I realized I'm experiencing a situation that doesn't make any sense to me at all.
Back at home, my room is on the top floor of our apartment building. There is a daily music playing fountain nearby and I can hear the songs as if the fountain is my room. I have always thought that there must be speakers somewhere nearby because it is impossible for the music to reach my room. I cannot hear the music when I leave the apartment (the apartment exit faces the same way as my window) in fact, I cannot hear it until I walk 4-5 mins towards the fountain. My room is on the 6th floor.
This year I started graduate school and as a new student, I was trying every floor of the library to see which one I'll like the best. After a while, I decided that I liked the top floor (10th) the best and kept studying there. The only thing I couldn't figure about the top floor has been the music I kept hearing which I didn't hear on any other floor. I was thinking of a secret 11th floor filled with musicians until today I found out the music is coming from the park nearby. The thing is that you cannot hear the music at all on the lower floors which are actually physically closer to the park! So instantly I remembered the similar experience I lived back in my apartment and realized there are probably no speakers hanging around to stream the music that is playing at the fountain.
Could anyone explain why that happens? The only explanation I could come up with was that the top floor is only isolated with the roof unlike other floors (which have other floors as isolation layers) so maybe the roof is not a good isolator? Because in my apartment my room is always warmer in the summer and colder in the winter with respect to the other floors. But I don't know if heat works similarly to sound.
Best Answer
I think the most significant cause (although usually acoustics problems are multifaceted/complex) would be that there is a suitable temperature gradient in the evenings that is causing these specific heights (floors) to receive more of sound waves from the source due to refraction.
The speed of sound will change with temperature (c = 331m/s + (0.6m/sC x T)), therefore in regions where the speed of sound is higher (in warmer temperatures) the wavefront or macro scale sound wave will seem to bend towards the lower speed of sound (cooler temperature). You can find demos of this effect and better explanations than mine from sources such as: http://hyperphysics.phy-astr.gsu.edu/hbase/Sound/refrac.html , https://www.acs.psu.edu/drussell/demos/refract/refract.html and a useful demo that you can test it out with: http://www.falstad.com/ripple/.
What I think is happening is that at night/in the evenings with the music playing there is a suitable temperature gradient formed so that the sound waves are bending appropriately in order to reach your apartment/library floor. Especially the low frequencies which will be attenuated less by losses in transmission should carry from even large distances.
If you want further information, try looking into the refraction of sound since there are a lot of explanations online from a variety sources.