Researchers deserve a round of applause for uncovering the science behind the acoustics produced from hand clapping that had not been clearly understood until recently.

The daily gesture that people make across cultures conveys support and sympathy, as well as expressing excitement and joy among other emotions.

Clapping is also used to move the hands to the beat while listening to music.

But there's more to the sound than meets the ear.

 X-MEN SUPERVILLAIN INSPIRED STUDY

A team of scientists from Cornell University and other U.S. universities now report that the percussive sound produced is not simply from the collision of palms, contrary to conventional wisdom. 

The sound emanates from a jet of air rapidly strewn out from the narrow gap between the index finger and thumb when the hands meet, in line with the Helmholtz resonance theory in physics.

The Helmholtz resonator is a phenomenon in which a certain sound is heard loudly when air in a container with an open neck vibrates at a natural specific frequency due to the springiness of the air inside, which repeats the cycle of compression and decompression.

The frequency of sound is variable, depending on such factors as the volume of the container and the size of the opening.

In a hand clap, the cavity between the palms corresponds to the container and the gap between the index finger and thumb to the opening of the neck.

A similar, well-known example of the theory is the tone created by blowing air across the mouth of an empty bottle.

One reason that the mechanism behind a clap had been a mystery for a long time is this sound generation is a “complex and unsteady process involving soft-body collision and acoustic wave propagation,” according to a research paper published recently in the online edition of the U.S. scientific journal Physical Review Research.

The research team conducted experiments involving human hands and silicone replicas of human hands to unravel how the sound of clapping is produced.

Sunghwan Jung, one of the authors and professor of biological and environmental engineering with Cornell University, said he was inspired for the experiment on hand clapping after watching the movie “X-Men: The Last Stand.”

In the 2006 Hollywood blockbuster, a mutant supervillain named Arclight emits powerful shock waves against her enemies when she claps her hands.

“We knew that we heard the sound from hand clapping,” Jung said in an email interview with The Asahi Shimbun. “However, it was not clear what frequency and how long this sound will propagate.”

AIR RESONANCE THEORY OFFERS CLUE

The experiment got under way in 2022, with Akihito Kiyama, a Japanese scientist who was working as a postdoctoral researcher at Jung’s lab, leading the project.

Kiyama was selected because of his experience in using audio and video recordings from his previous study on the relationships between bubbles forming and the cracking sounds produced when wet bamboo chopsticks were dipped in hot oil.

As for hand clapping, there was a prior study that suggested, though without concrete evidence, that the clapping sound might be explained by the Helmholtz resonator model.

However, Kiyama, now an associate professor of mechanical systems engineering with Saitama University in Japan, recalled being initially skeptical if the findings of their experiment would confirm the Helmholtz resonance theory.

“I was not really sure because hand claps involved soft materials like humans’ hands,” he said.

The scientists studied a person’s clapping motion and sound, changes in air pressure in hand cavity and air flow by recording them simultaneously with a high-speed camera, microphone, signal amplifier and pressure sensor.

The 10 participants enlisted for the experiment clapped their hands in three hand configurations--cupped, palm-palm and palm-finger.

In addition, silicone replicas of hands were also used for analysis.

The study found that little sound was produced at the very moment when the hands meet, but sound was generated when a jet of air spewed out of an opening between the index finger and thumb.

The researchers also confirmed that the frequencies of the recorded sound matched figures that were predicted based on the Helmholtz resonator theory.

'BEAUTY OF PHYSICS'

Yicong Fu, a lead author and a Cornell University Ph.D. student who succeeded Kiyama in the experiment, said he found the project fascinating because of the interdisciplinary effort that went into it.

“What got me interested in it personally was the multi-faceted physics involved in it,” he said in his email. “There are fluid flow, sound generation, tissue mechanics, dynamical systems, etc. I felt that I could synthesize my knowledge from various courses together to understand this complex physical process.

“Although it’s challenging, I think it was very fun to see how so many different subjects are connected to each other.”

Researchers noted that it took time to deduce why clapping also included sound that cannot be explained by the Helmholtz resonance model alone.

Fu and his colleagues finally unraveled the science behind the sound of clapping three years after the experiment began.

To Fu, the project turned out to be a fresh reminder of “the beauty of physics.”

“I was very amazed and humbled to see such an elegant theory is powerful enough to capture the frequency of this very dynamic and complex phenomenon,” he said. “It was really a time for me to appreciate the beauty of physics--it simplifies complex systems into an easy core that’s fathomable to everybody.”

He added that undertaking an experiment out of genuine curiosity can go a long way.

“I personally think this curiosity-driven research really shows the public that we should pay attention to our everyday lives, because when we think deeply, there are so many new things to learn,” he said.