I have an interior hollow bedroom door. The door seems to offer very little sound insulation, and I would like to add a bit more. I'm not expecting to get perfect silence or anything, but I would like to deaden down the sound some, so that way way I can listen to something in my bedroom at a reasonable level without my kids hearing it to the point that they can't sleep. I have in my mind that there might be something like an AB Foam type product that I could pour in to my door and make it a bit more sound resistant. Is there any such product? Any other suggestions to improving sound resistance in a room?
Reducing sound output from a typical bedroom can be extraordinarily difficult. The core problem is that sound travels through the point of least resistance -- you can get a good sound block in one area, but find it makes no difference since the sound just escapes from somewhere else.
If you have a hollow core door, then that is one of your biggest sound holes as a typical hollow core door has an STC rating of only 20 or so. That's not going to do a lot for any noise above a whisper. But wait, even that is not a true representation of the actual sound transmission. The problem is that rating assumes we are only talking about the door and not the air gaps around it. If you have a hollow core door, then I'm going to assume that it's not properly gasketed. If so, then the actual STC rating is substantially less.
Can you make that door more sound absorbent? No, not really. At least not in any efficient (money and time) manner. Your best bet would be to upgrade to a solid core door. If you have a standard door, then there is a good chance that the hinges will even line up and it'll be a straight swap for maybe $100.
Doing so will immediately jump your sound blocking to an STC rating of 25 or so. Eh? That doesn't sound much better than a hollow core door! Well, that rating is taking the air gaps into account, so it's actually substantially better.
But STC 25 is still not going to be good enough to make a big enough difference in your sound transmission. To do that, you're going to need to control the air gaps. If you add some acoustic door gaskets, then you'll be looking at an STC rating of 35 or so. That's actually quite good -- roughly on par with the STC rating of your wall. People will still be able to hear what happens in the room, but it'll be notably muted.
An alternate to buying an interior solid core door and adding acoustic door gaskets is to buy an exterior door, which is already going to be solid core and will have its own seal. That could be expensive, though, and the hinges will almost surely not line up.
The answer suggesting using a spray foam to fill the cavities in a hollow core door is totally wrong. Hollow core doors have a "honeycomb" card board baffle inside designed to keep the door faces from warping. The spaces created inside are probably 15 square inches or so. Once the foam filled the baffled off space, you wouldn't be able to put in any more. Just get a solid core door and be done. The improvement isn't spectacular but it's better than nothing. A "sound door" as we in the door business call them would be way better, but very pricey.
Best solution is to replace the doors. Don't know where you are from, but in UK normal 35mm plain doors cost about £20 each, when fire proof 30 min doors cost around £30-£40. And these fireproof doors are not only fire resistant, they are also much better in sound insulation - they are thicker and solid. Make sure you get them same size as the existing doors and replace them.
The trivial answer is a good pair of headphones. Even a crappy pair of headphones will suffice though.
But if you really want to do so, then I suppose you could
- Remove the door from the hinges.
- Drill holes in the top, bottom, and sides of the door.
- Inject a foam like Great Stuff into the door, letting it expand to fill the cavities. Be very careful here, as these foams expand greatly, and too much could damage the door.
Alternatively, buy and install a solid core door.
In any event, I expect you would find that once the door was improved that sound transmission still occurs through the walls. I think in the end, headphones are still a better idea.
I used seals around the inside of the door frame were the door closes on the frame. And at the bottom of the door I installed a rubber door sweep to cut down on the air gaps. And it cut back on about 40 per cent. It cost me around $15.00. To do this. And it works. And for the seal around the inside frame it comes in white or black. It looks good on a white door and white frame it hardly shows it is there .
Speaking as a sound engineer, and acoustics technician, of course you can fill a hollow core door to make it better for sound insulation. The front and back of the door are like a drum head and anything you can do to fill that gap will substantially quiet the noise transmission. You can use fiberglass poked in there with a broomstick, egg crate bedding style foam pieces, or even a bunch of crumbled up Styrofoam, the little beads which they used to fill beanbag chairs with.
At the top and bottom of your hollow core door are plugs; the thickness equals the thickness of the door and the length equals the width of the door. Pop that out of there and you'll see the entire width of the door open for you to fill. You might need a really long tube or straw to put the foam insulation in but it's a wonderful idea. And in addition, it's your door so you may do with whatever you would like with it and see how it turns out. Then you'll know for yourself instead of asking a bunch of people who hang out online.
Before you insulate the air gaps remember the air has to have a path to get back to the air return for ac and heat. That path usually comes from the gaps. If you plug up those gaps you will need to create another path for air return also creating another path for sound transmission. Not as easy as you thought, huh?
Learn how these cheap honeycomb cardboard hollow core doors are made and take off top and bottom inserts. Use a 40 inch or longer electricians arbor drill bit (or anything similar) and poke through that honeycomb. Use a spray extension and door/window foam only, as it's less expandable. Do not use Great Stuff expansion foam, it will bow the cheap door panels. When done, reseal the top and bottoms. Done. The better bet is to install solid cores. Still will cost the same due to time, hassle, and foam, or do what we do with each custom home we build and fabricate the doors yourself out of hardwood, poplar, oak, mahogany, sanded pine BC or whatever. It'll ease the pain if you just buy the solid core doors though. Slip the 3 hinge pins and replace with equivalent sizing...measure from horizontal door casing to door casing on either side of jamb stop. Door sizes are typically 2/0, 2/4, 2/6, 2/8, 3/0, 24", 28", 30", 32" and 36" respectively. Closets, pantry, turd rooms, hallway closet typically have no need for solid core doors. Bedrooms, master baths, laundry room, full baths, guest baths and all entry doors should all be solid core or hardwood.
Word of warning. Do Not put door seals/stops/thresholds below the door spacing to the finished floor. The A/C company has to specify air flow and returns from room to room with that air gap in mind to allow for proper air returns to the main unit. Doing so will cause some rooms to be like a vacuum and the doors will slam shut or you'll hear that annoying wind whistle around your doors.
So to answer your question simply: Yes, you can fill hollow core door, but No, I would not suggest it.