Question
My question of the week is about the acoustics of the didgeridoo. Could you please explain the science behind the cubic capacity of the internal chamber of the didgeridoo, in relation to it's length? Also, if they are tuned to a western key, are they louder than an off-key didgeridoo?
Answer
Neville Fletcher, Australia National University:
Over the past few years we've been doing a nice research project on the didgeridoo. It's the trunk of a small tree that's been hollowed out by termites, cleaned out, and somewhere between 1 and 1.5m long typically. It can either be pretty much cylindrical or flaring a bit depending on the sort of tree it's come from. It's played by blowing it very much as you would blow a trumpet or a trombone by vibrating your lips. The longer the didgeridoo the lower the note it will make. If it's about 1.5m long it makes a drone which is about 2 octaves below middle C. That's about 65 vibrations per second, if it's cylindrical. If it's conical so that it flares out at the far end then it plays a higher note. And if it's shorter it also plays a higher not rather like the fact that a trumpet plays a much higher note than a trombone, for instance.
The main thing about a didgeridoo is that you can change the actual sound quality and doing that the player changes the shape of his mouth by moving his tongue: very much as you would if you're saying vowels. If you go, "aaaeeeiiiuuu." In addition people have developed a technique called circular breathing where you fill up your cheeks with air to keep the drone going and then you quickly snatch a breath through your nose. You can keep the sound going for minutes and minutes at a time.
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