Analyzing Beats and Interference With Probeware

Author(s): George Lyle
Probeware / Instrumentation
SED 695B; Fall 2005

Research Question:

How do sound waves interefere to form beats?

Standards addressed:

California Physics Standards:

4. Waves have characteristic properties that do not depend on the type of wave. As a basis for understanding this concept:

f.

Students know how to identify the characteristic properties of waves: interference (beats), diffraction, refraction, Doppler effect, and polarization.

 
Independent variable
Dependent variables
Controls
Series
Mass on tuning fork Frequency    
Materials
Procedures
  • Pasco 750 interface with SCSI input to computer.
  • Sound Sensor
  • Two tuning forks mounted on resonance boxes.
  • Paper clips bent to fasten to top of forks to change frequency.
  • Prepare the tuning forks by attaching one or two large paper clips to the top of one fork.
  • Plug the sound sensor into port A of the Pasco 750 interface.
  • Place the sound sensor near the resonance boxes, with both boxes pointing at the sensor.
  • Continue following the instructions accompanying the pictures below
 
This is the equipment needed. Not shown is the Pasco 750 interface.
This is the older Pasco sound sensor. It functions as a microphone. Newer sensors act as sound level measuring devices and don't seem to have the rapid response needed for this task.
Reshape a paper clip to add a bit of mass to one of the two tuning forks.
You can put two clips on the fork to get an even greater difference in frequency.
This is how you position the sound sensor. It is best to keep it close and equidistant from the two resonance boxes.
This is the Science Workshop introductions screen. Click on the analog plug at lower right and drag it to Channel "A"
The following screen appears.
Select the Sound Sensor and click OK

The sound sensor icon appears below the "A" input.

Now drag the "Scope" icon to the "A" input
A small oscilloscope display appears.
Enlarge the Oscilloscope window.
Click on Experiment, then click on Monitor on the menu that appears.
An oscilloscope trace appears across the middle of the display.
Strike one of the tuning forks near the microphone. A waveform appears. If the waveform is too tall or too short, adjust it with the buttons in the upper right corner of the display.
If you strike the weighted and unweighted tuning fork simultaneously, you will see constructive and destructive interference on the oscilloscope.
If you slow down the sweep rate with the buttons at lower left, the interference is much more apparent.
Click on Experiment and use the menu that appears to stop collection of data by clicking Stop.
Close the oscilloscope window, clicking OK to confirm that you really want to do this.
Drag the FFT icon to input "A". A small Frequency Spectrum display appears.
Enlarge the Frequency Spectrum Display
Double-click on the horizontal axis and increase the number of points to 1024. This allows more accuracy at the expense of a slower refresh rate.
Here you can see the larger number of points.
Use the buttons at lower right to increase the maximum frequency to 500Hz.
Click on Experiment and select Monitor from the menu that appears.
Here you see ambient noise displayed.
Strike one tuning fork and you will see a sharp spike. The frequency appears at upper right.
Strike the weighted tuning fork and notice that the frequency is slightly different.
Strike both tuning forks and you will see two peaks if their loudness is the same.
Here is a display with a greater difference between the tuning fork frequency.
   

References & Links:

The manufacturer of the probeware.
An online Tutorial concerning interference and beats.
An Excel spreadsheet illustrating beats.
A Java Applet illustrating beats.