when I was in law school, our downstairs neighbour (who was... different), purchased for us this like sound insulating foam, as she claimed she could always hear our music and it shook her ceiling and kept her up at night. she lived on the main floor of the house and we had the top floor.
trying to be good neighbours, we apologized (we did have a few parties and were often up late at night loud and belligerent) and accepted the foam.
then one night, when all my roommates were either asleep or not home, she comes knocking on our door, complaining about our music. I politely tell her we are not playing music and my roommate is sleeping. she's not convinced, so she's come upstairs to investigate. sure enough, our house is basically silent.
turns out, she'd been hearing the basement neighbour's music all along.