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The sound of a bubble bursting

By David, 9 April 2020 News

Photo of a little girl blowing bubbles.

Scientists can predict the sound of popping bubbles using maths
Image: ©iStock.com/Monia33

What happens when a bubble pops? Does it make a sound? What causes that sound? A team of French scientists have all the answers.

Recording bubbles

Bubbles might be fun, but studying them is serious business. To do so, the team built a high-tech scientific rig. It included a syringe to blow bubbles, a high-speed camera for video, and an array of microphones to pick up sounds from different directions. And – to deliver the pop – the scientists used a needle aimed at the very top of the bubble.

They captured some great videos of bubbles as they unravelled from the pin prick at the top, all the way down to the bottom. The microphones recorded the sound of the pop, which was louder above and below the bubble, and quieter from the sides.

With all that information, the scientists came up with an explanation, and a set of mathematical formulas to predict the sound of bubbles popping.

Fine-tuning the formula

The team’s formulas for bubble popping were good, but not great. Here’s why.

A pin prick isn’t the only way to pop a bubble. If you leave them for long enough, bubbles get old and explode on their own. This usually happens because the bubble fluid flows down to the bottom of the bubble. When the top gets too thin, it breaks!

For a second round of experiments, the scientists blew bubbles and let them pop on their own. This time, the team noticed something in the videos that made them think. As soon as the bubble started to pop, a visible shock wave travelled quickly through the bubble.

The team worked out how the shock wave would affect the sound, and they came up with an improved set of formulas to predict the sound.

These formulas worked better than their first batch, for both old bubbles and for pin-pricked ones. That means there was also a shock wave in the pin-pricked bubbles that they just couldn’t see!

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