On the fifth day of Christmas – explore these scientists’ good vibrations of the bondi guitar and didgeridoo. Their relatively simple construction produces some surprisingly complex physics.
The Bondi Guitar
The bondi guitar is a rare and endangered instrument from Indonesia that can imitate the sound of metal bells and cowhide drums (kendang) in a traditional gamelan ensemble. The didgeridoo is a symbolic instrument associated with the culture of Aboriginal Australians that produces a low-frequency single tone which skilled players can sustain. Both instruments are subjects of scientific interest because their relatively simple construction yields some surprisingly complex physics. Two recent studies on their acoustic properties were presented at a meeting last December of the Acoustical Society of America, held in Sydney, Australia, in collaboration with the Australian Acoustical Society.
The Bondi Guitar
The bondi guitar originated with duck hunters in Indonesia as a means of protection from rain and other adverse conditions while working in the field, and also serves as a musical instrument to pass the time. It is a hemispheric structure made from bamboo branches to form an interwoven lattice, crossed at the top to form a dome. This dome is then covered with layers of bamboo branches secured with sugar palm fibers. Musicians typically sit cross-legged inside the dome-shaped sound chamber and scratch the strings and rods to play. The strings produce metallic sounds while the internal panels generate drum-like tones.
Recent Studies
Jia Oswa Fath Barikiset from Gadjah Mada University in Indonesia has been studying the physics and acoustics of the bondi guitar for several years now. And yes, he can play the instrument. He said during a press conference, “I had to learn how to do the research. It’s very difficult because you have two different playing patterns on the right and left sides. The right hand for the tone and strings, and the left hand for the rhythm and scraping the strings.”
Many of Barikiset’s previous research focused on the unusual metallic/drum-like sound of the strings, particularly the critical role played by the positioning of the bamboo clamps. He used computer simulations of string vibrations to glean insight into how the specific metallic sound is produced and how those vibrations change with bamboo clamps added in different locations on the strings. It was found that adding clamps produces vibrations at two different frequencies in various locations on the strings, where the longer part has a high-frequency vibration compared to the low-frequency vibration of the shorter part of the strings. This is key to producing the bell-like sound.
The Wet Instrument
This time, Barikiset was excited about the fact that many bondi guitar musicians noted that the instrument sounds better when wet. In fact, he attended a bondi guitar concert in Melbourne several years ago during the summer when it was very hot and dry – so the musicians brought their own misting bottles to ensure that the instruments stayed thoroughly wet. “The key element between the dry and wet versions of the bondi guitar is the bamboo branches – the material used to cover the walls of the instrument,” Barikiset said. “When the bondi guitar is dry, the bamboo branches open up and produce weaker connections between the adjacent branches. When the bondi guitar is wet, the branches tend to form a twisted shape, but because they are secured with ropes, they form strong connections between the adjacent branches.”
The resulting tension allows the branches to vibrate together. This significantly affects the sound of the instrument, as it gains a “dry” sound when it is dry and a bell-like tone when it is wet. Barikiset has tried making a bondi guitar from other materials such as paper, leaves, and even plastic, but those materials did not produce the same sound quality as bamboo branches. He plans to explore other musical instruments made from bamboo branches next time. “As an Indonesian, I have an additional motivation because the bondi guitar is part of our cultural heritage,” Barikiset said. “I try as much as I can to support the preservation of the bondi guitar and document and conserve other endangered Indonesian instruments.”
The Connection
With Human Vocal Tracts
In the meantime, John Smith from the University of New South Wales feels the same excitement about the physics and acoustics of the didgeridoo. The instrument is built from a trunk or large branches of a eucalyptus tree. The trick is to find a live tree with lots of growth activity, so the trunk is hollowed out, leaving only the living wood bark. Then, a suitable hollowed trunk is cut, cleaned, the bark removed, the ends trimmed, and the exterior shaped into a long cylinder or cone to produce the final instrument. The longer the instrument, the lower the tone or pitch.
Players will vibrate their lips to play the didgeridoo in a manner similar to that of lip-reed instruments like trumpets or moving pipes, except that the latter use a small mouthpiece attached to the instrument as a interface. (Sometimes a beeswax rim is added to the end of the didgeridoo’s mouthpiece.) Players typically use circular breathing to maintain this low, continuous sound for several minutes, inhaling through the nose while using the air stored in their puffed cheeks to keep producing sound. The interplay between the instrument and human vocal tracts makes the physics extremely complex, according to Smith.
Smith was interested in studying how the configuration of vocal tracts changes to produce temporal variations in the rhythmic pattern of the sounds produced. To do this, Smith said during the same press conference, “We had to develop a technique that could measure the acoustic properties of a player’s vocal tract while playing. This involved injecting a wideband signal into the player’s mouth corner and using a microphone to record the response.” This allowed Smith and his colleagues to record the transitions of the vocal tract in different configurations in the mouth.
The results showed that “we have shown that strong couplings in the vocal tracts can suppress frequency ranges in the resulting sound,” Smith said. “The strong residual ranges of frequencies, known as formant frequencies, are perceived by our hearing because they fall within the same ranges we use in speech. They are somewhat akin to a sculptor’s bite into marble, and we observe the remaining parts.”
Smith and others also noted that changes in timbre arise from players singing while playing or imitating animal sounds (like dingo or kookaburra), producing many new frequencies in the resulting sound. To measure the interaction between the vocal folds, they put electrodes on both sides of the player’s throat and shocked them with a small high-frequency current. They simultaneously measured lip movement with another pair of electrodes above and below. Both types of vibrations affect the airflow to produce the new frequencies.
The Desired Instrument
Regarding what makes the didgeridoo desirable to players, acoustic measurements were taken on a set of 38 of these instruments – each evaluated for quality by seven experts across seven different subjective categories – and a surprising result emerged. One might think that players would prefer instruments with very strong formant frequencies, but the opposite turned out to be true. Instruments with strong formant frequencies were rated the worst, while those with weak formant frequencies were rated higher. Smith believes, for example, that this makes sense. “This means their specific vocal tract frequency can dominate the timbre of the tones,” he said.
In the end, the bundengan and the didgeridoo are simple in construction, yet they carry within them complex and fascinating physics. These studies contribute to our understanding of the relationship between construction and sound and enhance interest in traditional music and indigenous cultures.
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