Didgeridoo acoustics/ yidaki acoustics

The physics of the didgeridoo (didjeridu/ yidaki/ yiraki), presented for non-specialist readers.c

Ben Lange The didgeridoo or didjeridu is the yidaki or yiraki in the language of the Yolngu, one of the peoples of Northern Australia, where the instrument originated. This page explains how the instrument works and gives a non-technical introduction to some research from our lab. (for which the formal publications are below.)

What is a didgeridoo or yidaki or didjeridu?

    scholarship ad The instrument is deceptively simple: it is just a wooden tube, about 1.2 to 1.5 m long, hollowed out by termites in the thin trunk or branch of a eucalypt tree, and with a ring of beeswax around the mouthpiece for sealing and player's comfort. It is a very unusual instrument: although it usually plays only one note, it is capable of a spectacular range of different sounds, and the rhythmic variation of these sounds is its chief musical interest. The painting on the instrument can have considerable cultural significance.

    The instrument is closed at one end by the player's lips and face. The difference between closed and open pipes is explained in Open vs closed pipes , which compares them using wave diagrams, air motion animations and frequency analysis. Go there for an explanation of the animation below. (The animation is for a simple, cylindrical pipe. However, most didjeridus are somewhat flared and have complicated surface geometries.)

How does the instrument work?

    The player's lips are alternately opened and closed by the higher pressure air flow from the lungs and the tension in the lips. This part of the mechanism is rather like that of a tuba or trombone, and is discussed in Introduction to brass acoustics. Like the brass instruments, the didjeridu or yidaki is a closed pipe.

    What makes its sound so unusual and varied is the strong interaction among
      - the sound waves in the instrument,
      - the sound waves in the player's vocal tract,
      - the motions of the player's lips and
      - the flow of air between the player's lips.
    In the brass instruments, the vocal tract has only a minor effect, but in the didjeridu

    it is very important.

How can it make such a range of sounds?

    A schematic diagram of the acoustic system (not to scale).

    The player's lips produce a sound wave that travels into the instrument, but it also travels in the other direction, into the vocal tract. The vocal tract is a resonator that, in normal speech, can assist the radiation of some frequency bands, but not others. In fact its resonances are what allow us to produce different speech sounds: see voice acoustics for an introduction.

    For a didjeridu player, the vocal tract is working backwards: it still has resonances, but the vibration is (usually) coming from the lips, rather than from the vocal folds. In either speech or in didjeridu playing, the frequencies at which the vocal tract resonates are determined by the shape of the tract, especially by the position and shape of the tongue, and the state of closure of the vocal folds.

      spectrogram of the sound sample given

    In the spectrogram at right, Lloyd Hollenberg illustrates several of the effects discussed above. The horizontal yellow line at the bottom of the spectrogram is the drone (at constant frequency.) The other horizontal lines are harmonics. The larger patches of light colour – the ones that vary – are formants. These are changing with time as Lloyd changes the shape of his tract.

    wav file 470k . . . Download didj sound file in .wav format (470 k) or mp3 file

    In a few places, he vocalises: that is, his vocal folds vibrate while he is playing. In this case, the vocal tract is being driven by vibrations at both ends, by the lip vibration and the vocal fold vibration. The frequencies are different, and the result is not just two pitches, but also notes with pitches corresponding to the sum of the two frequencies, and others as well. (Details in the experimental paper.)

What are the key findings of our recent research?

    Some background: Depending on how you position your mouth and tongue, vibrations in the vocal tract can be enhanced or inhibited at different frequencies. A band of enhanced frequencies is called a formant. They are important in speech, where different formants identify different speech sounds — so our ears are good at detecting them, especially when they change. Producing strong formants, and varying them, is important in didjeridu performance.
Lloyd plays while his vocal tract acoustic response is being measured

    The frequency response of the vocal tract is quantified by its acoustic impedance spectrum, a measure of how difficult it is to produce vibration at a given frequency. Our key technical advance has been to measure the impedance spectrum, inside the mouth, during playing. We do this by injecting a signal having hundreds of different frequencies, and measuring how the tract responds to those frequencies. It is a little bit like sonar, because we can use this information to work out the shape, but that analogy is limited. In the photo, Lloyd Hollenberg, physicist and didjeridu player, plays while having the acoustic properties of his vocal tract measured.

    This process is difficult because it is so noisy inside a wind player's mouth — over 100 dB. That's because the vibrating lips transmit a sound in both directions: into the instrument, and also into the vocal tract. The waves that go into the vocal tract interact with its resonances, and then some frequencies pass into the instrument to emerge in the output sound.

    To produce the strong formants characteristic of didjeridu performance, we found that the player produces two or more strong peaks in the impedance spectrum. These inhibit sound at their frequencies, and the uninhibited frequencies in between are heard as a formant in the sound. For instance, with the tongue very near the roof of the mouth, one can strongly inhibit the frequencies around 1.5 and 2.0 kHz. This results in a strong isolated band around 1.8 kHz that gives the insrument a sound a bit like "ee". (See the technical page for spectra, sound files and film clips.) Our hearing is very good at identifying formants in the speech range, so we are very conscious of their presence, especially when they change over time as the player moves his tongue.

How do experienced and novice players differ?

    It's easy to make a basic sound. Then you have to learn circular breathing. Learning to make these strong formants takes a while. To be able to produce the strong, clear formants requires learning how to produce strong resonances in the vocal tract. Other techniques involve vocalising and playing at the same time: one gets interactions between the vibrations from the lips and from the vocal folds.

    A virtuoso like William Barton (recent soloist with the London Philharmonic and Sydney Symphony) can produce an amazing range of sounds.

Implications for other instruments

    The vocal tract effects are much less spectacular in brass instruments, but still important enough that some composers, such as Luciano Berio, have used them. In a previous paper, we used an artificial system that 'played' the trombone to show the effect of the 'vocal tract' geometry on register, timbre and pitch. The vocal tract in this case was mechanical and therefore of known geometry and acoustic properties. We have since measured those effects on real, human players, during playing.

Have there been previous studies on the acoustics of the yidaki?

    Yes. Several papers have been published, including a few by members of this team. The sound of the instrument has been analysed, and the motion of the player's lips has been photographed.

    In one of our previous papers, we measured the acoustic response of players' vocal tracts while they mimed playing, and that certainly gave clues about the mechanism. But miming does not come naturally to players of this instrument, and we were especially uncertain about the position of the players' vocal folds when miming. Hence the importance of the measurements during performance.

Why did we do this study?

    Our group specialises in the physics of the vocal tract and musical instruments. So we had appropriate background.

    The yidaki or didjeridu is an iconic Australian instrument and, as an Australian research lab specialising in music acoustics, we thought that we should.

    Further, this instrument demonstrates the most spectacular case of the vocal tract influencing the sound. By understanding it, we hope to understand vocal tract effects in other instruments, where the effects are much smaller, but still musically important.

    Then there is curiosity. It is an unusual system with some subtle features and some interesting physics. Science and technology often benefit in unexpected ways from researchers looking into questions just from curiosity.

Circular breathing and vocalisation

    Circular breathing is idiomatic for the instrument (and also gives it its onomatopoeic Western name). For most of the time, the player exhales through the mouth in the ordinary way, but, when new air is needed, the cheeks are filled with air, the soft palate is used to isolate the mouth from the respiratory tract, and the air in the mouth is used to continue playing by contracting the cheeks. Meanwhile, the player inhales quickly through the nose. This requires coordination and a lot of practice. The result is an uninterrupted sound from the instrument. The timbre changes, however: during the normal part of the cycle, the whole vocal tract is connected via the lips to the instrument. During the inhalation, only the mouth itself is involved. There are great differences in the resonances in the mouth for these two configurations, so the timbre of the sound produced changes dramatically. Far from being a disadvantage, players use this timbre difference, along with differences in loudness, to establish the rhythm of the performance.

    Vocalisations are much more common in modern styles than in traditional performance. When the vocal folds vibrate during play, the vocal tract is driven by two vibrations, at different frequencies, from the lip vibration and the vocal fold vibration. The result is not just two pitches, but a complicated set of frequencies corresponding to the sum and difference of the two frequencies, and others as well.

    We discuss the physics of circular breathing and vocalisation in this paper and the effects of vocalisation in this paper.

Other questions

    Does a skilled player use the glottis subconsciously when playing?

    Apparently. None of the players to whom we have spoken are aware of it, except during vocalisations.

    Did your method of sound measurement impair the players' ability to play?

    Sometimes. Many players play with the instrument a bit to one side of the mouth. It was easier for such players to deal with our system.

Why are some didjeridus better than others?

    This is a subtle question and there are several subtle effects. However, there is one over-riding effect: players prefer instruments that do not have very strong resonances in the range 1-2 kHz. This is easy to understand in terms of the story that we tell above: this frequency range is the one in which players can manipulate the formants using the resonances of their own vocal tracts. They prefer instruments whose resonances that fall in this range are weak compared to the resonances of the vocal tract. This is explained in more detail in this paper.

Sounds and pictures

Links for the didjeridu/yidaki and related material

  • The Didj Shop site has a broad range of information about the didjeridu/ yidaki. Its manager, Svargo Freitag is collaborating with this lab on the acoustics of yidaki.
  • Yidakiwuy Dhawu Miwatjnurunydja, from the Yolngu people of North Eastern Arnhem Land, has information about the didjeridu and Yolngu culture in text and in video clips.
  • Home page of William Barton, one of Australia's best known yidaki/didjeridu soloists.

Several students and staff have worked various aspects of the yidaki project:
Alex Tarnopolsky [1], Neville Fletcher [1,2], Benjamin Lange [1], Lloyd Hollenberg [3], Guillaume Rey [1], John Smith [1] and Joe Wolfe [1].

[1] The University of New South Wales, [2] The Australian National University and [3] The University of Melbourne.

This research was done on Bidjigal Country, supported by the Australian Research Council. We thank the Didj Shop and our volunteer players for their involvement.

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