Pitch: How does the thickness of a vibrating string affect sound?
(From Experiment Central, published by U·X·L and available online in Science Resource Center.)
Purpose/Hypothesis
In this experiment, you will explore how the thickness of the vibrating object affects the pitches it produces. You will use different sizes of rubber bands to test this effect. Before you begin, make an educated guess about the outcome of this experiment based on your knowledge of sound. The educated guess, or prediction, is your hypothesis.
A hypothesis should be brief, specific, and measurable. It must be something you can test through observation. Your experiment will prove or disprove your hypothesis. Here is one possible hypothesis for this experiment: "Thicker bands will produce lower pitches." In this case, the variable you will change will be the thickness of the rubber band, and the variable you will measure will be the pitch of the sound. You expect a thicker band to produce a lower pitch sound.
What Are the Variables?
Variables are anything that might affect the results of an experiment. Here are the main variables for this experiment:
- the thickness of rubber bands
- the length of bands
- the strength with which the band is plucked
- the sound produced by plucking
- the experimenter's ability to detect different pitches
Level of Difficulty
- Easy.
Materials Needed
- 8-inch-square (20-centimeter square) metal baking pan with straight sides
- 5 rubber bands of different thickness but the SAME length
- ruler
Approximate Budget
- $5, if you need to purchase rubber bands; other materials should be available in the average household
Timetable
- 1 hour
How to Experiment Safely
- Try not to snap yourself with the rubber bands
Step-by-Step Instructions
- Arrange the rubber bands in order from thinnest to thickest.
- Measure the width of each rubber band with your ruler. Record these numbers on your data sheet, as illustrated.
- Keep the bands in order, stretch each one over the pan, which acts as a sound box. Be sure to stretch them the same amount so the portion of the band over the open part of the pan is under the same tension as the rest of the band. See illustration.
- Pluck each band, beginning with the thickest one, and listen carefully to the pitch it produces.
- Describe each tone as you pluck the band and record on your data sheet what you hear.
Summary of Results
Study the results on your chart. How did the thickness of the band affect the pitch it produced? Did a thick band produce a lower pitch or a higher pitch? Thick bodies vibrate more slowly than small ones, and slower vibrations produce lower pitches. Is this what happened in your experiment? Was your hypothesis correct? Write a paragraph summarizing what you learned.
Troubleshooter's Guide
Problem: You cannot hear the differences in the pitches.
Possible cause: Your rubber bands are too similar in size. Try to find bands that are several millimeters different in width. Check an office supply store or an art supply store.
Problem: You cannot hear much sound at all.
Possible cause: The pan is absorbing the vibrations. Be sure the pan is metal, with straight sides, and deep enough so the bands are free to vibrate.
Change the Variables
You can vary this experiment in several ways. Try using bands with even greater differences in thickness. Record their width and see what happens. Try putting the same size bands on a larger pan and plucking the two instruments next to each other. What do you hear? Experiment with different size pans and you can create an entire orchestra.