Have you ever heard an out-of-tune orchestra or a duet that isn’t quite right? Usually it’s just an assault on the ears but if you get past that and listen closely, you can hear some interesting physics. In this activity, you’ll use the music you usually make with soda bottles to explore what happens when two notes are close but not quite right.
Student Guide
Beat Frequency
Experimenting with air and water to create pitch.
What do you hear when there are two sounds that have similar, but not identical, pitches?
This resource was originally published in PhysicsQuest 2017: Sound.
What do you hear when there are two sounds that have similar, but not identical, pitches?
- Two clear plastic tubes
- Two cups
- Water
Intro
Before the experiment
- What causes a sound?
What causes a sound?
- If you did Activity 1
If you did Activity 1, think back to what you learned. What does it mean to say sound is a wave?
Setting up
- Fill each cup
Fill each cup with 1” of water.
- Place one plastic tube
Place one plastic tube in each cup of water. Make sure the tubes are flat against the bottom of the cups.
During the experiment
Collective data
- Blow over the top
Blow over the top of one of the tubes until you hear a sound.
- What do you hear?
- Why?
- Blow over the top other tube
Blow over the top of the other tube.
- What do you hear?
- How does it compare to the first tube?
- Can you hear a difference?
- With a friend
With a friend, blow over both tubes at the same time.
- What do you hear?
- Add a little bit of water
Add a little bit of water to one of the cups. With a friend, blow over the two tubes at the same time.
- What do you hear?
- Add 1/16” of water
Add 1/16” of water to one of the cups.
- Blow over the tops
Blow over the tops of two tubes at once.
- How many notes do you hear?
- How does the third tone compare to the other two?
- Add another 1/16”
Add another 1/16” of an inch of water to the cup and blow over the tops.
- How many notes do you hear?
- How do they compare?
- Keep the first cup
Keep the first cup at 1” and add water until it is 2 ⅜” deep in the second cup.
- How many notes do you hear?
- How do they sound together?
Analyzing data
- What happens when you blow
What happens when you blow on two tubes with the same length full of air (length from the top of water to the top of the tube)?
- How do they sound together?
- What happens when the lengths
What happens when the lengths of the air columns in the tubes is a little different?
- What did you hear?
- What happens when you change
What happens when you change that length a little bit more?
- What did you hear?
- What about
What about when the lengths were very different, like in step 4?
- In this activity
In this activity you learned that sound is a wave and the length of the wave and therefore the pitch you hear is based on the length of the air column vibrating.
- How do the pitches of the same length tubes compare?
- Since sound is a wave
Since sound is a wave, two different sound waves can add and subtract, just like water waves.
- How are the sound waves of different pitches interacting when the pitches are close but not the same?
Conclusion
- Why do you think
Why do you think you heard a third pitch when you “played” two similar, but not the same, pitches using two tubes of close, but not identical, lengths?