Input impedance curves for the reed woodwind instruments

Abstract

Equipment has been developed which will automatically plot the input impedance curves of the reed-excited musical instruments as a function of frequency. Sound from a regulated driver unit is passed through a high acoustical impedance into an adapter unit attached to the mouthpiece of the instrument. This impedance is in the form of a capillary of annular cross section, and is designed to have an approximately constant impedance magnitude over the frequency range used. The sound-pressure amplitude generated in the instrument is measured by means of a microphone whose output is applied to a chart recorder, so that a graph of input impedance versus frequency is obtained. Representative impedance curves were run for selected notes on the clarinet, oboe, bassoon, and saxophone. Input impedance values range from 400 to 1600 cgs acoustic ohms for the various instruments. For the low notes on the clarinet, the resonance frequencies are compressed in relation to the harmonics of the fundamental mode, the higher resonances having low frequencies than the corresponding harmonics. For low notes on the bassoon and oboe, the situation is the opposite, the resonances being stretched in relation to the harmonics. For high notes on the oboe and bassoon, the frequency of the blown tone may be considerably more than a semitone below the frequency of the resonance on which the instrument is operating.