Sound Synthesis for Fluids With Bubbles | Two Minute Papers #97

Dear Fellow Scholars, this is Two Minute Papers with Károly Zsolnai-Fehér. We have had quite a few episodes on simulating the motion of fluids and creating beautiful

footages from the results. In this work, the authors created a simulator, that shows us not only the motion of a piece of fluid, but the physics of bubbles within as well. This sounds great, but there are two huge problems: one, there are a lot of them, and

two, they can undergo all kinds of deformations and topology changes. To conjure up video footage that is realistic, and relates to the real world, several bubble-related effects, such as entrainment, splitting, merging, advection and collapsing, all have to be simulated faithfully. However, there is a large body of research out there to simulate bubbles, and here, we are not only interested in the footage of this piece of fluid, but also what kind of sounds it would emit when we interact with it.

The result is something like this.

The vibrations of a bubble is simulated by borrowing the equations that govern the movement of springs in physics. However, this, by itself would only be a forlorn attempt at creating a faithful sound simulation, as there are other important factors to take into consideration. For instance, the position of the bubble matters a great deal.

This example shows that the pitch of the sound is expected to be lower near solid walls, as you can see it marked with dark blue on the left, right side and below, and have a higher pitch near the surface, which is marked with red. You can also see that there are significant differences in the frequencies depending on the position, the highest frequency being twice as high as the lowest. So this is definitely an important part of the simulation. Furthermore, taking into consideration the shape of the bubbles is also of utmost importance.

As the shape of the bubble goes from an ellipsoid to something close to a sphere, the emitted sound frequency can drop by as much as 30%. Beyond these effects, there were still blind spots even in state of the art simulations. With previous techniques, a chirp-like sound was missing, which is now possible to simulate with a novel frequency extension model. Additional extensions include a technique that models the phenomenon of the bubbles popping at the surface. The paper discusses what cases are likely to emphasize which of these extension's effects. Putting it all together, it sounds magnificent, check it out! But still, however great these sounds are, without proper validation, these are still just numbers on a paper. And of course, as always, the best way of testing these kinds of works if we let reality be our judge, and compare the results to real world footage. So I think you can guess what the next test is going to be about! The authors also put up a clinic on physics and math, and the entirety of the paper is absolutely beautifully written. I definitely recommend having a look, as always, the link is available in the description box. Also, you'll find one more link to a playlist with all of our previous episodes on fluid simulations. Lots of goodies there. As always, we'd love to read your feedback on this episode, let us know whether you have found it understandable! I hope you did. Thanks for watching, and for your generous support, and I'll see you next time!