University of Virginia
Physics Department

## The Frequency of Sound

A Physical Science Activity

### Student Activity

Materials

• Two tuning forks, preferably 30 - 70 Hz difference.
• CBL system
• TI-83 with PHYSICS software package
• Vernier microphone

Procedure

1. Connect the Vernier microphone to the CH1 input on the CBL. Use the black link cable to connect the CBL unit to the calculator. Firmly press in the cable ends.
2. Turn on the calculator and the CBL unit. Start the PHYSICS program and use the microphone setup for 1 probe (for info on how to download the PHYSICS programs go to http://www.vernier.com).
3. Choose the Waveform collection mode.
4. Choose one of your tuning forks and hit on a surface (not a hard surface, but something like the bottom of your shoe) SOFTLY to get it ringing. Note: the tuning forks do not need to be hit too hard, just enough to hear a solid tone.
5. Place the fork ends (without touching anything) near the microphone and start recording.
6. After the graph is displayed, record the y value for one of the peaks and then one of the troughs. (You only have to do this for one of them because they should all be about the same height. If the graph does not show sharp peaks and troughs, retake data.)
7. Calculate the total change in amplitude by adding the peak and trough amplitude values and dividing by 2. Be sure to make all of the values positive before adding.
8. Next find out how much time it took to go a given amount of cycles. One cycle is the amount of time it takes to go from one peak to the next, so put the cursor on one of the first peaks and mark down the time (the x value), then move the cursor over to one of the last peaks and measure the time. Every time you go from a peak to the next peak, that's a cycle! Count all the cycles between peaks and mark this number down on the Data Sheet.
9. In order to find out the time for just one cycle, divide the time it took for all of the cycles by the number of cycles you passed over. This is the period T.
10. Take 1/T to find the frequency.
11. Now, using the same fork, hit it harder to perform the same analysis as in steps 5-10
12. Perform the steps 5-10 for a soft hit and a hard hit of the other tuning fork.
13. Now select the Frequency collection mode from the program main menu. This feature measures the frequency for you! See if it does a good job by measuring the frequency for the loud and soft hits for both tuning forks and comparing them to your calculation of the frequency.

Data Sheet

 Fork # 1 1 2 2 Type of hit Soft Hard Soft Hard A Peak amplitude (V) B Trough amplitude (V) C Total amplitude (V) {(A+B)/2} D Initial time (s) E Final time (s) F Time interval (s) {E-D} G # of Cycles between times H Period (s) {F/G} I Frequency (Hz) {1/H} J Calculator Frequency (Hz)

Questions:

1. Does it matter how hard you hit the fork? Does the frequency change when you hit it harder? How about the amplitude?
2. Which tuning fork has a higher tone? How does the frequency of the high tone tuning fork compare with the frequency of the lower tone tuning fork?
3. To obtain the period and frequency, we measured the time over a lot of cycles, then divided. Why didn't we just measure the time over one cycle?
4. The frequency of the tuning fork is probably written on the handle. How well does your calculated value compare with this frequency?