Use a good audio amplifier and a good microphone, build an electronic circuit that should be between the pré-amplifier and the amplifier. This circuit should invert the entrance sign in 180 degrees. That is used in electrocardiogram to eliminate it collapsed of 60 Hzs / 50 Hzs.
"Luhan M> > How far has sound cancellation technology come, for use in a room area?
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