You’ve probably seen an instrument played, and if you are lucky you’ve been able to play an instrument yourself. In instruments like the violin or guitar, it’s easy to see that changing the length of the string changes the pitch. But is this true for all instruments? Do this experiment and find out!
Student Guide
Straw Trombone
Experimenting with instruments and their sounds.
How does changing the length of an instrument change the pitch?
This resource was originally published in PhysicsQuest 2017: Sound.
How does changing the length of an instrument change the pitch?
- Smaller diameter straw
- 2x large diameter straw
- Ruler
- Scissors
Intro
Before the experiment
- How does changing
How does changing the length of an instrument change the pitch?
Setting up
- Take one of the white bendy straws
Take one of the white bendy straws and cut the top in a “v” with the pointy end at the top.
- Slide the black straw
Slide the black straw in the bottom of the bendy straw. It might be a bit hard to fit it in, but with a little help, it should.
- Slide the second white straw
Slide the second white straw on the bottom of the black straw.
- Made a straw trombone
You have now made a straw trombone.
During the experiment
Collecting data
- Take the black straw
Take the black straw out of the cut, bendy straw and blow through the cut end. You should hear a buzzing sound. If it doesn’t work, keep trying. It’s a bit tricky to get to work, but once you do, you’ll know.
- Put the black straw back
Put the black straw back in the end. Blow on the straw. How did the pitch change?
- Add the second white straw
Now add the second white straw to the end of the trombone and blow. How did the pitch change?
- Move the straw up
Move the straw up and down, making the trombone longer and shorter. What makes the pitch higher? What makes it lower?
- Start with the straws
Start with the straws as far apart as they can go so that your trombone is as long as possible. Measure the length. Blow and listen to the pitch. If you have access to a smartphone you can download pitch-detecting software and measure the pitch directly. I recommend the Physics Toolbox app for this, but use whatever pitch detecting software you’d like. It will give you the frequency of the sound you are making.
- Now take out the black straw
Now take out the black straw and make your trombone as short as possible without cutting the straw. Listen to the pitch. If possible, use the app to again measure the frequency of the sound you are making.
- Not using a smartphone
If you are not using a smartphone, rank the highest pitch you hear as “10” and the lowest pitch you hear as a “1”.
- Put the trombone
Put the trombone together again so it is as long as possible. Move the straw up by 1 cm and blow again.
- Rank the pitch
Rank the pitch on a scale from 1 to 10 or record the measurement from the app.
- Repeat step 4
Repeat step 4 until you can’t make the trombone any shorter by cutting it. Cut 1 cm off the bottom of the top straw and record the pitch.
- Do this
Do this until you can’t make any noise. You should have a very short straw.
Analyzing data
- Qualitatively, how did the pitch change
Qualitatively, how did the pitch change as the length of the staw changed?
- Graph the frequency
Graph the frequency of the sound (pitch) vs the length of the straw.
- What does the graph
What does the graph look like? Is it a line? A curve?
- As the length
As the length of the straw changes, how does the pitch change?
Conclusion
- How does changing
How does changing the length of an instrument change the pitch?