From: k-sub-d@ix.netcom.com
Date: Mon May 19 2003 - 09:08:34 PDT
Message-ID: <7063154.1053360832795.JavaMail.nobody@gonzo.psp.pas.earthlink.net> Date: Mon, 19 May 2003 09:08:34 -0700 (PDT) From: k-sub-d@ix.netcom.com Subject: Re: Pinhole Digest #1219 "Variations on Frequency"
Steve,
It seems to me that you are having two problems getting the experiment to work well for you. The first addresses the actual lengths at which you will find resonance which has been discussed twice already.
The second is the method you stated you were using with the ninth graders. You stated you are blowing across the tube to make a tone. When you do this, the tone you hear is actually the fundamental resonant tone for that length of closed tube. The reason the pitch goew down as you lift the tube is that you are making a longer resonance chamber. You are working the tube in much the same way as your mouth and vocal cords serve to pick out a frequency and amplify it through resonance when speaking or singing. In order to use this experiment to calculate the speed of sound you really need a tuning fork or some other device that creates a vibration of a known frequency. Then change the length and you will hear the increased volume at the resonance length. It is quite noticeable and it's an experiment that my students always seem to enjoy.
If you have a long enough tube and a high frequency tuning fork you can actually find the second and occasionally the third resonance although the higher resonances produce smaller effects. If you want really accurate results for the speed of sound make sure to account for the small height (approx 1cm usually) that you hold the tuning fork above the tube. If you don't have tuning forks, ask the music department if they have some or a student who has an electric guitar tuner.
Hopefully this will help. I love this lab. Do it on days with different temperatures and you'll get slightly different results and can show the students that compressional waves really are medium dependent for their speed. Good luck! Have Phun with PHysics!
Kevin DOyle
Aragon High School
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