Retrofitting sound proofing in any buildings, especially apartments is very difficult.
I first studied this problem in 1974 and it hasn't got any easier.
The physics of sound transmission are very simple:
sound needs a medium for transmission, hence the "nobody hears you scream in space" expression.
Sound will pass through the smallest of holes as well as be passed through solid floors if they are not dense enough.
Think about how a drum works
The use of hollow core floor slabs in apartments has been a particular issue, especially when these cores were used for carrying pipework such as sewage pipes.
The final point is that sound is so personal/psychological as well as being environment related. Sound at 03:00 is a lot more "invasive" than at 15:00.
Hardwood floors in apartments are another nightmare for the people underneath and while many leases forbid them, they get put in and nothing gets done.
Wall vents, trickle vents in windows, fireplaces and just bog standard poor workmanship all add to the problem.
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