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
If you have time ...
- What is the speed of sound in bulk aluminum?
What is the speed given by the formula above?
Use the latter speed for the rest of the experiment.
- 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?
- 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?