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Two apartments were formerly joined as one with a door that's now permanently closed, had its knobs removed, and is hidden behind bookcases for good measure. The building has good thick walls so the only noticeable sound leakage is through the secret door.

How can I soundproof the door? I'm assuming the solution is some kind of foam, which I can conceal behind the bookcase, but what kind of foam? I'm guessing it doesn't need to be the kind of sound-absorbing panels used in recording studios because I'm trying to reduce sound going through the door between rooms, not sound within the room. Better if it's not something permanent like expanding foam insulation in case someone ever wants to move a bookcase.

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    Can you access the other apartment for this work? Because if so, you could remove the door and put up drywall across the gap with normal insulation (e.g., fiberglass) in between the two layers of drywall. If you use any kind of foam, keep in mind that many types of foam are dangerous in a fire Not an issue in small quantities to seal up holes in walls, but as an entire wall section this can be a serious concern. Aug 7, 2023 at 0:56
  • I agree with @manassehkatz-Moving2Codidact... Also a door like this is highly illegal in the US.
    – DMoore
    Aug 7, 2023 at 3:31

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Rather than foam (which can be flammable, to varying degrees) I'd suggest rockwool between the bookcase and the wall, and/or filling the doorframe behind the bookcase if the door is inset, not flush. It's used for sound isolation and it's not flammable at all. In fact, it can help to block fire.

Depending on the details of how "reversible" this needs to be, you might also apply "fire caulking" around the joint between the door and the frame (top bottom left and right - all the way around) if that part can be considered permanent. If not, there is removable caulking available that would seal the joint, can be removed, but lacks the fire performance of fire caulking.

If the frame is inset and the only concern is moving the bookcase, rockwool and enough framing to hold drywall and make it like a door never was - the only reason to preserve the doorway at all would be if there was some prospect of ever opening it again. But if it's a rental rather than an ownership situation you'll have to get the landlord to do that.

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  • This sounds promising but is rockwool safe to be exposed in open air? I mean it will be concealed behind a bookcase but not actually inside a wall. Aug 10, 2023 at 0:26
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Just sealing the inevitable gaps where the door meets the frame (and where the handle was) will make quite a difference, especially for higher pitches like speech.

Lower pitches will need something quite heavy. Assuming the bookcase does all the concealing you want, I'd screw plywood (not really thin, more like 12-20mm or 1/2-3/4") to the frame, and seal that all round. If the door is quite recessed, leaving a gap, mass-loaded vinyl or rubber is sold for blocking sounds. You'd need to check a specific product against fire regulations where you live.

Of course, doing this from both sides is better than from just one. But the walls are probably pretty thin too if they were internal to one property before, and thin walls will let some sound through, over a much larger area than the door.

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The easiest would be to hang something on the wall to cover the door, like a thick quilt. That would be decorative and you could take it with you when you leave. This is published in several reference sources: "Typical sound absorbing materials are fiberglass, rock wool, open cell polyurethane foam, cellular melamine foam, heavy curtain blankets and thick fabric wall coverings." As mentioned foam can be a fire problem. Good Luck.

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  • You need to link to whichever of your several reference sources you quoted. Or provide a bibliography-style footnote if they were on paper, not the web.
    – Ecnerwal
    Aug 7, 2023 at 16:55
  • Nothing to quote as nothing was given. You can look up sound absorbing materials. Here is one of many. soundproofcow.com/product-category/sound-absorption-materials
    – Gil
    Aug 8, 2023 at 3:31

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