How can I soundproof an interior door?

Question

I'm looking to soundproof some interior doors.

My plan was to drill a 1/2" hole at the top and fill it up with foam, but I'm not sure what type of foam to get. I've used tiny foam cans before, but I'm not sure they'd be up for the task. My fear with them is that the first few seconds foam will get stuck to the sides of the door halfway down, and then all of the foam that follows will fall on top of that foam, leaving the bottom portion of the door uninsulated.

I suppose another option (and probably a better one) would be to drill a tiny hole at the bottom, in which a straw from a can of foam would be placed and sealed with silicone. Another hole could be drilled at the top of the door, serving as a vent to get rid of the air that's being displaced by the door.

Anyways, any tips/advice/product recommendations are appreciated.

PS: I will deal with the walls later. More care has to be given to them due to the high risk of fire. Foam puts off gases that are not only flammable, but I believe perhaps also self-oxidizing. I say this because I once knew a guy whose house burned down due to spray foaming the interior walls (during the actual application, not afterwards), and rumor has it that the entire house was engulfed in flames in minutes (too quickly to save a $100k painting) and burned as if it were doused in gasoline.

PSS: The windows are not a concern, as external noises are not typically the problem. IT'S COMING FROM INSIDE THE HOUSE!!! (Obligatory movie quote)

Answer 1

Hollow core doors are actually not totally hollow, they typically have cardboard glued inside which just serves to stop the panels from warping.

Sound moves through both air and structure.

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Your main concern should be air - are the doors totally sealed, especially the gap at the bottom? The door should shut tight, and ideally be air-tight.

You should also consider vents joining the rooms, as sound can travel there as well. On the flip-side, you do need vents in the room (both a way in and out), or the air will get stale, it won't be heated/cooled, and worst case can actually lead to mold growth. It is only fairly recently that they started building homes with return vents in most rooms -- in older homes, the gap under the door is actually the only way air circulates when the door is closed, and usually there is just one return vent per floor.

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You can only minimize sound travelling through structure. Using a foam core door or an interior door designed for soundproofing would be a good option. A solid door should also transmit less sound than a hollow one. An exterior door may also be an option.

There are of course other concerns like sound travelling through walls and other structure, but that's beyond the scope of this question and also fairly well covered by other questions.