James P. Cottingham
Phys. Dept., Coe College, Cedar Rapids, IA 52402
The interaction of an air jet with a resonator is fundamental to the operation of several musical instruments and a significant topic in musical acoustics courses. Recent studies of a Helmholtz resonator excited by an air jet have explored the dependence of the frequency and amplitude of the Helmholtz mode on jet speed and other parameters. [R. Khosropour and P. Millet, J. Acoust. Soc. Am. 88, 1211--1221 (1990); J. P. Cottingham et al., J. Acoust. Soc. Am. 92, 2380 (1992)]. It has been observed that domains of jet speed, for which a single-frequency Helmholtz mode occurs, are separated by narrow transition regions. Investigation of this phenomenon, which is audible as well as measurable, has been adapted as a set of laboratory exercises for use in musical acoustics courses. Using a homemade air jet with the laboratory compressed air supply, students are able to study the internal sound field of a jet-excited resonator with a small tie clip microphone. Investigation of a significant nonlinear phenomenon is thus possible with simple equipment.