[iaw]

What do you mean by "current constraints?"

The properties of the medium (air), being the core constraint, is fairly well rooted in the laws of physics and observed experimental results.

What constraints are there that can actually be changed?

[dingo_bat]

What if you could locally change the relevant characteristics of air?

[iaw]

You can, it just makes the environment inhospitable for humans and requires orders of magnitude more energy than just what would be used to transmit the sound waves.

If you mean hyper-localization, that would be a new scientific phenomenon and one worth a great deal more to the world than as a wireless charging technique. The best industrial manufacturing techniques utilizing ultra-sonic arrays can do some amazing things but they:

1) Make the environment inhospitable to humans

2) Don't actually change the "relevant characteristics" of air so much as force a specific resonant behavior to allow them to target the energy at specific locations.

What specific "relevant characteristics" of air would you be changing? How do we then leverage changing of said characteristic to achieve our desired result? It would help if you utilized the terminology from the "Gas" Wikipedia page, a cursory glance and it appears to be comprehensive (from a concept perspective) [0]

[0] https://en.wikipedia.org/wiki/Gas