Can sound be used to change trajectory of objects? I'm doing a fictional history in near future, where there is a very advanced army but i like to keep things real, and my knowledge of physics is very limited...but i know sound has been used to levitate objects before so i'm thinking about a device that can generate sound to make bullets deviate enough to not hit the target(mostly drones with low armor and soldiers).
I'm already using EMP generators to fry the electronics missiles systems incoming( it's not active all the time, it just generates a strong pulse when missiles are detected for energy reasons) this would be for me limited knowledge the most real and close we can get from sci-fi "force fields" but if anyone have a better and possible real way to block/deviate/fry objects with sound i would like to know.
 A: Here's an article about acoustic version of laser named SASER.
http://en.wikipedia.org/wiki/Sound_amplification_by_stimulated_emission_of_radiation
It can emit very directed acoustic field. As far as I know, present versions have quite limited posibilities, but if you're writing SF it may be inspirational.If SASER's wave can propagate in human body, which isn't crystalline, than MAYBE enough reasearch would make it work in the air.
Generally it is possible to emit sounds that deviate objects, for example blast waves or supersonic booms, so it's enough to have SASER powerful enough.
Moreover scientists from reasearchGate agree that directing of sound is possible.
http://www.researchgate.net/post/Can_we_direct_sound_like_a_laser_beam_means_can_we_make_sound_linear
But be careful with using such devices in cities, since diffraction may make the sound rebound into your troops.
A: I tend to disagree. Acoustic levitation is based on the standing waves phenomenon and therefore it needs a bit time to be established. Emmiting a ray of sound to hit the bullet is not a good way to deviate it from its path.
If I was a sci-fi engineer in the field of acoustic defense, I would create walls of standing waves with knots and antiknots organised to divert the bullet (so probably a series of walls of difference knots organization). But it would still need a hell of the distance to make the bullet harmless.
Shockwave proposed in comments is another interesting way to go. That actually could be aimed, but a timing is the key: too soon or too late and the wave won't be strong enough to divert the bullet. And again, it probably has to be a series of shockwaves and it would need a distance.
For bullets as high-velocity objects I would go with the good old electromagnetism. Maybe you should read something about Lorentz force.
If you want something acoustical, then for fun consider


*

*a slow moving doomsday weapon to be stopped by acoustic levitation.

*acoustic trap (explosive or so is suspended by acoustic levitation and by walking nearby you destroy the field and make it fall).

*use something like horns of Jericho - use a sound of carefully chosen frequency to destroy solid standing objects

A: i have an idea to that!
just shoot a burst of 10 hz amplified to the max an try it with paper
then you should x exponentialy. and convert to a military grade then try to shrink the tecnology for civilian use. my source of knowlege is bass frequencys. sine wave. squere wave. its not have a deflector all the time just activated by radar incoming threat. what im trying to say is should be like a invicible flash o bass
that my take 
