Interference
Light, Sound and Waves

# Noise-cancelling tuning fork

Practical Activity for 14-16 16-19

Students listen to how the loudness of a tuning fork varies as they rotate it. An introduction to destructive interference.

## Equipment

Each pair of students will need:

• A tuning fork

## Procedure

1. Strike the tuning fork on a suitable surface and hold the fork upright next to their ear.
2. Repeat but rotate the fork slowly about its axis while they listen. They should hear the loudness vary.
3. Repeat but this time identify the number of times there is silence per rotation.

## Discussion prompts

• How many sources of sound does a tuning fork have?
• How can two sound waves cancel each other out?

## Teaching notes

The tuning fork has two identical prongs. As they vibrate, each act as a source of sound.

Sound waves from two sources can arrive in step (in phase) and constructively interfere to produce a louder sound or they can arrive out of step (out of phase) and destructively interfere. As students rotate the tuning fork, they should hear four regions of silence.

It is challenging to draw diagrams to illustrate destructive interference for a tuning fork. The distance between the two prongs (a few centimetres) is smaller than the wavelength of the sound (typically a metre or so). If you want to draw diagrams to illustrate overlapping wavefronts use an example in which the sources are separated by a distance greater than a wavelength (eg two loudspeakers).

Students are likely to be familiar with noise-cancelling headphones. They could research how these work. Like the tuning forks they use destructive interference to cancel sounds. The headphones include a microphone which receives sound waves from the environment and an electronic circuit generates an inverted version of these sound waves so that when this is played into the listener’s ears, the two sets of waves cancel out.

## Learning outcome

Students describe how sound from two sources can cancel out through destructive interference.

This experiment was safety-checked in March 2020.

###### Interference
can be exhibited by Progressive Wave
can be analysed using the quantity Path Difference