Difficulty: 10/10 Effectiveness: 10/10

What you need is:

-Bluetooth speaker that turns on when plugged in and left on all the time, or can be turned on via non-IR remote -Large Surface Transducer (it’s like a speaker, but just vibrates whatever it’s attached to, turning that surface itself into a “speaker). -Possible need for small amplifier -“Old work box” - it’s a replacement for the electrical box behind a light fixture, that is also removable.

Step1: Drop a light fixture and expose the electrical box. Bust out the electrical box from behind the light fixture. Pull the wires down for access, leave ceiling hole open.

2: Wire the surface transducer to the speaker output of the Bluetooth speaker. You’ll have to make sure you have adequate power supply to the transducer for maximum effect, so you may also need a small amplifier as well.

3: The hole behind where the light was should expose your upstairs neighbors subfloor above. Attach the surface transducer to their subfloor. Use short screws so you don’t penetrate through.

4: Take the plug for the Bluetooth speaker and strip off the plug part, pull wire and the wires for the light fixture down into the old work box. Hard-wire the Bluetooth speaker into the power wires.

5: Push up and install the old work box. Reattach light fixture. Turn on power to light.

You’ve now got a completely hidden device that turns the floor above into a speaker.

Feel free to play various sound effects at inconvenient times, such as bloodcurdling screams in the middle of the night, cricket sounds every 45 minutes, whispering at low volume occasionally, footsteps, a childs voice, “infrasound” to induce hallucinations, several seconds of a Barney song intermittently throughout the day so it gets stuck in their head but can never pinpoint where it’s from.

For the truly despicable and dedicated, make one of these for every room which has a light fixture. That way you can synchronize whispering and make it sound like it’s either surrounding them or moving room to room, footsteps moving through house when one of them is home alone, etc.

If done tactically, I don’t see how they’d ever figure it out, and they’d be squarely convinced their apartment is haunted. If maintenance is called, all they’re going to see is that you don’t have any speakers in the house, and be just as confused as the people in the apartment.

Bonus points: With some technical savvy, you could also try to attach a microphone to their subfloor, especially in the bedroom. Set it to record when it picks up a certain volume of sound. Try to get them talking, or fighting if you’re really lucky. Then play their own voices back to them over the speakers, never in the same room as them though. Talk about psychological damage.