Peter L. Hoekje
Phys. Dept., Univ. of Northern Iowa, Cedar Falls, IA 50614-0150
Two demonstrations illustrate the mechanism of sound production in musical wind instruments. First, a ``water trumpet'' inspired by A. H. Benade produces waves in a water-filled trough. The wave action is generated at one end of the trough by a feedback valve which regulates the inlet flow of water according to the height of the water waves at that same end of the trough. This is analogous to the generation of sound in lip reed brass instruments and cane reed woodwinds in which the flow of air through the reed valve is affected by the pressure inside the instrument mouthpiece. The second demonstration again mimics the feedback mechanism of the reed with a simple electronic device which allows a wind instrument to sound itself. The air in the mouthpiece is excited by a small speaker which is driven by an amplified signal from a microphone sensing the pressure in the mouthpiece. The playing frequency depends on the resonances of the instrument and corresponds to a normal note of the instrument. Thus the demonstrator can finger a scale on the instrument and the instrument will play along.