Imma Perfetto
Cosmos science journalist
How does the way we clap our hands effect the sounds we produce? A new study has figured out the physical mechanisms and fluid dynamics of clapping hands, which its authors say could be used one day to identify individuals by their claps alone.
“Clapping hands is a daily, human activity and form of communication,” says Sunny Jung, a professor of biological and environmental engineering at Cornell University in the US and senior author of the paper in Physical Review Research.
“We use it in religious rituals, or to express appreciation: to resonate ourselves and excite ourselves.”
Jung was inspired to explore the physics of clapping after watching the 2006 film X-Men: The Last Stand. In one of the scenes a mutant, Arclight, claps her hands to send out shock waves that take out an opposing army’s weapons.
“It made me curious about how the wave propagates when we clap our hands,” he says.
Yung and his team used high-speed cameras to track 10 participants who clapped using cupped hands; a more rigid flat handed technique; or the demure fingers to palm.
(Nicole Kidman’s viral seal-like method from the 2017 Oscars was not included in this study.)
They assessed the size, shape and texture of the hand, the sound produced, and the force, speed and pressure of the clap.
By sprinkling baby powder onto the hands they could even visualise the airflow produced by clapping them together. They found that the sound of the clap comes from air being forced through the cavity between the palms and the opening between the thumb and index finger.
“It’s the air column pushed by this jet flow of air coming out of the hand cavity that causes the disturbance in the air, and that’s the sound we hear,” says first author Yicong Fu, doctoral student in the field of mechanical engineering.
The larger the cavity between palms, the lower the frequency of the clap – clapping with cupped palms creates a deeper sound than with flat hands or fingers.
“This is also a fundamental principle of the musical instrument,” Jung says.
“Depending on the size of the cavity and the length of the neck opening, you create a different sound – we showed that this also applies to hand clapping.”
The geometry of clapping hands and the resulting fluid patterns of the air resembled that of Helmholtz resonators, the simplest example of which creates sound by blowing air across the opening of an empty bottle. The more liquid, and therefore the smaller the cavity, the higher the pitch of the sound produced.
“We confirmed both experimentally and computationally that the Helmholtz resonator can predict the frequency of the human hand clap,” Fu says.
Claps are much shorter than the sound made through a traditional resonator, and the researchers figured out why. The soft tissues of the hands vibrate after impact, absorbing energy and dampening the sound.
“When there’s more vibration in the material, the sound attenuates much more quickly,” Fu says.
“So, if you want to get the attention of another person very far from you, and you want the sound to last longer, you might want to choose a certain type of hand clapping shape that makes your hand more rigid.”
The research also opens the intriguing idea that a clap could be used to identify the person doing it.
“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,” Fu said.
“Now that we understand the physics of it, we can use the sound to identify the person.”
But not if they are clapping with one hand.