Soundproofing Door Core
I’m looking to build a semi sound proof door for a child’s room (for sleeping purposes). I am not looking for recording studio level soundproofing, or party-wall sound blocking (the wall is what it is; I’m not adding extra drywall or anything like that). I’m looking rather to build a door that can muffle normal conversations and noises in an adjacent room.
My understanding is that most important will be creating tight seals around the perimeter.
Beyond that, is there a material or construction method that is worth using for the core of the door itself that is appreciably better than just a solid wood door?
It is fine if the door is a bit heavy or a bit thick, but I don’t want it to feel like it’s made of lead or by as thick as a wall.
The reason I am looking to build and not buy the door is because it is a slightly larger opening and in an awkward location that doesn’t work well with a regular or even french door. I will be making it a bi-fold door, which of course comes with added challenges in terms of sealing the door.
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In addition to the air-sealing you mentioned, I think your best bet would be a door filled with gypcrete for fire resistance. Not that you need the fire resistance, but the mass will dampen sound waves.
A few years ago I built an addition that included a mechanical room next to a bedroom (not ideal, long story) and I used an exterior door for a full air seal to keep noise down. I just used a regular, foam-filled exterior door and was disappointed in the sound attenuation, but I'm glad I at least used a reasonably airtight door.
I did this recently for my nursery room. Used a regular solid door, for door seal, went with those foam kerf door seals. You can get these pre-made with the door stop or kerf your own door stop.
Bifold is a very bad idea though. Hard to air seal and annoying to use.
With an air tight door, make sure there is still a return air path.
A solid-core door with decent air sealing is going to be the best bang for the buck for just muffling sound -- everything beyond that is going to be diminishing returns.
What matters for something as thin as a door is mass -- the more of it the better, in that limited space. If you're building your own doors, then consider making your own solid-core rather than "pure" solid doors. That is, create a frame out of hardwood for hinges and such, but fill in the bulk of the door with MDF. MDF performs notably better than wood for sound attenuation.
All of that is for "normal" doors, though. Using a bi-fold door for sound control is going to really mess up the works. The door edges are always the weak spot for sound (and air) movement and bi-fold doors add three extra weak points since each will mean missing mass AND a much higher chance of air leakage.
I would also be concerned about a bi-fold door — especially one with a lot of mass — in a child's room. *Lots* of opportunities for pinched/smashed fingers.
+1 for a solid door. Typical commerical doors are solid wood, and they are much better about blocking sound compared with the usual hollow interior residential doors. Commercial frames can also be ordered with weatherstripping pre-installed, although you can get prehung exterior doors for residential purposes that are a decent tradeoff and a lot cheaper. A double rubber "sweep" type weatherstripping on the bottom the door ("double" meaning two sweeps, one on either side of the door) will help to seal the gap at the bottom of the door, which is otherwise a "sneak path" for sound. The mass of the door helps deaden sound, the gasketing keeps sound from sneaking out aroung the edges. You want compression style weatherstripping here for this purpose, not the brush style stuff.
BTW, I used to work in a recording studio years ago. All the doors to the really well soundproofed areas (like the vocal booths) were actually two doors, opening in opposite directions -- one swung into the soundproof room, the other swung out into the hallway or larger room. Hinges were on the same side. The walls are all double studwalls about a foot thick. The doors set into heavy gasketed frames. You can go into one of those vocal booths and scream your head off, all people on the outside will see is you looking goofy through the window -- no one can hear you :-)
One last thing: if you want a window in the door to look through, you want *laminated* glass in a door being used for soundproofing.
Bill
+1 on the sweeps and the points on massing.
You may also want to pop the architrave off around the door opening and check that there isn't a big air gap between the door frame and the rough opening. If there is, then either closed cell foam (window and door type so it doesn't expand and deform your frame) or pack it tight with something else. Otherwise a good flexible (or acoustic) caulk if the joint is tight will be value add in any event.
We built a film studio as part of my previous life in my commercial building...and as Bill suggests, the studio had double doors which is bit hard to do in this case.
Bifold will be nearly impossible to manage with sound attenuation, so I'll add the recommendation for a solid door and the best door seals you can find. You can also take a standard door and apply 3/4 or 1" MDF to one side of it using Green Glue in between the door and MDF. That works very well and for a child's room you could finish the MDF in chalk paint for a bit more fun :-) Our studio and adjacent spaces used automatic door sweeps which close down snugly when the door is closed. They work very well to seal up the door bottom: https://www.amazon.ca/Pemko-Aluminum-Automatic-Bottom-PemkoPrene/dp/B009UWK98M
That is a mortise version, but they also come in surface mount flavours for steel doors etc.
The final challenge will be supply and return ducts with respect to sound attenuation. My go to there is either acoustic duct lining, or using inline duct mufflers. You can buy these, or do a good DIY version quite easily.
I'm building a 'media room' with greater sound mitigation efforts than you describe, but the door for me is definitely the weak link. One helpful thing I found was that jeld-wen publishes STC ratings of their doors. I went with the Monroe door for my room, and will be adding Pemko automatic door bottoms to them to get a good seal with the floor (link is to door ratings):
https://cmd-jeld-wen.s3.us-east-2.amazonaws.com/assets/documents/2870436722.pdf
Check out avsforum (website) if you really want to dig in on this. There's some good threads there on soundproofing doors, etc.
Thanks all, these are all helpful thoughts.
I don't really *want* to build a bifold, it just seems like it would fit the space best (when open). But perhaps it is worth rethinking as it will certainly add a lot of complexity and perhaps not be user friendly.
It seems around here, bifolds are mostly reserved for closet doors and such, but from searching online, it looks that parts of Europe more commonly use bifolds for exterior doors and are set up so they can face seal (which a closet door couldn't do with the pivot being located where it is). I am definitely starting to lean towards a normal door, even if more intrusive when open, to keep things simple.
The other big aha here is the return air path. I have almost exclusively lived with hydronic heat until moving into this house so am not accustomed to thinking about that. Alas there are no returns in this room which is currently quite open to the adjacent room (there is also a window type opening that I will be closing off).
I wonder if my best option is to run a new dedicated return in that room? Would the existing return in the adjacent room then be considered oversized since I would have added additional return volume without increasing distribution volume? I can't imagine how to cut a return back into that adjacent room without it transmitting a ton of sound and undermining all the door sealing efforts.
I have designed some nice bifold doors using Hafele hardware with high quality custom doors. They are much nicer to operate than typical bifold doors. I've also done trifold doors using regular leaf hinges. The trifold with leaf hinges could close against a stop, which is what you would need for soundproofing. Even the Hafele bifold hardware wouldn't work with door stops.
Mechanicals are not my strong suit but I believe it would be ok to be a bit oversized on the return air; the system will just draw a little less from the other returns.