Physics finally explains the sound of clapping

A round of applause, please. Scientists have finally figured out what’s behind the sound of hand-clapping.

It’s the same concept at work when you make sound by blowing across the top of an empty bottle. Helmholtz resonance is its formal scientific name.

New experiments used baby powder to map the flow of air emitted by hand-clapping. This confirmed that clapping hands become Helmholtz resonators. Pressure measurements and high-speed video of clapping backed up those findings.

Researchers shared their results in a paper accepted in Physical Review Research.

To make a Helmholtz resonator, all you need is an enclosed pocket of air with an opening connected by a “neck.” The inside of a glass bottle works. So does the space between clapping hands.

As air vibrates back and forth within the neck, it creates sound waves. The pitch of those waves depends on the volume of the air pocket. It also depends on the dimensions of the neck and opening.

When someone claps their hands, a jet of air streams out of a gap where the hands meet. It goes between the thumb and forefinger. “This jet of air carries energy,” explains Yicong Fu. That’s “the initial start of the sound.” The jet kicks off vibrations of the air.

Fu is a mechanical engineer at Cornell University in Ithaca, N.Y. His group saw a similar effect when they used cup-shaped silicone models to mimic palms clapping.

The team studied different types of claps. Some tests smacked together cupped hands. Others clapped flat hands. Still others involved fingers hitting a palm. The pitches of those claps matched what would be expected for a Helmholtz resonator.

For instance, Helmholtz resonators with bigger air pockets make deeper sounds. And cupped hands produced lower-pitched claps than flat hands. That makes sense, since cupped hands created a larger air pocket than flat hands do.

Understanding the physics of hand-clapping, Fu says, could help identify people by their claps. Someday, users might log into a device based on their unique clap. The findings could help musicians fine-tune songs with the perfect hand-smacking beat.