Imma Perfetto
Cosmos science journalist
New research which “observed” sound has revealed the intriguing physics behind one of orchestras’ simplest instrument – the triangle.
Made of a single metal rod bent into the shape of a triangle with one open end, the triangle hangs suspended from a string or wire at the top closed corner. Striking the metal with a beater creates the sound.
This slightly absurd percussive instrument produces a distinct “twinkling” sound of indefinite pitch.
A new study in JASA Review Letters, a peer-reviewed journal which publishes research from all fields of acoustics, has discovered how its triangular shape causes this surprisingly powerful sound.
“The triangle instrument produces enchanting and beautiful tones, raising deep and profound questions about the connection between music and physics,” says Risako Tanigawa of Japan’s Waseda University and lead author of the paper.
Tanigawa and collaborators measured the 2-dimensional sound fields around triangles using acousto-optic imaging.
As sound waves move through the air, they cause vibrations which change air density and therefore the speed at which light travels through the air. Acousto-optic imaging can capture the differences in laser light travelling through this air to study sound vibration patterns.
“Optical sound measurement has only been applied to limited subjects until now,” Tanigawa says.
“By observing the sound field of a triangle for the first time, we captured phenomena not previously explored through microphone observations.”
They found that acoustic resonance occurs in the semi-open triangular air created by the musical triangle.
“Immediately after the impact, the sound waves propagated through the rod of the triangle,” the authors write in the study.
“When the wave reached the bend, the direction of travel changed according to the triangular shape, and the first sound wave interfered with the subsequent sound wave.
The identical sound waves, moving in opposite directions through the instrument, become superimposed on top of each other to produce higher amplitude standing waves.
“These results suggest that the standing waves were formed by the combinations of radiations from the 3 sides of the triangle,” the authors write.
Resonance occurs when the frequency of an external stimulus, such as a strike by a metal rod, matches an object’s natural frequencies. This increases the amplitude of the sound waves to create a much louder sound.
For example, a wine glass could be made to shatter when made to vibrate at its natural frequencies by the sound of an opera singer’s voice.
The researchers found that sounds produced at the triangle’s natural frequencies were louder and lasted longer.
“There is still room for discussion, but the observation of standing waves in a semi-open space is intriguing because it is generally known that resonance occurs in shapes with closed sides,” Tanigawa says.
“Our experimental results suggest it may occur in semi-open space as well.”
Next, the team wants to further investigate the impact of the open space at the corner of the triangle on this resonance.
“Musical instruments play a significant role in human lives, serving cultural, social, and physical functions,” Tanigawa said.
“Understanding the physical properties of an instrument provides benefits such as preserving traditional musical instruments and providing insights for instrument makers, players, and development of electronic instruments.”
