Ring the rod

The speed of sound in bulk aluminum can be found in your textbook. However, waves travelling through a long, thin rod will have a slightly different speed; it is more accurately given by a formula involving Young's Modulus:

  1. 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.

  2. 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.

  3. 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 frequency.

  4. 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?

  5. Using the wavelength and speed of sound, calculate the (theoretical) freqency of this standing wave. Compare it to the frequency of the actual sound wave recorded by the microphone. Do they agree?

  6. Repeat steps 3-5 while holding the rod at other places: try L/4, L/8, L/3.
If you have time ...

Have one person whistle into the microphone. Record the frequency. Try to figure out the (effective) size of the person's mouth. Does it make sense? Then have the person whistle a higher tone. What must the person do with his mouth to increase the frequency?