Two tuning forks with similar frequencies; one fork is variable in frequency to tune beating.
What it shows:
The interference of waves from two tuning forks of slightly differing frequencies gives rise to beating, that is, a modulated wave of frequency.
νb = (ν1 - ν2)
How it works:
Using two tuning forks of 256Hz, with one of the pair having small clamps (see figure 1) attached to the fork's limbs. These alter the fork's resonant frequency, and adjustment of the clamp positions allows you to adjust the beating frequency of the forks.
figure 1. Tuning fork clamp. Dimensions depend upon limb dimensions.
Setting it up:
Provide a rubber hammer to strike the forks. Set the clamps to give a reasonable beating so the lecturer doesn't have to struggle to find the right settings.
The technique of listening to the beating of two frequencies was used during WWII by pilots of twin engines aircraft; a much more accurate way of matching the plane's engine revs than reading the dials! At a beat frequency greater than 20Hz the beats merge to a tone called a difference tone.