How loud does a sound need to be to make pressure waves detectable by a camera? I've seen Veritasium's video that describes how sound can be picked up by a camera by observing small movements of objects sensitive to pressure changes (bag of chips, aluminium foil, paper...).
That got me wondering, how loud does a sound need to be to make pressure waves detectable by a camera ?

The example above is extreme because waves are adding up due to the bullet being supersonic, but it shows that a pressure wave with a high enough amplitude can be detected by a camera. So let's set up our virtual experiment :


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*Finding pressure waves in an image isn't obvious to humans, but suppose we have advanced image processing techniques that eliminates that factor.

*Our camera, perfectly static, is looking for apparent deformations of black and white grid. 

*Suppose our camera has a frame-rate sufficiently high so it doesn't affect the experiment.
At what point will our setup see the sound ? Does the quality of the audio only depend on the quality of the camera ? If so, assuming camera technologies always improve, at some point, robots will be able to see people speak only by looking thew the air ? Are any of my assumptions broken ? 
If I had to ask a single question, what are the limitations of this technique, if any.
 A: Like you said, it all comes down to a lot of factors and variables, especially the sound amplitude. A sound wave or pressure wave is a stack or multiple stacks of compression and rarefaction, and if the gradient of both is high enough, the light passing through each will be distinguishable enough because of density difference. This idea is what is used to film a shockwave propagation from a supersonic object like the one above by shinning a light source through a target region towards a mirror behind the target region and the reflection converges towards a point to get full magnification at the scene. You can check out the the YouTube channel Get Smarter Everyday for videos showing something like that.
To answer your last question, I'll disclaim I'm not an expert at this, but based on logical physics, "with enough light shining through the pressure wave as it propagates, and all that light converging towards the camera, and the camera being a high resolution high speed camera, one can see a lot of details, even down to little disturbances like turbulent and laminar flow, and pressure concentration and sparsities. The only limit is the human visual perception, our eye has evolved to notice contrast better than uniformity, and if there isn't much gradient between the compression and rarefaction of the wave, you are limited to seeing so much details from a footage like that, but increase the light intensity, and you might just see a bit more.
