formula "velocity = wavelength
x frequency" to find the frequency of each tone at the resonance point.
This really works great in getting them to grasp the formula, and privides
practice in finding experimental calculations.
However, the next question: Assuming once again that you do hit the
resonance point, what part of the wavelength is it? Is it half the wave
length or one fourth? Or is it something else?
Steve Miller
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