The range of sounds a skilled didgeridoo player can get from a simple wooden tube is astounding. The underlying mechanisms of this acoustic versatility have eluded researchers, but clever use of technology has now revealed one of the tricks of the trade.
Didgeridoos are traditionally played by Australian Aborigines in ceremonial dances. Usually, these instruments can only play one note. The various tone colors result from characteristic pitches--called formants--overlaying the fundamental note. The same phenomenon distinguishes vowels in speech. But how players produce these formants inside a digeridoo was a puzzle.
To see how it's done, a team of physicists led by Alex Tarnopolsky and Joe Wolfe at the University of New South Wales in Sydney introduced a synthesised mix of many sound frequencies into players' mouths while they played the didgeridoo. The team then measured the patterns of sound reflected back to reveal which frequencies the vocal tract--the vocal cords, throat, mouth, and nasal cavity--enhanced and which it impeded. The formants that emerged from the didgeridoo were associated with strong resonances in the vocal tract, the researchers report 7 July in Nature.
As in speech and singing, such strong resonances could only be produced by partially closing the gap between the vocal cords, known as the glottis, say the researchers. Usually left open during breathing, partially closing the glottis prevents sound from escaping into the lungs and enhances reflection of sound from the vocal cords. Skilled didgeridoo players appear to have mastered the art of glottal control, albeit subconsciously.
The study shows that the "didgeridoo is a unique hybrid between the human voice and a musical instrument," with the vocal tract and didgeridoo forming one big tube, controlled by the vocal tract, says Tecumseh Fitch, a bioacoustics expert at the University of St. Andrews in the United Kingdom. As with studies of Central Asian throat singing, "this is another example of ... ethnomusicology really leading to some important scientific breakthroughs," he adds.
Listen to didgeridoo sounds