Science for All: The physics of human handclaps

Bollywood actor Shah Rukh Khan, left, claps as cricketer Virat Kohli looks on. (Image used for representation only)
| Photo Credit: Bikas Das

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What’re the odds you clapped your hands in the last 24 hours? Probably very high. Clapping is a common activity for people — so common that we take the sound it makes for granted. But do you know where that sound comes from?

In a study recently published in Physical Review Research, researchers used real-life and experimental data and theory to find the answer.

At the heart of the sound is a Helmholtz resonator — a device that produces sound by trapping and vibrating air within a cavity. When you push air into or out of it, the air molecules oscillate and create sound of a particular frequency. This is also why blowing over an open bottle produces a whistling sound.

The researchers said the air enclosed between the palms is the cavity in a Helmholtz resonator. In an experiment, they deposited dry baby powder into a palm cavity to render the airflow visible. Then as the hands clapped, the researchers tracked airflow behaviour, acoustic signal, cavity pressure, and soft material deformation.

The team found that the first and strongest jet of air was heard at the first impact of the two palms, after which the jet dissipated. They recorded a few secondary and tertiary jets as well but they created little sound. This established the clapping sound was caused by air flowing between the palms, not by the palms’ vibration. Each clapping sound dissipated in about 10 ms.

Not all claps were the same. The team found that different palm shapes made different sounds. The frequency, and therefore the pitch, increased as the shape changed from cupped to palm-palm to palm-finger contact.

Claps by some individuals also had two distinct frequency peaks: a lower one associated with the Helmholtz resonator and the higher one with sound from the finger grooves, when the fingers met the receiving hand. These grooves acted like a pipe open at one end, and the sound was made by air vibrating inside.

Because people clap differently and because fingers can bend to different degrees while clapping, the real-life sound peaks varied from theory.

The team also reported that the extent of skin deformation didn’t affect the frequency much, it could still affect the sound’s intensity and how long it lasted.

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