[andars]

Sorry, I'm not qualified to give a good answer.

I would guess that the relatively long wavelength of ultrasound could potentially cause resolution issues.

[kakarot]

Not only resolution issues, but accuracy issues as well.

I doubt ultrasound can provide the same fidelity as a highly-tuned laser high-frequency laser beam when it comes to keeping something in one place.

[klintcho]

Thanks for both your answers. Though I was also thinking perhaps you could just suspend the particles in mid-air, and use the movable prism in a laser projector to actually "render" the projection (thus not needing to move them at at a high frame rate or at all). However I guess, you probably do need to move the particles as well to be able to make it a 3D volumetric shape in space (seen from all angles etc).