Thomas B. Gabrielson
NAWC Aircraft Div., Code 5044, Warminster, PA 18974
David L. Gardner
CDR, NOAA Pacific Marine Environmental Laboratory, Seattle, WA 98115
Resonant tubes are often used in teaching laboratory experiments to demonstrate a wide range of topics from standing waves to lumped-parameter approximations and electroacoustic analogues. The popularity is understandable given the ease of construction and the broad applicability outside the laboratory, ranging from impedance tubes to musical instruments. While the measurement of standing waves and modes is common, the relationships, in an open-ended tube, between the tube, the radiation impedance, and the driver impedance is less frequently addressed. In this experiment using inexpensive components and a Smith chart, the resonances of a small loudspeaker and of an open tube are measured. The radiation impedance at the tube end is transformed to the driver location and the resulting reactance is compared to the loudspeaker reactance. The tube resonances are thus demonstrated to occur when the two reactances are equal, but of opposite sign.