Round of Applause

Scientists may have found a way to recognize people by the way they clap their hands.

A new study researched the physical mechanism and fluid dynamics of clapping hands to understand how different people clapping produces different sounds.

Senior author of the paper Sunny Jung was inspired to research the physics behind hand clapping after watching the film “X-Men: The Last Stand,” where one character claps her hands to send out shock waves and takes out the opposing army’s weapons.

“It made me curious about how the wave propagates when we clap our hands,” said Jung.

Clapping hands is a common human activity and form of communication. To study it the researchers used high-speed cameras to track 10 participants who clapped hands in different ways – by cupping their hands, by keeping them rigid and flat, and by clapping their fingers to the opposing palms, Cosmos Magazine explained.

They also considered the size, shape, and texture of the hands, the force, speed, and pressure used to clap, and the sound produced. They sprinkled baby powder on the participants’ hands to see the airflow produced by the clapping.

Researchers discovered that the sound of a clap is produced by air being pushed through the space between the palms and the gap between the thumb and index finger.

Clapping with cupped palms leaves a larger gap between the palms, which results in a lower frequency and a deeper sound compared with other methods.

The geometry of clapping hands and the resulting airflow patterns resemble those of Helmholtz resonators, which create sound by blowing air across the opening of an empty bottle. The more liquid in the bottle, and therefore the smaller the cavity left for air to circulate, the higher the pitch of the sound produced.

The team confirmed both experimentally and computationally that the Helmholtz resonator can predict the sound of clapping.

However, claps are much shorter than the sound made by the resonator because the soft tissues of the hands vibrate after impact, absorbing energy and dampening the sound. The more vibrations in the material, the quicker the sound attenuates.

“The hand clap is actually a very characteristic thing because we have different sizes of hand, techniques, different skin textures, and softness – that all results in different sound performances,” said first study author Yicong Fu. “Now that we understand the physics of it, we can use the sound to identify the person.”