University of Virginia
Physics Department

## Interference

A Physical Science Activity

### 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?

Part II.

Materials

• 2 computers with tone generators

Procedure

1. Play both tone generators at the same frequency.
2. Have students walk around the room to hear the constructive and destructive interference.
3. Play the tone generators with different frequencies (approx. 10 Hz apart).
4. Have the students hear that beats produced.
5. Reduce the difference in frequencies by 1 Hz at a time until you only have a 1 Hz difference.
6. 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?