The sound of clapping, explained by physics

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

The research pinpoints a mechanism called a Helmholtz resonator — the same acoustic concept that underlies the sound made when you blow across the top of an empty bottle. Experiments using baby powder to map the flow of air, alongside pressure measurements and high-speed video, confirm that explanation, researchers report in a paper accepted in Physical Review Research.

A Helmholtz resonator consists of an enclosed cavity of air — like the inside of a glass bottle, or the space between clapping hands — with an opening connected to the cavity by a neck. Air vibrates back and forth within the neck, creating sound waves of a frequency that depends on the volume of the cavity and the dimensions of the neck and opening.

When a person claps their hands, a jet of air streams out of a gap where the hands meet, between the thumb and forefinger. “This jet of air carries energy, and that’s … the initial start of the sound,” says mechanical engineer Yicong Fu of Cornell University. The jet kicks off vibrations of the air. Fu and colleagues saw a similar effect using cup-shaped silicone models designed to mimic palms slapping together.

The researchers studied clapping in different configurations: cupped hands, flat hands with palms clapped together and fingers hitting a palm. The frequencies of sound the team recorded matched the predictions of the Helmholtz resonator theory. For example, cupping the hands when clapping produced a larger cavity — and a lower-pitched sound — than clapping with flat hands. 

Understanding the physics of hand clapping, Fu says, could help develop methods to identify people by their claps — for example, allowing users to log into a device based on their unique clap. Or it could help musicians fine-tune songs with the perfect hand-smacking beat.