I’d wonder if there’s a weird issue where sound is traveling through a duct or something.

A white noise generator could be interesting to try - if you’ve already tried having a civil conversation about it anyway. Otherwise, record mundane conversations and leave them on repeat all day when you’re not at home.

There are definitely microphones that can do it. You ever seen one of those microphones that look kinda like a little satellite dish? They pick up sound from long distances. They can also sometimes pick up stuff through walls. They also make microphones that you can put up to things to listen through them. But that might not even be it. Your neighbor could have something in your apartment, . Get something high high pitched, start whispering so your neighbor turns up their listening device, then blast the high pitched sound. Like hit the test button on your smoke detector. Maybe first go from room to room whispering to see if your neighbor can hear in every room or not. That’ll help determine what they’re doing to listen, and you’ll want to blast the sound in a room they can hear it in.

You should also report them for banging on the walls. You can tell your landlord, or make a police report, or both. Get some recordings of it. Record for a week or two, then complain with the recordings. It’s harassment that prevents the peaceful enjoyment of your property. Once you put your landlord on notice about it, they have to handle your neighbor, or they are in breach of the lease. I usually wouldn’t recommend the cops unless the landlord fails, but with this situation, the neighbor is sus, and I’d worry about them retaliating, and I’d want to get he cops involved because of the potential eavesdropping/wiretapping. If you get the police on it now, then if someone happens in the near future, your neighbor will be the top suspect.

Could it be coming through the duct work? I lived in a duplex once that had this issue. The walls were thin enough but you could hear neighbours conversations thru the ducting.

Get a Dayton Audio Exciter and a cheap white noise machine with audio jack. Strip the wires and attach to the exciter. Mount exciter onto shared wall and turn on. If they are listening they will be blanketed with a solid hum. You should be able to turn it low enough as to not disturb you.

We have a neighbor that creates music or some crap. He’s nice and all but he blasts that stuff. We live in a brick building, so don’t hear much unless it’s really loud. Maybe try noise canceling foam pads on the side where they are? It’s not really unattractive…but, maybe it would help relieve some.

Maybe have a polite conversation with the neighbor first to see what the issues are. You can explain that it seems they react to light sounds at times and you want to work together to find an acceptable level. Then you can both experiment so see what things sound like standing in both apartments.

If they don’t want to do anything like that then tell the landlord to get involved.

It seems weird that someone would go through the trouble of putting a microphone in place to eavesdrop on your whispered conversations only to bang on the wall in objection to unfriendly comments. That kind of defeats the purpose of being sneaky and nosy, doesn’t it? Either that or maybe your test commentary actually isn’t all that quiet.

ask them if you can come into their apartment to observe the sound levels from your place. Have your GF conduct different sound experiments. You should monitor the sound with a sound level analyzer.

Ideally your landlord is present during the entire test.