The sounds of the water music of Vanuatu

Randy Hurd – randyhurd@weber.edu

Weber State University, Department of Mechanical Engineering, Ogden, UT, 84408, United States

Additional author: John Allen

Popular version of 5aMU3 – Acoustics of the Vanuatu Water Music
Presented at the 189th ASA Meeting
Read the abstract at https://eppro02.ativ.me//web/index.php?page=Session&project=ASAASJ25&id=3981726

–The research described in this Acoustics Lay Language Paper may not have yet been peer reviewed–

Women in the island nation of Vanuatu create music in a unique way. Standing waist deep in a pool, they strike the water with their hands creating a unique variety of tones (see Figure 1). While the acoustics of inanimate objects entering water (such as spheres and raindrops) have long been understood, the mechanisms governing human hand strikes have received less attention. For this study, we replicate and simplify these musical techniques in a controlled laboratory environment to analyze the physical properties—the hydrodynamics and the resulting acoustic profile—of the sounds produced.

Figure 1: Women from the Leweton Cultural Group in the Banks Islands of Vanuatu dance together while interacting with the water surface to create music. (Image courtesy of The Secrets of Vanuatu Water Music. Directed by Marc Hoeferlin, ARTE France and ZED, 2015)

To isolate and measure these effects, we recreated the water-slapping motions in a transparent water tank. We used a high-speed camera to capture the subsurface cavity formation in detail (see figure 2), and recorded the sounds with both an in-air microphone and an underwater hydrophone.

Figure 2: A series of high-speed image sequences portray simplifications of four different techniques used by the women of Vanuatu to create music. a) A flat-handed slap produces a wide and shallow entrained air cavity. b) A cup-handed slap produces a slightly deeper cavity. c) A plunge with a deep hand produces a deep cavity that collapses in the final image. d) A horizontal plowing motion entrains air behind the hand (50 ms between images).

The key finding of this work is the establishment of a direct link between the physical motion of the hand, the shape and size of the air cavity created, and the acoustic characteristics of the sound produced. We find that the way the hand interacts with the water creates different subsurface cavities and control the volume and tone of the sound produced. Even hand-shape upon impact is shown to affect the resulting tone. In essence, the research demonstrates that the tone and duration of the sound are primarily controlled by the size and shape of the entrained air cavity. The larger the cavity, the deeper and longer the resulting sound.

The women of Vanuatu are incredibly sophisticated in their approach to creating music. They manipulate the sound spectrum without needing different instruments, simply by varying parameters like hand pose, curvature, and depth of penetration. This is a powerful demonstration of how multiphase flow, water entry and acoustics can produce an enriching and aesthetically complex experience.

Measuring the sounds of microbubbles for ultrasound therapy

T. Douglas mast – doug.mast@uc.edu

Instagram: @baglamist
University of Cincinnati, Cincinnati, OH, 45224, United States

Popular version of 2aBAa7 – Measure for measure: Diffraction correction for consistent quantification of bubble-related acoustic emissions
Presented at the 188th ASA Meeting
Read the abstract at https://doi.org/10.1121/10.0037529

–The research described in this Acoustics Lay Language Paper may not have yet been peer reviewed–

Microscopic bubbles, when caused to vibrate by ultrasound waves, can be powerful enough to break through the body’s natural barriers and even to destroy tissue. Growth, resonance, and violent collapse of these microbubbles, called acoustic cavitation, is enabling new medical therapies such as drug delivery through the skin, opening of the blood-brain barrier, and destruction of tumors. However, the biomedical effects of cavitation are still challenging to understand and control. A special session at the 188th meeting of the Acoustical Society of America, titled “Double, Double, Toil and Trouble – Towards a Cavitation Dose,” is bringing together researchers working on methods to consistently and accurately measure these bubble effects.

For more than 30 years, scientists have measured bubble activity by listening with electronic sensors, called passive cavitation detection. The detected sounds can resemble sustained musical tones, from continuously vibrating bubbles, or applause-like noise, from groups of collapsing bubbles. However, results are challenging to compare between different measurement configurations and therapeutic applications. Researchers at the University of Cincinnati are proposing a method for reliably characterizing the activity of cavitating bubbles by quantifying their radiated sound.

A passive cavitation detector (left) listens for sound waves radiated by a collection of cavitating bubbles (blue dots) within a region of interest (blue rectangle).

The Cincinnati researchers are trying to improve measurements of bubble activity by precisely accounting for the spatial sensitivity patterns of passive cavitation detectors. The result is a measure of cavitation dose, equal to the total sound power radiated from bubbles per unit area or volume of the treated tissue. The hope this approach will enable better prediction and monitoring of medical therapies based on acoustic cavitation.

Figure 1: In an experiment simulating drug delivery through the skin (left), a treatment source projects an ultrasound beam onto animal skin. A passive cavitation detector (PCD) listens for sound radiated by bubbles at the skin surface, while the skin’s permeability is measured from its electrical resistance. Measured bubble activity is quantified using the sensitivity pattern of the PCD within the treated region (highlighted blue circle).

The researchers reported results from two experiments testing their methods for characterizing cavitation. In experiments testing ultrasound methods for drug delivery through the skin (Figure 1), they found that total power of subharmonic acoustic emissions (like musical tones indicating sustained vibrations of resonating bubbles) per unit skin surface area consistently increased when the skin became more permeable, quantifying the role of bubble activity in drug delivery. In a second experiment (Figure 2), the researchers quantified bubble activity during heating of animal liver tissue by ultrasound, simulating cancer therapies called thermal ablation. They found that increased bubble activity could indicate both faster tissue heating near the treatment source and reduced heating further from the source.

Figure 2: An ultrasound (US) array sonicates animal liver tissue with a high-intensity ultrasound beam, causing tissue heating (thermal ablation) as used for liver tumor treatments. Increased bubble activity was found to reduce the depth of treatment, while sometimes also increasing the area of ablated tissue near the tissue surface.

This approach to measuring bubble activity could help to establish standard cavitation doses for many different ultrasound therapy methods. Quantitative measurements of bubble activity could help confirm treatment success, such as drug delivery through the skin, or to guide thermal treatments by optimizing bubble activity to heat tumors more efficiently. Standard measures of cavitation dose should also help scientists more rapidly develop new medical therapies based on ultrasound-activated microbubbles.