The triangle is a small instrument made of a metal rod bent into a triangle shape that is open at one corner. While small, its sound is distinct, with multiple overtones and nonharmonic resonance. But what causes the surprisingly powerful sound?

"The triangle instrument produces enchanting and beautiful tones, raising deep and profound questions about the connection between music and physics," author Risako Tanigawa said. "Optical sound measurement has only been applied to limited subjects until now. By observing the sound field of a triangle for the first time, we captured phenomena not previously explored through microphone observations."

In a paper published in JASA Express Letters, Tanigawa and colleagues at NTT Corporation and Waseda University in Japan captured sound fields around musical triangles.

Sound moving through the air causes vibrations that affect the air density and the speed at which light travels through the air. Specialized photographic methods known as acousto-optic imaging can capture the differences in light intensity to study detailed characteristics of sound vibration patterns.

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