A Can Noise Cancelling Theory Be Used to Send Secret Messages?

[quicksilver123]

Hi, I had kind of a cool idea and was wondering if someone would try and refute it or tell me if it would be possible.

So let's say you have three people sitting three metres apart, and you need to deliver a message via sound to person B, who is the person in the middle.

Would it be possible to send this audio to person B without person A or C hearing it?

Say you could have a speaker transmitting a longitudinal wave but you surround that wave with an interference pattern to cancel out any noise outside of a certain cone with a base radius given as a function of the distance person B is from the source?

I know this sounds like a mad scientist schizophrenic idea but wouldn't it be kind of cool to send secret messages in that no one else could hear (yes I know this is highly impractical method haha)

 

[russ_watters]

You are on the right track with your interference pattern logic. You can use a phased array to send beams of sound along the constructive interference lines. Radar is steered that way:
https://people.ece.cornell.edu/land/courses/ece4760/FinalProjects/s2012/tcj26_ecs227/tcj26_ecs227/

You can't cut the beam off at a certain length though.