Unfortunately flats are a real mixed bag sound insulation-wise.
Usually large modern blocks have concrete floors and barrier walls between apartments for fire regulations, so sound proofing is normally excellent. The last flat I was in was amazingly well sound proofed. Never heard more than a slight murmur from anyone else and then only during parties, and I used to crank my system without any comment from anyone and I specifically asked them.
Conversions are totally different and often very poor however. Personally, if you want to be able to enjoy your system without worrying then I consider looking at some proper sound insulating products to do underfloor and possibly use a suspended ceiling, assuming you own rather than rent of course.
Steve aka 'Twelvebears' (it's a long story)
System: Technics 1210 Mk5, Jelco 750 arm, AT33EV via MF X-LP2 Phono Stage, Oyaide mat and record clamp. SB Touch via Marantz PM-11S1 amp and Wilson Benesch A.C.T. speakers. Mark Grant cables and PS Audio Power Plant Premier mains regenerator.
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