The didjeridu is the principal musical instrument of the world’s oldest continuous culture and its sound is an acoustic icon of Australia. This lip-valve instrument usually produces a single, sustained, low drone note. The musical interest comes from rhythmic changes in timbre, including those associated with ‘circular breathing’, wherein exhalation from lungs to instrument with raised velum is alternated with expulsion of air from the mouth and inflated cheeks while air is quickly inhaled through the nose with the velum lowered. In an additional technique, players phonate at frequencies usually above that of the instrument, producing complicated heterodyne components. This paper reviews research from our lab. Measurements of the acoustic impedance spectrum in the mouth during playing show that impedance peaks produce antiformants or minima in the output sound spectral envelope. The instrument’s slight flare and irregular bore geometry produce resonances that do not fall in harmonic ratios. Instruments judged good by players have particularly weak resonances in the formant frequency region so that the vocal tract impedance peaks can dominate the sound spectrum more easily.

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