I have an understanding of how bubbles work. They encapsulate air (or other fluids) in a membrane caused by surface tension. When they pop, there is often a sound. Sound is a type of energy, kinetic to be precise, that usually occurs from collisions. When a bubble pops I would assume that a sound implies that air rushes out due to a pressure change. Why is there a pressure change? I wouldn't expect the bubble to exert enough pressure to compress air. If the sound is caused by the air now being able to move into the rest of the room due to Brownian motion, then why wouldn't I hear air moving in a still room?
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$\begingroup$ I think if you could find some slow-motion (with audio!) of a bubble popping, it might be interesting. I suspect the air is not the cause but the tension in the water film. I'll bet the sound is either of the film "tearing" or of the tension accelerating the water together at the end where it "claps" together. I was unable to find good audio for a bubble pop to see if it gave evidence for or against either of these ideas. $\endgroup$– BowlOfRedCommented Jul 22, 2014 at 6:35
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$\begingroup$ you can create your own pops with with you mouth and feel how much pressure is really needed to create an audible pop $\endgroup$– ratchet freakCommented Jul 22, 2014 at 20:10
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$\begingroup$ I would think (based on how I make that sound) that it involves further exhalation, after examination, you seem correct. (@ratchetfreak) $\endgroup$– Thoth19Commented Jul 22, 2014 at 23:30
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4 Answers
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The air pressure inside the (intact) bubble is larger than in the surrounding. This pressure difference is called Laplace pressure and is caused by the surface tension between the soap film and the air. When the bubble pops the compressed air expands, thus creating a pressure wave, which you ultimately hear as the typical popping sound.
answered Jul 22, 2014 at 2:25
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$\begingroup$ Hi David Zwicker, and welcome to Physics.SE! We usually like our answers to be a bit more self-contained, or at least link to some useful resources. I believe your answer might even be right, but it's bery hard for me to judge that from what you've written. For example, why is the pressure inside the bubble different? $\endgroup$– ACuriousMind ♦Commented Jul 22, 2014 at 2:52
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$\begingroup$ I added a link to Wikipedia, which explains Laplace pressure much better than I ever could. $\endgroup$ Commented Jul 22, 2014 at 3:22
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$\begingroup$ Is there an intuition I can gain for why the pressure is greater on the inside? I'd assume the very fact that it is contained by the bubble itself right? Off to look up a pressure wave. Thank you. $\endgroup$– Thoth19Commented Jul 22, 2014 at 4:34
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7$\begingroup$ The way I think of it from an intuition point of view is that bubbles want to be small, the surface tension is pulling all the parts together. This means that at that surface there is the outside air pressure pushing in, the inside air pressure pushing out and the surface tension pushing in. Thus the air pressure must be greater inside to balance that surface tension. $\endgroup$– ChrisCommented Jul 22, 2014 at 12:33
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3$\begingroup$ @Cruncher: That would depend on how the bubble was formed. In the case of soap bubbles created by blowing on a film then in woolly terms the blowing is applying a force that is compressing the air (a bit) while causing the film to buckle out. When it snaps into a bubble then the compressed air remains in the bubble, thus higher pressure. Use a balloon as an analogy and it might be more obvious in some ways (as long as you accept the analogy). $\endgroup$– ChrisCommented Jul 22, 2014 at 16:28
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This page quotes the pressure inside a soap bubble as $\frac {4\gamma }R$, where $\gamma$ is the surface tension, about $25\text { dyne}/\text{cm }$ for soapy water, and $R$ is the radius of the bubble. For $R=1$ cm, the pressure is then $100 \text { dyne}/\text{cm}^2 = 10 \text{ Pa}$. This is released when the bubble pops. It doesn't seem like much with the atmosphere being $101 \text{kPa}$ but it doesn't take much.
answered Jul 22, 2014 at 2:32
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The surface formed by the bubble is such that its energy is minimized. Since increasing the interface between a liquid and air increases its energy due to surface tension, the bubble tends to reduce its radius, which implies that the pressure inside it must be higher than the pressure outside, and following this reasoning you may also get a quantitative result that relates this pressure difference with the principal curvatures of the interface (Laplace equation). Since the pressure is higher inside the bubble, if the bubble bursts, energy in the form of sound will be propagated.
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Cavitation is the formation of bubbles in a liquid when a sufficiently strong negative pressure is applied. A point in the liquid experiences a “negative pressure” if the local pressure goes below the average pressure in the liquid. This can happen when water in a pipe has a very abrupt turn, near the propellers of ships and submarines, in presence of a high-intensity sound (you can imagine sound as an oscillation of pressure).
In the Cavitator, sound is pulling the water apart, leaving a bubble of gas which is due to the excess pressure inside which is why smaller bubbles tend to pop with a greater sound (more pressure)
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$\begingroup$ So the thought is that sound causes popping? I was thinking more along the lines of soap bubbles blown from a jar (with a wand). So would my blowing by the initial sound in this case? $\endgroup$– Thoth19Commented Jul 22, 2014 at 2:12