If the speakers (or their combined phased-array effect) are directional, you can project antinoise with different effects in each direction.
So for the person inside the apartment it sounds like a wall in the way, attenuating sound from outside, but for the neighbour across the street, it sounds like a hole absorbing the sound instead of reflecting it like a wall.
With enough points in a directional array it starts to resemble an acoustic hologram, with some remarkable one-way properties.
If you wanted to go really far with this, you could cover the exterior walls of buildings with dense arrays, and it would dampen street sounds for everyone in the neighbourhood, indoors and outdoors. In the street it would sound a bit like there were no buildings either side to reflect sounds back into the street, yet at the same time the buildings would still block sound from neighbouring streets.
I am suddenly day dreaming of a button you can press so that walking through a city sounds like walking through a forested valley on a windless day, as steady snowfall deadens all the sounds around you.
So for the person inside the apartment it sounds like a wall in the way, attenuating sound from outside, but for the neighbour across the street, it sounds like a hole absorbing the sound instead of reflecting it like a wall.
With enough points in a directional array it starts to resemble an acoustic hologram, with some remarkable one-way properties.
If you wanted to go really far with this, you could cover the exterior walls of buildings with dense arrays, and it would dampen street sounds for everyone in the neighbourhood, indoors and outdoors. In the street it would sound a bit like there were no buildings either side to reflect sounds back into the street, yet at the same time the buildings would still block sound from neighbouring streets.
Perhaps in future cities will do this.