A lot of apartment construction must be either poorly converted or poorly constructed. I've lived in multi-unit buildings in a few places and sound isolation is pretty good. In London, I met a family at the lift and the mother apologized for how loud her children had been that weekend. My bedroom was against their living room. I honestly hadn't heard a peep.

Then here in San Francisco my particular unit is next to the garbage chute and I haven't ever heard someone putting their garbage down it. My wife and I run the 3D printer through the night and our neighbor hasn't said anything yet. It's about 57 dB from 1 m away so that's why I suppose. We do rarely hear their kids when they wail, as kids do, but not otherwise.

One of the things I do when we consider a place to live in, though, is that I play music at max volume on my wife's phone and then check from various parts of the home. I also talk to yell till my wife notices on the other side of bedroom doors and so on. To be honest, many places can be built to be quite quiet. My daughter sleeps above the work / office and it's about 29 dB right now with the printer running.

Naturally if one cannot sleep at 29 dB our home wouldn't work or you'd have to turn off the printer overnight, but overall it seems fine for me.