It is easy to produce a simple sound but virtuoso didgeridoo playing involves tricky breathing and a certain way of opening the vocal tract, Australian physicists say.

The instrument, which originated in northern Australia and is made from the trunk of a tree hollowed out by termites, usually plays only one note.

But skilled performers can produce a range of resonating sounds.

Scientists at the University of New South Wales studied how the masters do it by using a small tube and a microphone to detect the sounds inside their mouths.

The researchers discovered that the opening of the glottis - the part of the larynx that contains the vocal cords - was the key.

A didgeridoo is about 1.5 metres long and was traditionally used to accompany chants and songs. Its unusual sounds are produced by interactions among the sound waves of the instrument and in the player's vocal tract and the motion and air flow between their lips.

The movement of the lip sends a sound wave into the instrument but it also travels back into the vocal tract that can act like a resonator, boosting some sounds and repressing others.

The scientists measured the frequency response of the vocal tract inside the mouth of the performer.

"We do this by injecting a signal having hundreds of different frequencies, and measuring how the tract responds to their frequencies," Dr Wolfe said.

Skilled didgeridoo players subconsciously reduce the opening of the glottis to produce the instrument's haunting sounds.

- Reuters

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