Physicists have figured out how exactly the characteristic sound produced when opening a bottle of beer with a twist-top cork is produced, which immediately follows the pop. It turned out that the main source was the oscillating gas column in the bottle neck. Scientists came to this conclusion when they analyzed high-speed video and audio recordings obtained when uncorking a bottle. The authors of the work also compared the results of the experiments with numerical calculations and the Helmholtz resonator model, obtaining almost complete agreement with the experiment. The study was published in Physics of Fluids.
To date, physicists have studied beer from a variety of unexpected angles: they've counted the number of bubbles in a bottle, examined the stability of foam when poured from the bottom up, and even examined a peanut dancing in a glass. The point is that, from a hydrodynamic perspective, beer is a very interesting liquid: it's under pressure in a bottle or can and also contains dissolved carbon dioxide.
For example, it is already known how gas flows out when a bottle is opened: when the pressure in the vessel exceeds the ambient pressure by 1.83 times, the outgoing gas flow reaches the speed of sound, becoming an underexpanded jet stream. Although physicists have already modeled the uncorking of champagne and even saw Mach diamonds during their experiments, no one has studied the sound made by opening a beer bottle with a toggle stopper (sometimes called a flip-top). In this case, we are not talking about the pop itself, but what follows it - a characteristic tone with a frequency of about hundreds of hertz.
A group of physicists from Austria and Germany, led by Max Koch from the University of Göttingen, found that when a bottle of beer with a flip-top cork is opened, a standing wave arises in the neck of the vessel, reverberating for 70 milliseconds and giving the sound a characteristic tonality. To do this, the scientists used home-brewed ginger beer, a camera with a recording frequency of 8 to 16.8 thousand frames per second, as well as sound recording equipment that made it possible to obtain spectra with a resolution of 25 hertz after the Fourier transform.
Physicists analyzed the data obtained and divided the bottle's opening into four stages: the first is the release of gas with a shock wave, the reflection of this wave, and the condensation of water vapor; the second is the formation of a resonating gas-condensate column in the bottle's neck; the third is the release of dissolved carbon dioxide from the liquid; and the fourth is the splashing of the liquid due to the rising gas-liquid boundary.
During the second stage, the gas column in the bottleneck oscillated up and down with high amplitude for 70-100 milliseconds—this was the source of the main acoustic signal. The spectrum of the sound itself had a single strong peak at a frequency between 640 and 870 hertz (depending on the bottle's filling: from the smallest to the largest volume of liquid in the vessel), indicating a sinusoidal signal received by the microphone. However, the supersonic speed of the process remained questionable, as the condensate front emerged at a speed of 50-150 meters per second. This is less than the speed of sound in air (approximately 330 meters per second under normal conditions), but the study's authors noted that locally, the flows could still exceed the supersonic limit.
Scientists numerically simulated the process of uncorking a bottle, representing the outgoing gas as adiabatic, non-viscous and weightless. The physicists considered the process adiabatic due to its short duration, and also did not take into account the effect of gravity, since the Froude number in the experiment was approximately 635 units (accordingly, the gravitational term in the Navier-Stokes equation was negligibly small). As a result, the simulation very accurately coincided with the experimental data. In addition, the authors of the article considered the Helmholtz resonator model, which also described the observed resonance well from a theoretical point of view.
The scientists emphasized that their study was the first to disprove the widespread belief about the nature of the sound of champagne or beer being opened, but that several unanswered questions remained. For example, about what happens in the first milliseconds of opening a bottle, which could not be recorded with sufficient resolution by either a camera or a microphone.
Physicists love not only beer and champagne, but also other carbonated drinks: we wrote earlier about how scientists blamed surfactants for bubble chains.
