Perhaps it's just the neighbors I've had, but it seems that at my apartment, I can't hear the people next door but I can hear the people above and below. Used to wake up to the upstairs neighbors stomping and door slamming before they moved out. Also used to have neighbors below that would engage in consensual activities regularly at 3 am. Adjoining walls don't seem to have the same issues, no matter who the neighbor has been, and voices nearly have to be at full scream to be heard from any direction. Apartment building was built in the '80s, used to be used as condos.