Consists of a pair of U-shaped tubes. One of them fits over the other, so that the length of tube on that side may be adjusted in a way that's similar to a trombone slide. In the center of the bottom tube a speaker is mounted, with a thick piece of rubber tubing used to make the join. In the center of the top tube a horn is mounted.
This demo illustrates very clearly the phenomenon of interference in sound waves. It's particularly effective because it does so in a way that can be easily heard by an entire lecture hall. It can be thought of as an 'acoustic interferometer', so it would work well to explain how laser interferometers work.
Make sure the trombone is firmly set on top of the speaker and connect the speaker to a function generator. 3kHz works well1)
When the input signal is of the frequency of 3 kHz, the first destructive interference happens at the distance of 3 cm, and the first constructive interference happens at the distance of 6 cm , the second destructive interference is at the distance of 9 cm, all those three points can be easily demonstrated from the trombone.
By sliding the trombone out, you can observe where destructive interference occurs. A neat thing you could have students do is given the speed of sound and frequency, have them find this sliding distance.
Demo room information