The trick is to use an algorithm that controls the speakers precisely and uses constructive and destructive interference of the sound waves to place sounds where they are wanted, moment by moment. (newscientist.com)
As a consequence, a complete cancellation of counter-propagating beams by destructive interference is not possible anymore. (sciencedaily.com)
A second light grating to recombine the paths was randomly added, which led to constructive or destructive interference as if the atom had travelled both paths. (sciencedaily.com)