What actually works for soundproofing a room?

Sound travels as vibration through structure and as airborne pressure waves through gaps, and effective treatment addresses both. Almost everything sold as soundproofing addresses neither, which is why so much money is wasted on it.

The first correction: acoustic foam is not soundproofing. Foam panels absorb reflections within a room, improving how it sounds for recording or speech clarity. They do essentially nothing to stop sound passing through a wall — and this confusion is the single most common and expensive mistake in the subject.

The four principles that genuinely work:

Mass. Heavy materials resist being vibrated. Adding dense layers — acoustic plasterboard, mass loaded vinyl, a second leaf of board — is the foundation. Doubling mass gives a modest improvement, which is why mass alone rapidly hits diminishing returns.

Decoupling. Breaking the physical path so vibration cannot pass directly. Resilient bars, independent stud walls and isolation clips are far more effective than mass alone, because they interrupt transmission rather than resisting it.

Absorption. Mineral wool inside a cavity damps the air within it, stopping the cavity resonating — a cheap and substantial improvement when combined with the other two.

Sealing. Airborne sound follows air. A small gap can undo an entire wall build-up: gaps around sockets, at skirtings, under doors, and around pipework. Sealing is the cheapest and highest-value step available, and it is routinely skipped.

What to treat first, by problem type:

Airborne noise — speech, television, music — needs mass, sealing and decoupling.

Impact noise — footsteps, furniture — needs isolation at the source: carpet, underlay, resilient floor layers. Treating the ceiling below is far less effective than treating the floor above.

Doors and windows are almost always the weak point, since a hollow internal door blocks very little regardless of the wall around it.

Flanking transmission — sound travelling around your treatment through adjoining structure — is why partial treatment frequently disappoints.

Realistic expectation: meaningful reduction, not silence.