The best sound source we’ve found is a simple metal clicker, such as those made for dog training. They are made from spring steel, and emit a single sharp click when the steel is bent. Check a pet store and ask for a “dog training clicker.” We’ve seen them between $2 and $5 each.

Hold the clicker near the mouth of the tube, start data collection, and press and hold the clicker. Don’t release it, as you’ll then get a second click.

If a dog clicker is not readily available, a few people have found that a small wooden mallet sticking another piece of wood is best. Next best is a good finger snap.

Clapping and metal on metal do not work as well.