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Some musical instruments are just wonderful – they respond
quickly and easily, produce a good sound with little effort and
they just ‘feel good’. Others, well, they look nice
on the wall. This summary is as true of the didjeridu as for other
instruments. But why choose the didjeridu to answer the important
question ‘What makes a good musical instrument?’
On most musical instruments, the question is quite difficult.
Because of psychological factors, the experiments must be done ‘double
blind’ – neither the player nor the experimenter should
know which instrument is being tested, so that expectations do
not influence judgment. For a cheap model, the instrument may take
the blame for a poor sound while, for a famous instrument, the
player may try to meet expectations and compensate any shortcomings.
The didjeridu has the advantage that neither player nor experimenter
knows the details of the highly complex bore produced by the appetites
of termites!
In a recent study, we measured various physical properties of
38 didjeridu, whose overall quality was assessed by 7 experienced
players. We found that higher quality instruments had a lower acoustic
impedance in the range 1 to 2 kHz. Previously we showed that the
characteristically varying timbre is produced by the player using
resonances in his own tract to modify the spectrum of the instrument.
Thus an instrument with a low impedance in this region would allow
a greater contribution from the player’s vocal tract and
allow a player more control of the formants in the sound.
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Fig. 1: The
quality rankings of these instruments (from top to bottom)
were 5, 6, 7, 13, 21, 35, 37 and 26, out of a set of 38 instruments.
There is no simple relation between quality and appearance,
which makes the didjeridu ideal for a double blind studies. |
Fig. 2: The
acoustical impedance spectrum is the ratio of sound pressure
to sound vibration, for each frequency. Each peak corresponds
to a resonance. |
John Smith, Guillaume Rey and Joe Wolfe
For more information about our work on didjeridus,
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