Sonic booms can occur in fairly routine settings: for example, it is a sonic boom you hear when a whip cracks. But in your bathtub? Apparently, whenever a hard object falls into a pool of water, a jet of air is produced that briefly reaches supersonic speeds.

To study this, physicists at the University of Twente, in the Netherlands, and at the University of Seville set up an experiment in which they plunged a disc-shaped object flat down into water at the relatively leisurely speed of one meter per second. 

The disc displaced the water and created an air bubble in its wake as it sank.

As the water closed in to form the bubble, it pushed air up through a tighter and tighter neck, making it accelerate.

"It’s like a little nozzle which closes," explains Twente's Detlef Lohse, similar to what happens in a rocket engine. To track the air's motion, the team filled it with glycerin droplets produced by a smoke machine of the type used in dance clubs.

Using a high-speed camera and with the aid of computer simulations, the researchers estimated that the jet reached a speed of 350 meters per second at its peak, or just above the speed of sound, the team reports in an upcoming issue of Physical Review Letters.

Although for objects of different shapes and sizes many details may change, "the physics is the same," Lohse says. "Dropping a stone into the water you create a supersonic jet."

The researchers now plan to study the sound waves produced by the air jets created in the plunging object experiment.

Credit: Detlef Lohse