# Coupled Tuning Forks

#### Abstract

Two matched tuning forks are mounted on resonance boxes. Hit one and the other vibrates too.

Yes

#### Principles Illustrated

The transfer of energy from one tuning fork to another that vibrates at the same frequency is easy. The diving frequency of the first tuning fork matches the natural frequency of the second tuning fork, giving rise to resonance.

For a more careful discussion for resonance and its relationship with natural frequency, see hyperphysics: http://hyperphysics.phy-astr.gsu.edu/hbase/sound/reson.html.

#### NCEA & Science Curriculum

PHYS 3.3, PHYS 3.4

#### Instructions

The matched (same natural frequency) tuning forks mounted on boxes will couple to each other effectively. In other words, put them near each other and hit one. The other will vibrate as well. This means energy has been efficiently transferred from one to the other, a resonance effect. On the other hand, a tuning fork with a different natural frequency will not vibrate when placed near the vibrating fork. Energy is not transferred efficiently.

You can actually illustrate this with tuning forks that are not mounted on resonance boxes, although students have to hold the forks up to their ears to hear the sound. Hold the forks near the end of the handles and press the ends together as suggested in the photo above. Again, with different tuning forks there is little or no transfer of energy. Avoid integer multiple frequencies such as 256 and 512 Hz. A 256 Hz tuning fork will drive a 512 Hz fork, but not the other way around.

One can also cause a tuning fork to vibrate using a signal generator and speaker tuned to the natural frequency of the fork. Have students verify that the fork is not vibrating, turn on the sig gen, and then turn off the sig gen. The fork will be vibrating.

#### Safety

Individual teachers are responsible for safety in their own classes. Even familiar demonstrations should be practised and safety-checked by individual teachers before they are used in a classroom.