variations on frequency

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From: Steve Miller (nanodog2@hotmail.com)
Date: Sat May 17 2003 - 11:01:32 PDT 

From: "Steve Miller" <nanodog2@hotmail.com>
Subject: variations on frequency
Date: Sat, 17 May 2003 11:01:32 -0700
Message-ID: <BAY2-F136d3OJXsNQpt00005c37@hotmail.com>

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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