University of Virginia
Physics Department

## Interference

A Physical Science Activity

2003 Virginia SOLs

• PS.1
• PS.8

Objectives

Students will

• understand that two sound waves can interact, or interfere with each other;
• hear how two sounds with slightly different frequencies interfere with each other to form "beats";
• understand that the ear can distinguish multiple frequencies at once.

Background Information

When waves occupy the same spatial region they add their amplitudes. This may result in a new type of wave depending on the frequencies of the individual waves. The addition of two similar waveforms may indicate a pulse called a "beat." The frequency of beats is constant and is determined by how close the two original frequencies are. The closer the frequencies are, the slower the beats are.

### Student Activity

Part I

Materials

• 2 tuning forks (same frequency)
• Modeling clay
• Wood block

Procedure

1. Strike both tuning forks on the wood block or the heel of a rubber-soled shoe.
2. Place the stems of the tuning forks on a table, and listen to the sound they emit.
3. Stop both tuning forks and stick a small piece of modeling clay onto a tine of one of the forks.
4. Now strike both forks again and place their stems against a table. If there is no difference in the sound generated as before, then move the clay either up or down along the tine, or add more or less and repeat until a different sound is heard.

Data Sheet

 What does the different sound generated by the forks sound like? What is it called?   How do you think the modeling clay affected the tuning fork?   Why do you think you were asked to place the tuning fork on the table?

1. The sound generated is called a beat.
2. The modeling clay changes the tuning fork just enough to slightly change its frequency. This difference in frequency is what causes the beat.
3. Putting the tuning fork on the table amplifies the sound.

Part II.

Teacher note: You will probably need to download a tone generator program. A good one that we found can be downloaded for free from the following site. www.nch.com.au/action/index.html Another good resource for wave interference and sound in general is The Soundry. The Soundry has information on the ear and how we perceive sound, the physics of sound, history and applications of sound, and even interactive labs. These labs include Java applets that cover topics such as interference and beats, the Doppler Effect, harmonics, and sound waves in general.

Materials

• 2 computers with tone generators (have only one speaker working on each computer)

Procedure

1. Open the tone generator program on both computers.
2. Set the frequency on each computer to 440 Hz. Start the tone generators on both computers.
3. Have students walk around the room to hear the constructive and destructive interference.
4. Stop both tone generators.
5. Leave one of the tone generators on 440 Hz, but change the other one to 430 Hz. Start the tone generators on both computers.
6. Have the students hear the beats produced.
7. Change the second tone generator to 431 Hz (thereby decreasing the difference in frequency by 1 Hz).
8. Continue increasing the frequency of the second tone generator gradually until it is at 439 Hz.
9. Have students hear the change in the beats as the frequencies get closer to each other.

Data Sheet

 What did you hear in different parts of the room when the two frequencies were the same?   What did you hear when the frequencies were 10 Hz apart?   How did the sound change as the difference in frequencies decreased?

1. There were places where the sound was louder and places where there was almost no sound.
2. There were beats.
3. The beats were slower (or farther apart).

Extensions

Have the students move the clay or add more clay to change the pitch more. Have them compare how the beats sound at frequencies that are closer together and farther apart.

Have two students who play musical instruments (preferably the same instrument) demonstrate how they tune to each other by listening to beats. They may not be experienced enough to do this, however.

Using the tone generators on the computer, play the same frequency from the two computers. Place students at the points of maximum constructive and maximum destructive interference. Try to find a pattern in this.

Students with Special Needs

Adapt as needed to children who have physical disabilities.

Click here for further information on laboratories with students with special needs.

Assessment

Data sheet to be completed during the laboratory.

Extensions

Have the students move the clay or add more clay to change the pitch more. Have them compare how the beats sound at frequencies that are closer together and farther apart.

Have two students who play musical instruments (preferably the same instrument) demonstrate how they tune to each other by listening to beats. They may not be experienced enough to do this, however.

Students with Special Needs

All students should be able to participate in this activity, although some students may not be able to hear the beats very well.

Assessment

Data sheet to be completed during the laboratory.