Helmholtz resonator experiment for Project Listen Up.

Abstract

The behavior of sound waves and resonant effects can be observed using Helmholtz resonators. Resonators are built from identical round wooden boxes purchased from a craft store. The tops are glued and drilled with hole diameters from 2–3 cm. We can study the resonant behavior as a function of hole diameter only. The end effect is large compared with the thickness of the orifice (0.6 cm). In the experiment, a swept tone (100–900 Hz) from a small speaker drove the resonator. One studied the output of the microphone located near the orifice. One could use a swept spectrum analyzer, a function generator, or tuning fork with varying weights to measure the resonant frequency of the box. A plot of hole diameter versus frequency can be compared with the theoretical Helmholtz resonant frequency prediction, which depends on the volume of the box, the cross sectional area, the sound speed of air, and the effective length of the hole. One can model the effective length of the resonator as the thickness of the top plu...