Nothing ends a good night faster than a knock at the door. It happens more than people admit, and almost all of it is avoidable with a small amount of forethought.
Start with the legal reality, which varies by place but follows a common pattern. Most jurisdictions have quiet hours, typically starting somewhere between ten and eleven at night and running until early morning, with a second window in the afternoon in some countries. Inside those hours, the standard is not silence, it is not being audible in a neighbour’s home to a degree that disturbs them. That is a vague standard and it is applied by whoever answers the complaint.
Which means the practical question is not what is legal, it is whether anybody complains.
Telling neighbours in advance is the highest value thing you can do, and it takes ten minutes. Knock on the doors that share a wall, a ceiling or a floor with you. Say there is something on, say roughly when it will finish, give a phone number. The effect of this is out of all proportion to the effort. A neighbour who has been told will usually absorb an hour of noise they would otherwise have reported. A neighbour who was not told has no reason to be generous.
Understand which sounds actually travel. This is where most people guess wrong. Voices and treble stop at walls reasonably well. Bass does not. Low frequencies move through the structure of a building and arrive in the flat below as a thud with no music attached, which is far more irritating than hearing the actual song. So the fix is not necessarily turning everything down, it is turning down the low end specifically. Most speakers have a bass control and most people never touch it.
Speaker placement matters for the same reason. A speaker sitting directly on the floor transmits vibration into the structure. Lift it onto a stand, a table, anything with a bit of isolation underneath, telegra.ph and the transfer drops noticeably. A folded towel under a speaker is a real solution, not a joke.
Then there is the part nobody thinks about: the noise that is not music. Guests on a balcony at midnight are louder to the neighbours than anything inside. Doors in a stairwell. People saying goodbye in the street for twenty minutes. The music can be perfectly reasonable and the complaint can still arrive because of a conversation outside at one in the morning. Close the balcony after a certain hour and ask people to leave quietly. It sounds like nagging. It works.
Have a plan for the cut off. Decide in advance what time the volume drops, and actually do it. An evening where music comes down to conversational level at eleven and continues for another two hours is an evening that does not get interrupted. Fighting for another hour at full volume usually costs you the rest of the night.
If someone does complain, the response matters enormously. Open the door, apologise, turn it down immediately, thank them. Do this and it almost never escalates. Argue, and the next call goes to the authorities rather than to you.
If you are hosting something larger and the building is likely to notice, consider a venue instead of your home. The arithmetic changes once you factor in the cost of a soured relationship with the people you live next to. A small hall for four hours often costs less than people expect and removes the entire category of problem.
One last thought. The people most likely to complain are the ones with young children and the ones who work early. Knowing who those are in your building tells you exactly which two doors to knock on first.