I know that the title sounds ridiculous so let me explain.
A friend lives in a ground floor condo. He complains about the noisy neighbor above him. We were trying to find a solution that would help my friend. We discussed what we know about how the building was constructed and where the HVAC ducts run, etc. I asked if the upstairs neighbor ever complained about noise from his home below her. He told me no that she can’t hear him because sound only travels in a downward direction.
This is not true but what would make a person think this? Is there any link to reality here?
A gaping hole in their education?
Yes but I was wondering if there is something that this belief may be based on. An odd bit of folk wisdom or an old theory.
Isn’t it a fact that you can hear the footsteps of your upstairs neighbor through your ceiling much better than those from your downstairs neighbor through your floor?
I had an upstairs neighbor who had a very squeaky bed. She’d bang her bf about 2x a week, making her bedroom sound like a machine shop. I’d wait until her bed started banging the wall then I’d take a broomstick and bang like shit on the ceiling right below her bed. She heard it. I proved sound travels up too.
There is a slight basis in reality in that many of your sound emitters, like the TV, footsteps, and the vacuum cleaner, are coupled mechanically more to the floor than the ceiling. The TV is usually sitting on a solid piece of furniture which is then sitting on the floor, so sound vibrations from the TV will be coupled into the floor fairly easily. If you bolted your TV to the wall instead, then the sound would be most audible by your next door neighbor, instead of anyone either above or below you.
Someone living in a downstairs apartment or condo will also easily hear footsteps from someone above them, but someone in an upstairs apartment or condo won’t usually hear footsteps from someone below them.
So while sound tends to radiate outwards in all directions, your friend isn’t completely off-base here.
Yes, it may be I did not understand him. In my experience, the sound from connected apartments depended on the source of the sound. Yes I could hear the people above walking. The people on the side I could hear the TV well. From below I was often blasted with the deluxe stereo system. I do think it is just wishful thinking on his part, that the upstairs neighbor can’t hear him at all. They don’t speak to each other and are in a continual state of trying to annoy each other into moving. They are fighting a war with no diplomacy.
Your friend is totally correct, just for the wrong reasons. Yeah, sound is omnidirectional.
But, as engineer_comp_geek points out, reason 1 is that with concrete floors in the room above, the coupling is very good. There’s little to damp it - footsteps reverberate right through the concrete to the floor below.
Lower frequency sounds are also much less likely to be damped - sound deadening materials used in materials work much better on higher frequency sounds like voices.
Anyways I’ve experienced what he’s talking about. The amount of noise bleedthrough is basically proportional to the floor an apartment is on. Basement apartments have it the worst. Sounds like a herd of giants up there.
It is very loud and very obvious, this isn’t me complaining about a pea under a mattress.
On the other hand, basement apartments have lower heating and cooling costs generally.
It’s also true that you can hear your downstairs neighbor banging on the ceiling through your floor better than you can hear your upstairs neighbor bang on the ceiling through your ceiling.
Sure. But in my experience this happens a lot less frequently (and only when you really deserved it)! 
I’ve lived in basement apartments, and I’ve lived on the top floor of apartments.
As at the time, I was quite a night owl, and wasn’t always wanting to be silent, I found that basement apartments were actually preferable.
It had its annoyances, as I’d be trying to sleep at 10am, when the person in the apartment above me was practicing on his drum kit, but it had the benefit of me not having to tiptoe around my apartment all night for fear of disturbing my neighbors. I had a noise complaint once, when I had some friends over after work around 2am. We weren’t being loud at all, we were just watching a movie, but the complaint wasn’t over music or shouting or anything like that, it was specifically over people “walking around.”
The proper comparison is between the noise made by your upstairs neighbor clomping around on the floor versus your downstairs neighbor walking heavily on their ceiling. I think you’ll find they make an equal amount of noise when they do this, so I can’t imagine where your friend gets the idea that the upstairs neighbors make all the noise.
Whom else could he blame, living on the ground floor? The morlocks? Cause you know, you don’t mess with the Devil!