Georg
von Hippel

As everybody knows, Australia is the land of cangaroos, koalas and emus. It is also the country that gave the world the didgeridoo (or didjeridu), which is possibly the world's oldest wind instrument. For those who haven't encountered this bizarre-sounding (at least to classically-trained Western ears) instrument, a didgeridoo (called yidaki, or mago, by its Aboriginal inventors) is a wooden pipe of of 1.2 to 1.5 meters length, which is traditionally made from a tree that has been suitably hollowed out by termites (though mass-produced modern didgeridoos are often hollowed out by hand, or even made from PVC pipes).

It is played using the technique of circular breathing (breathing in through the nose while breathing out through the mouth using the tongue and cheeks to expel the air) while continuously vibrating the player's lips to produce the instrument's typical drone. What most pointedly distinguishes the didgeridoo from Western wind instruments, though, is the role that the player's vocal tract plays, and a team of Australian physicists (who else?) at the University of New South Wales have investigated this using microphones inserted into player's oral cavities.

The vibrating lips of the player produce a sound wave that travels into the instrument, but which is also reflected back into the player's vocal tract. The vocal tract is an enormous complicated resonator that can boost some frequencies and suppress others, thus enabling human speech and singing. Normally, the vocal tract operates as a modifier for sounds produced by the vocal folds, but for a didgeridoo player, the vocal tract is working backwards: it is still a resonator, but now the sound waves are coming in from the lips instead of going out. No matter whether speaking, singing, or playing the didgeridoo, though, the frequency spectrum of the vocal tract (i.e. the frequencies at which it resonates, and how strongly it resonates at each of them) is determined by the vocal tract's current shape, which is most strongly controlled via the position and shape of the tongue.

Depending on the position and shape of the tongue, vibrations in the vocal tract can be enhanced or inhibited at different frequencies, creating bands of enhanced frequencies called formants. Formants play a crucial role in human speech, where different formants identify different speech sounds, and thus our ears have evolved to recognize them easily.

By measuring the resonance spectrum of didgeridoo players' vocal tracts with a device consisting of a sound generator and a microphone that were inserted into the players' mouths, a team of physicists at the University of New South Wales were able to determine that the secret to effective didgeridoo playing was to create a very strong formant, which requires to keep the vocal fold almost closed, much like in speech.

While the shape of the vocal tract also plays a role in other wind instruments, its influence there is relatively minor, making the didgeridoo the cangaroo of wind instruments: yet another thing that is different "down under".