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so what stops people intercepting these audio messages, using the same idea you could use a parabolic mic to track and hear the individuals audio

Shape of the wavefront. It is very directional elliptical, you would need very high sensitivity microphone to attempt a capture off angle.

Directivity is about -50 dB off axis potentially. You can further enhance it some with active noise cancellation.

What about capturing sound as it scatters on the listener's head?

I guess you need a good model of their head to post process a readable message out of the distorted audio.