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Instead of cancelling annoying sounds (I'm thinking of techno played on earphones on the quiet carriage), why not get a head-mounted receiver transmitter using the principle of modulated ultrasonic waves. The incoming signal is boosted to the ultrasonic range. Two transmitter send out a highly focussed
beam of ultrasound which can be made to intersect at any distance (using a laser dot would add extra revenge vibes). The signals differ by the sound of the source and where they intersect they cancel each other except for the difference tones, i.e. the techno high-hat. This is then converted to the audible range at high volume inside the head of the target and drives the headphone user screaming from the carriage.
The Audio Spotlight
These guys might be of interest to you. It's similar technology, if not the same. [wagster, Mar 31 2009]