Home > Physics, Playing techniques, Yidaki > The sound of a Didgeridoo 

The sound of a Didgeridoo 

The didgeridoo is a musical instrument that, for thousands of years, was known and played only by the Australian aboriginal people. I have already written much about this instrument on this blog, but since I have not written anything here for a long time I thought I should reintroduce the instrument and explain why I find it so fascinating.

The didgeridoo, which actually has various names coming from the different languages of the many aboriginal groups that share it, is a simple tube of wood or sometimes bamboo or hollowed pandanus palm trunk. In the case of wooden instruments they originate from termite hollowed eucalyptus trunks. They are instruments that are found in nature. The work of the artisan is in the discovery of appropriately hollowed trees, the cutting down of the tree, cleaning of the interior and sculpting of the exterior to follow the internal shape. Clearly the sound of every instrument is different although each artisan generally searches for instruments that have a characteristic timbre.

To play the didgeridoo one produces a basic, or fundamental, tone by sealing the mouthpiece with pursed lips that then vibrate as air flows through them thus producing standing waves that resonate with certain frequencies depending on the geometry of the tube.  This basic tone is predominantly of a low frequency between about 65 and 90 Hz, between a C and an F. Traditionally slightly higher and lower fundamental frequencies are also used though they are not as common.

My initial and continuing fascination with this instrument is related to the richness of its sound and the personal expressiveness available to every didgeridoo player. The supreme expressive musical instrument is arguably the human voice. The didgeridoo displays many elements of this expressiveness that interact with the intrinsic musical characteristics of the instrument being played. 

To understand better this claim it is useful to consider the acoustics of the instrument. Of course at a basic level the didgeridoo is simply a tube with resonant frequencies and playing it with vibrating lips will cause this air-column to vibrate with these frequencies which are determined by the physical characteristics of the instrument. Obviously the most important resonance is the the fundamental one, the basic note that the instrument plays and this is roughly dependent on the length. The actual timbre of the instrument is then determined by the series of higher resonances and they can vary dramatically. To understand this point in a little more detail it is useful to have a basic understanding of the harmonic series.. 

The harmonic series is most intuitively understood as the frequencies of vibration of a taut string of fixed length. The lowest frequency harmonic has frequency f. The subsequent harmonics then all have frequencies that are simply multiples of f, i.e. 2f, 3f, 4f, …. In musical terms, if the lowest frequency corresponds to the note A, then the higher frequencies in the harmonic series correspond to another A (an octave above the first), then E then another A (two octaves above the first), then in order C sharp, E, G, A,…

Getting back to the didgeridoo, a perfectly cylindrical didgeridoo has as resonances only the odd-harmonics, i.e. f (A), 3f(E), 5f(C sharp), 7f(G) ..etc.. whilst a conical didgeridoo has resonances corresponding to flattened harmonics of the cylinder (depending on mouthpiece radius and how conical it is). In a typical  traditional instrument one does not have any of the frequencies of the harmonic series above the fundamental frequency f. 

Up to this point there is nothing in particular that distinguishes the didgeridoo from other musical instruments. Actually it seems to have a very big disadvantage as it can produce a series of notes that have no particularly nice musical relationship to each other. Like a badly out of tune alphorn. 

Physically the crucial difference is easily observed. It is the mouthpiece! Of course in size it is just like the mouthpiece of say a trombone or a bass tuba, but there is a crucial difference. Brass instruments have mouthpieces that have the form of a small cup with a hole in the bottom, didgeridoo mouthpieces have almost parallel walls and although in some cases there is a slight narrowing in the first 10-20cm (typically yidaki have mouthpieces of this form) it is negligible compared to the extreme narrowing of brass instrument mouthpieces. The cup shape of the brass mouthpiece is necessary to align all of the harmonics of the instrument into a full harmonic series, without the mouthpiece the instrument does not have a nice harmonic series, and this is one of the almost magical properties of the cup (effectively a Helmholtz resonator). This cup with small hole also quite effectively isolates the player’s vocal cavity from the instrument while it is precisely the coupling of the player’s vocal cavity to the didgeridoo sound column that allows the player to produce incredible variations in the sound of the instrument. I like to think of the didgeridoo as a resonant extension of the vocal/oral cavity and the sounds produced are a consequence of the acoustic properties of the instrument plus vocal cavity of the player.

At a very simple level this coupling between vocal cavity and didgeridoo means that when you use your voice in the instrument it can easily be heard together with the notes of the didgeridoo. The voice thus is a second sound generator and when the sung note is in is not the same as the didgeridoo note one begins to create interesting effects akin to ring modulation used in in synthesisers. However the more interesting effects are related to the beats and vibrations that one can hear when making certain tongue movements. The simplest tongue movements produce simple sounds, tongue movements like ta, da, ki, ka together with the simplest tongue and cheeks movements used during the inbreath phase of playing. The more unique didgeridoo sounds are those produced by the turbulent and pulsing movement of air inside the mouth, a little like waves that bounce around inside a breakwater compared to waves that travel freely in the sea. These produce all the additional pulsations and embellishments of the basic sound and the true masters of control of these effects are the aboriginal Australian custodians of this instrument. For them the basic tongue movements are more complex than those mentioned above, coming from their spoken language, often incorporating also the tongue movement for breathing. These movements create often drastic changes of pressure inside the players mouth which in turn modulate the sound of the didgeridoo producing the complex characteristic sound of the instrument.  

In future blog posts I would like to return to this discussion and carefully analyse a couple of the simplest traditional sounds, and also the wobble – a contemporary technique nevertheless originating from aboriginal players Mark Atkins and Alan Dargin – as examples of the above described effects.  

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