Ring the rod
The speed of sound in bulk aluminum can be found in your
However, waves travelling through a long, thin rod
will have a slightly different speed;
it is more accurately given by a formula involving
- Look up Young's modulus and the density of aluminum.
Use them to compute the speed of sound in an aluminum rod.
- Set up a computer and LabPro interface.
Attach a microphone to the CH1 input.
Download this file to your desktop and double-click
it to start LabPro.
- Hold the rod at its midway point (see figure above)
and tap it gently on the floor.
Record the sound waves.
Analyze the sound waves to find their
- Draw a picture showing the rod,
and a standing wave which can occur
when you hold the rod in the middle.
What is the wavelength of this wave?
- Using the wavelength and speed of sound,
calculate the (theoretical) freqency of this
Compare it to the frequency of the actual sound
wave recorded by the microphone.
Do they agree?
If you have time ...
- Repeat steps 3-5 while holding the rod
at other places: try L/4, L/8, L/3.
Have one person whistle into the microphone.
Record the frequency.
Try to figure out the (effective) size of the person's
Does it make sense?
Then have the person whistle a higher tone.
What must the person do with his mouth to increase the frequency?