A soap bubble’s swan song is a quiet
“pfttt.”

Put your ear next to a soap bubble, and
you might hear a high-pitched sound as it bursts. Now, scientists have characterized
that sound using an array of microphones and analyzed the physics behind the sound of popping bubbles, scientists report in the Feb. 28 Physical Review Letters.

A bubble’s burst begins with a rupture
in its soapy film (SN: 1/12/17). The rupture grows as the
film retracts, altering the forces from the film pushing on the air within the
bubble, physicist Adrien Bussonnière and colleagues report. The shifting forces
cause pressure changes — sound — that microphones can pick up. Additionally, as
the film retreats, soap molecules get packed together near the edge of the film,
changing surface tension in the area, which also alters the forces on the air
and affects the sound.

To study blink-and-you’ll-miss-it events like bubble bursts, scientists typically turn to high-speed video. But the new technique illustrates how acoustics can reveal the changing forces that produce certain sounds, including potentially the rumble from within a volcano or the buzzing of a bee, says Bussonnière, of Université de Rennes 1 in France. “Images cannot tell the whole story.”