How a Himalayan singing bowl produces soothing tones, much like a wine glass, has long been a mystery. Now, a team of American researchers have finally solved the enigma behind those rapid knocking sounds, or "chatter".

Recently, a group of undergraduate researchers from Rollins College, Florida, revealed the origin of the "chatter" that occurs when wooden sticks called "puja" are played against the bowl.

The bowls, which originated in the Tibetan mountain region and are made of metal alloys, have been used for meditation and worship for three millennia since around 500 BCE. However, they have found new audiences in contemporary music only recently.

While the complex stick-slip motions responsible for the "chatter" have been extensively studied in other instruments, few studies have investigated this action in the Himalayan singing bowl.

"As the puja moves around the rim of the bowl, it switches very quickly between sticking to and slipping on the metal, which is called 'stick-slip motion'," said Chloe L Keefer, one of the researchers.

This motion is responsible for producing sounds in a wide variety of musical instruments, including the violin and cello, and many additional non-instrument vibrational processes.

Using a laser Doppler vibrometre — a scientific instrument for making non-contact measurements of vibrations of a surface — Keefer's team measured vibrations at several points on the inner rim of the bowl near where the puja contacted the bowl.

"The puja's motion excites the vibration of the singing bowl, causing a unique deflection shape," explained Keefer.

The experiments showed the puja forces a point of zero vibration called "a node" on the bowl, which lies in the vicinity of its contact point.

"The interesting part of the puja's motion is people would expect it to lie on the node of the bowl's vibratory motion, but in fact it doesn't," said Keefer.

Rather, this node lies within two millimetres of the puja.

As the puja rotates around the rim, the node follows behind it, and as the "puja" rotates faster and faster, the displacement or the vibration amplitude of the rim increases accordingly.

When the amplitude of the vibration is large enough, it briefly knocks the puja off the bowl, producing the chatter.

In addition to instruments, the research the team performed can also advance an understanding of brake squeal in automobiles.

The study was presented at the meeting of the Acoustical Society of America (ASA) in Jacksonville, Florida, this week.