Since this is a question related to a “classic” column, it may have been answered on this board before, but I was wondering.
I was reading the column regarding why there isn’t an audio equivalent to the laser. http://www.straightdope.com/classics/a2_114.html
Unless, I didn’t read closely enough, Cecil didn’t seem to answer the question.
Would it be possible to build a device that could focus sound waves such that a person could speak to someone a couple hundred feet away while not being heard by someone nearby?