When you strike a Tibetan “singing bowl” it gives off a long harmonious sound; when you rub it with a little mallet, the sound is emitted constantly, until a pulse phenomenon is observed (the bowl begins to produce a kind of hooting sound that is much appreciated by certain people as part of their meditation practice). But if you rub the bowl while it is filled with water, waves appear on the surface of the liquid, followed by droplets being formed, as if the water were coming to a boil. What exactly is happening with this bowl?

The story begins with Denis Terwagne, at the time when he was a doctoral candidate in physics working in GRASP (Group for Research and Applications in Statistical Physics), a unit at the University of Liège. He went to work in the laboratory of a certain Professor Bush for three months, as a visitor at MIT in Cambridge, Mass. (USA) The director of a neighbouring therapy centre, whose therapeutic methods were based on relaxation, showed up one day at the laboratory, bowl in hand. “Could you study what is happening with this bowl?” she asked Professor Bush. The professor quickly gave the task to the young doctoral candidate from Liège. Mission accomplished, with the publication of the results in  Nonlinearity (1).

Tibetan bowls are apparently not Tibetan. It seems that the bowls really come from Nepal (or from the Himalayan region), and the appellation “ Tibetan” has simply gotten attached to them for commercial reasons, the Western public being attracted to anything that is supposed to be related to Tibet. Their use has also been a topic of controversy. Are they musical instruments? Are they part of some sort of worship practice? No doubt they are both. In the West, they have become popular as aids to relaxation or meditation. What is certain is that they have remarkable acoustic qualities, being made up of an alloy of up to twelve different metals. They vibrate harmoniously when they are struck and produce an enchanting melody when they are rubbed with something like a bass drum mallet. When the bowls are filled with water another phenomenon is produced. If one rubs the lip of the bowl, the surface of the water ceases to be flat (see the video). Waves appear, and if the vibration is increased, droplets will bounce up on the surface of the water, as if the water was being brought to a boil. These droplets appear to levitate above the surface of the liquid in the bowl. These waves were described in 1831 by the English physicist Faraday, and are now known as Faraday waves.

(1)Tibetan singing bowls, Denis Terwagne, John W. M. Bush, Nonlinearity 24 (2011) R51-R66