Louder than an airplane: Scientists analyse 'pop' of swing-top bottle

The popping sound of a swing-top bottle brings a small joy to many beer drinkers - but also to a group of physicists in Germany who have proven how various factors combine to create a sound that is deafening for a split second.

When the "pop" sound happens, the gas in the neck of the bottle vibrates at a frequency of about 800 hertz, and the sound pressure reaches 180 decibels for about 70 milliseconds, say the authors of a new study.

This makes the sound in the bottle's neck briefly louder than standing next to a jet engine, according to research leader Max Koch, a physicist from the University of Göttingen.

In research published in the journal Physics of Fluids, the team used microphones and high-speed cameras to study home-brewed ginger beer in swing-top bottles.

For the effect to occur, the bottle must contain carbon dioxide and be under significant pressure. When opened, the pressurised carbon dioxide rapidly expands.

"When gas expands, it cools," Koch explains. In bottles with internal pressures of 3 to 5 bar, temperatures can drop to as low as minus 50 degrees Celsius for a brief moment. This effect can be even more pronounced when opening champagne bottles, though it also lasts only a fraction of a second.

Additionally, the pressure above the liquid drops suddenly when the bottle is opened. This causes the previously dissolved gas in the liquid to form bubbles, increasing the liquid's volume and causing it to rise. This fizzing effect is further enhanced by the jolting motion of opening the bottle, as well as the cap striking the neck from the outside.

For what he describes as a "fun project," Koch and his team - who happen to also be hobby brewers - used 0.33-litre bottles of home-brewed ginger beer with internal pressures of 2 to 5 bar. The aim was to simulate a liquid-gas mixture and analyse details of the "pop" sound.