Two oscillators in the double-wall bottle

The double-walled water bottle demonstrates two distinct resonances. When held at the neck and struck on its side, it makes a low-pitched, long-lasting tone that is due to the inner and outer walls oscillating in opposite directions. A second distinct tone, the well-known Helmholtz resonance, can be excited by blowing across the neck or striking the base of the bottle with a soft mallet or heel of the hand. In a lab setting, students can use free audio software to determine the frequencies of these two resonances. These frequencies shift in opposite directions as students vary the amount of water in the bottle, and students may hear beating of the two frequencies when they cross. As a subject for a laboratory activity, the double-wall bottle illustrates the interplay between experimental data and theoretical models in a familiar object.