When Max Koch opened a bottle of homebrew beer one afternoon in Göttingen, Germany, he wasn’t just cracking open a drink. He was launching a scientific experiment into one of the most familiar yet misunderstood sounds in modern life: the fizzy pop of a pressurized swing-top bottle.

Armed with a high-speed camera and a sharp ear for acoustics, Koch—who studies biophysics at the University of Göttingen and moonlights as a home brewer—set out to understand what gives that sound its character. The answer, it turns out, is surprisingly complex.

The team’s findings, now published in Physics of Fluids, show that the soothing sound is not a single burst of pressure. Instead, it’s a short-lived standing wave—an acoustic “ah”—that echoes briefly within the bottleneck.

More Than a Pop

The experiment began almost playfully, with Koch recording his bottle-opening ritual using a high-speed camera. But the footage revealed a cascade of unexpected physics.

Encouraged by Robert Mettin, head of the Ultrasound and Cavitation group at the university, Koch and colleagues expanded their home experiment into a full-fledged scientific study. Alongside visual data, they captured high-fidelity sound recordings and used computational fluid dynamics to simulate what was happening inside the bottle.

“The pop’s frequency is much lower than the resonation if you blow on the full bottle like a whistle,” Koch explained.

So what causes the difference?

According to the team, when the swing-top lid is released, carbon dioxide and air rapidly expand through the narrow neck of the bottle. The sudden decompression triggers a chilling effect—temperatures plunge to about minus 50 degrees Celsius. That, in turn, lowers the speed of sound inside the neck and creates conditions for a standing acoustic wave to form.

“This is caused by the sudden expansion of the carbon dioxide and air mixture in the bottle, as well as a strong cooling effect… which reduces sound speed,” Koch said. “The decibels it emits are high—inside the bottleneck it’s as loud, or even louder, than a turbine of an airplane within one meter, but it doesn’t last long.”

In other words, while the sound may seem like a mere pop to our ears, it’s actually a split-second concert of gas dynamics and acoustic resonance, performed within the narrow bottleneck.

The Secrets of the Bottle Neck

The visuals captured by the high-speed camera show more than just the moment the cap is released. After opening, carbon dioxide begins to emerge from the beer, prompting the liquid level to rise. At the same time, the jolt from opening causes the beer to slosh—again, creating visible waves that pulse inside the neck.

The team also observed something curious: the momentum from the metal lid striking the glass after release could play a role in what brewers call “gushing.” That’s the sudden foamy overflow that sometimes follows opening.

“The momentum transfer of the lid hitting the glass with its sharp edge after popping might also trigger gushing,” Koch noted, “due to the enhanced formation of bubbles.”

But not everything matched up perfectly. While their simulations predicted an initial acoustic peak just before the “ah” sound, this spike never appeared in real recordings.

“It was a challenge to explain the low frequency of the ‘ah’ sound emitted by the opening and find a simple model to explain the values,” Koch said. Still, the core phenomenon was clear. A standing wave within the bottleneck—a kind of tiny, transient beer organ—creates the familiar note we associate with a drink well-deserved.

As for the toughest part of the research? Koch laughed: “Another great challenge was drinking the homebrewed beverages and still maintaining clarity during the experiment.”