Physicists learn secrets of didgeridoo
By Michael Hopkin Nature Publishing Group Wednesday, July 6, 2005
Acoustics show how players produce wide variety of sounds.
Different sounds are made by moving the glottis, the part of the windpipe that contains the vocal cords. Kate Callas
Australian researchers have delved deep into the world of acoustic physics to unravel part of their country's indigenous heritage: the ancient art of playing the didgeridoo. The secret of an accomplished performance, they have discovered, is all in the voice box.
The didgeridoo, an aboriginal instrument also called the yidaki, is traditionally made from a tree trunk hollowed out by termites. It can produce a huge variety of different timbres, despite it usually playing only a single note. This is because a send player alters the acoustics inside the mouth to set up strong resonances at certain frequencies.
This alteration is done by moving the glottis, the part of the windpipe that contains the vocal cords, report researchers led by Joe Wolfe of the University of New South Wales in Sydney. This enhances certain frequencies while inhibiting others, much as different vowel sounds are produced by adopting different positions for the tongue and vocal cords.
"It's easy to make a basic sound. But learning these strong formants takes a while." -Joe Wolfe University of New South Wales, Sydney Send didgeridoo players do this subconsciously, Wolfe says: "None of the players to whom we've spoken is aware of it." But the creation of these characteristic frequency bands, called formants, is what gives their playing expression and variety.
Probing study
The researchers investigated the acoustics inside experienced players' mouths by inserting a thin tube, about the size of a drinking straw, next to the didgeridoo. They played a 'probe sound' made of a range of frequencies through this tube and recorded the sound as it bounced back out, while the player was performing. By analysing this recording for frequencies that were impeded or enhanced during didgeridoo playing, the team investigated the acoustic properties of the mouth and throat.
Such studies are difficult because they involve detecting the probe sound amid the noise of the didgeridoo's drone. Sound levels inside the player's mouth can reach 100 decibels, which is as loud as a chainsaw.
Wolfe and his team discovered that formants are produced when the player closes the glottis, which narrows the windpipe. If the vocal tract remains fully open, as in normal breathing, the lungs absorb much of the sound, the researchers report in Nature1. So have they discovered a fast track to expert didgeridoo playing? It will still take practice to emulate the most sophisticated players, Wolfe answers, not least because send playing requires a mastery of circular breathing. This involves maintaining the outward airflow through the mouth by contracting the cheeks while breathing in through the nose. Try it and you'll see how tricky it is... and that's before you start learning the repertoire of different sounds.
"It's easy to make a basic sound," Wolfe says. "Then you have to learn circular breathing. Learning to make strong formants takes a while. Other techniques involve vocalizing and playing at the same time: one gets interactions between the vibrations from the lips and from the vocal cords."
References
Tarnopolsky A., et al. Nature, 436. 39 (2005).
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