Acoustic levitation shape question (Japanese scientists link)

[ethereal_int]

http://sploid.gizmodo.com/amazing-scientists-make-things-fly-around-in-space-usi-1492880964


I wanted to ask is there a specific reason for the completely horizontal X shape of the levitated particles. If we already have such precise control over the standing waves, couldn't it be possible to make more unique shapes?

Like a 3d one with depth (a bigger thickness, or more levels), or a square pattern with 4 walls and an empty center? Or does it have to be a variation of a 2d star because of the orientation of the speakers? Would 2 more speakers give the X 2 more lines? What about a speaker ring or a sphere?
Ultimately that would allow us to control more objects at once independently, and use it in automated construction better.

 

[Bobbywhy]

The patterns visible in the video are caused by the pressure nodes in the acoustic field. It appears there are four speakers that contribute to the total sound field. Then when their driving signals' phases are changed it causes the 4-sided group to rotate. Yes, if you mounted more speakers around the circumference the sound field would be generated with more complex (and potentially more controllable) nodes. I've never heard of a "speaker ring", but many speakers evenly spaced around the center would begin to approximate that. My guess is that yes, that would allow more complex motions of suspended particles. Those little white beads are Styrofoam, the LED, and tiny resistor are all very light weight. Suspending heavier objects would require HUGE amounts of acoustical power. Why not experiment yourself and then come back here to Physics Forums and show us your results?

 

[Bobbywhy]

Here is a paper describing the experiment:

"Three-dimensional Mid-air Acoustic Manipulation by Ultrasonic Phased Arrays"
Authors: Yoichi Ochiai, Takayuki Hoshi, Jun Rekimoto
http://arxiv.org/abs/1312.4006