I've been thinking, SLS/SLM printers currently use a roller to spread 3d printing substrate, but wouldn't ultrasonic vibrations spread the substrate more cleanly, accurately, and with greater density than a roller?
The problem is twofold. Resonance and Granular convection
Let's start with an empty box. We toss in some powder to create the first layer and use an ultrasonic to create a first layer. What happens? The bed starts to resonate depending on the sound you send into it in patterns - and the powder starts to form valleys and ridges along them as one can see in this video.
What happens if one shakes a box of fine granulate that contains larger items? Granular convection happens! All items raise simultaneously and the small items start to fall first, resulting in them getting under the larger ones, so as a result end up pushing the large items up.
Because of both effects, there won't be an even layer and it would raise the items printed, even if we managed to get good layers.