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Author Topic: Filling the hollow doors for better sound insulation  (Read 33406 times)

goblinsly

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Filling the hollow doors for better sound insulation
« on: May 12, 2013, 04:23:39 PM »

Hey everyone,

I read a bit about cheap hollow doors online and how people filled them with different materials. There are many problems with that, like weak hinges, cardboard inside the door, weak exterior of the doors and soo on. But lets leave all of this aside, i would still like to know how things in theory would work ...

Let us say we have hollow door that weights 13kg (28 pounds). I know from experience that if we hang about 30 extra kg on them, the hinges will survive, soo lets say that would be 40kg together. That is 3 times as much mass as the door had at the beginning. Doubling the mass should add about 5dB STC if i am correct, soo that should be noticed, dont you think ?

Filling the door with only sand would be bad, that would add about 80kg to the door which would either break the hinges or break the door on the bottom. About half the door could be filled with sand and another half with high density rock wool (80kg per cubic meter). There could be 1 layer of rockwool and 1 layer of sand, and polyurethane foam would be used to reinforce the door on several points. Soo like i said, let us put aside all those scenarios where something goes wrong, if all went well and the door went from 13 to 40kg, filled with rockwool and sand, what do you think the sound insulation improvement would be, for instance how well would it insulate against speech ?

Randy S

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Re: Filling the hollow doors for better sound insulation
« Reply #1 on: May 13, 2013, 03:39:54 PM »

Mass law states that for every time you double the weight of a partition should equal around -6db over the average range of frequencies, however this is normally for scenarios that do not have low frequency bass or high sound pressure levels(spl). Absorption panels are glued to door to help absorb some of this.

In the end you are left with the jamb itself as the transmission path and you also find a quick diminishing point of return. This is where a double back to back door system(complex structure) is used to void mass law.

This is why we do not attempt to soundproof hollow core doors.. better to start with a solid core door and add mass and absorption to the door.. far cheaper and better results.





Randy Sieg

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888-942-7723
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goblinsly

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Re: Filling the hollow doors for better sound insulation
« Reply #2 on: May 14, 2013, 11:34:32 AM »

Thank you for your reply !

Some time ago i added insulation to hollow doors, 5 cm of DP8 mineral wool and covered it by 3 cm thick wood panel and added 2 layers of drywall. Now i am thinking whether i would get better insulation by installing some cheap massive doors and then just add a few layers of drywall directly on it, making a better contact then there is now ( maybe add some foam type of material between doors and drywall to prevent easy sound way). The problem would be though that i would most likely have to replace the frame also. I will have to check ... maybe i could get some cheap second hand doors ... the frame itself i could even make myself ..
For 2 x 0.65 cm door, what do you say would be a good mass to begin with, like 30, 40kg ?

johnbergstromslc

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Re: Filling the hollow doors for better sound insulation
« Reply #3 on: May 14, 2013, 03:03:29 PM »

I attempted to dismantle and insulate a hollow-core door back in the day.  The time-cost/benefit ratio was comical....

Better to pony up the $50 bucks and get a solid-core door slab.

Alternatively, you can get a couple sheets of 1/2" MDF, cut and attach panels on both sides, allowing for opening clearances.  This would run about the same as a solid slab, but will be much easier to install - no hinge mortises to rout.  You wouldn't even have to use fasteners - double sided foam tape will work nicely.  That makes it removable later as well.

You would need to replace all of the hinge screws to accomodate the extra weight.  If you have 3 hinges per door, you could upgrade to 2-3" wood screws and I don't think you'd have to worry about the extra weight, which would be around 27 kg/60 lb. 

The longer screws will penetrate into the studs, so be careful about tightening them too much and warping the frame.  Might be a good idea to pop off the door casing and make sure you have adequate shims behind the hinges before installation.....

goblinsly

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Re: Filling the hollow doors for better sound insulation
« Reply #4 on: May 14, 2013, 06:49:21 PM »

johnbergstromslc: i just found out yesterday that filling the hollow door is a big pain .. but at least i didnt lose much time and basicly spent no money.

The door mass increased from 13kg to 26kg. They are filled with sand and mineral wool. I have to say that i am very happy with the result, the thing i tried to insulate from was gas furnace. I tried putting my ear on the door today and it was a lot better, basicly it was the same as if i put my ear on the double drywall wall with fiberglass insulation between ( wall is almost 3 times thicker then the doors ). A thing to say is that insulation is better then it was, even though i removed the old seals around the doors and didnt apply new ones yet. I plan to do that tomorrow when i will see the final result. While i am happy with the results, i dont think i will ever be doing this again. Next time i am buying some second hand massive doors and then i will just put layers of drywall over it.

ps: i was wondering ... lets say you are very limited with space and want to add mass to something like the door. If you would compare drywall and extremely thin metal plate, and i mean really thin one, and they would weight the same ... would the sound insulation also be the same ? This kind of plates are used for big roofs and therefor it would probably very cheap to cover 1 small door with it. It would also leave some space for mineral wool ...

johnbergstromslc

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Re: Filling the hollow doors for better sound insulation
« Reply #5 on: May 15, 2013, 09:39:34 PM »

Whatever panel you attach to the outside of the door, I'd still recommend attaching it with foam tape.  You'll get some decoupling versus having a rigid mechanical connection.

1/16" steel is roughly equivalent to 1/2" drywall in terms of density.  A thin plate will vibrate more, though....

Randy S

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Re: Filling the hollow doors for better sound insulation
« Reply #6 on: May 15, 2013, 09:55:00 PM »

You can staple or glue mass loaded vinyl 1lb or 2lb to the door to increase the mass with ease.
http://www.supersoundproofingsales.com/products.asp?dept=28
Randy Sieg

Super Soundproofing Co
www.soundproofing.org
888-942-7723
Ph. 760-752-3030
Fax.760-752-3040

goblinsly

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Re: Filling the hollow doors for better sound insulation
« Reply #7 on: May 15, 2013, 11:10:35 PM »

I did look for mass loaded vinyl before, i hope we even have it in Slovenia ... i dont really know what it is, is it cheaper in terms of money per mass then drywall ? How heavy is it anyway, i suppose for square meter, for same weight, vinyl will be thinner then drywall, and i guess in terms of vibrations, it will be better since it is somewhat softer, not to mention like you said, ease of installation. I do have some leftover drywall which i have to use also ...

foam tape ... again, i will have to look for that .. could you describe it to me ? I have double sided duct tape and that i guess would be similar only thicker with some foam between. Are you sure it will hold all those panels in place ? I plan on putting 3 or 4 sheets of drywall on my door ... i was thinking of using normal silicone also ... offcourse it is not near as good as greenglue but i guess it is better then nothing.

johnbergstromslc

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Re: Filling the hollow doors for better sound insulation
« Reply #8 on: May 17, 2013, 09:47:38 PM »

Drywall is about 1/6th the cost of MLV....

Foam tape will not hold 3-4 sheets of drywall.  In my opinion, that is massive overkill - the sound might not get through the door, but it will leak around the frame.  I wouldn't go to all that trouble.  A single piece of 5/8" (16 mm) will work fine.

goblinsly

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Re: Filling the hollow doors for better sound insulation
« Reply #9 on: May 19, 2013, 04:33:12 PM »

Btw as far as mass goes ... i have 2 big pieces of glass ... they are too scratched to be used as a window or something like that but if i attached them to the door and put a nice wooden sheet over them, they would not be visible. Would they be a good mass material, i would also put some silicone between them and the door. They are not as big as the door, soo there would be a few cm missing on the edges of the door, there i would just put something else like wood. What do you think ?

As for only using 1 sheet of drywall ....we will see, the most importaint thing is to really seal the doors well to prevent any cracks, but still i think 1 sheet wont do it. Sometimes the speech is directly on the other side of that door and as it is now it wont even block normal speech (hollow doors + 5cm DP8 mineral wool, 15mm wood, 3x12mm drywall).

mr browne

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Re: Filling the hollow doors for better sound insulation
« Reply #10 on: February 22, 2014, 02:16:04 AM »

Drywall is about 1/6th the cost of MLV....

Foam tape will not hold 3-4 sheets of drywall.  In my opinion, that is massive overkill - the sound might not get through the door, but it will leak around the frame.  I wouldn't go to all that trouble.  A single piece of 5/8" (16 mm) will work fine.

Dont mean to sound rude here, not trying to be but if you think simply adding a single piece of 5/8" (16 mm) dry wall to the back of your door is going to acheive much then i would read up more on soundproofing, unless the noise you are trying to reduce is very minimal then you might have ok results but it's not nearly enough. Piling on the drywall is physical overkill, that's where MLV comes into it's own, thin, easy to use and extremely effective. If you are only doing a door it's not overly expensive if you take into account time vs money. consider the clean up time of cutting down 4 sheets of drywall, finishing the edges, cleaning the mess, taking off the handling, making the hole in the drywall, refastening it. you wont be able to simply put it back on because it'll need a longer barrel or whatever it's called that holds the two doorhandles together blah blah blah or you could just glue up some MLV that cost 6times the cost of drywall and be done in 1hr. i know what i would prefer...

I suppose you have to take into consideration where the door is located, what the purpose of the door is and what your budget is. I personally can't understand why anyone would simply add drywall to their door!!! Crazy. Perhaps if you were adhering it with soundproofing green glue [instead of stapling anything or using foam tape or any there tape, you need some decoupling so nailing or screwing is defeating the purpose here] and if the dryway was acoustic drywall. This is not the cheapest way but it's going to add the least bulk/weight to your door and yield the best soundproofing results.

But you still have to tackle the problem of the door jamb itself!!!!!
Is it wood? Is is metal? Is it hollow metal?

I love FELT so i'm bias here but i have had great results with gluing 1/2" wool felt to the front of the door [assuming the door opens inwards] and 3/4" wool felt to the rear of the door. The front side covers only the door itself. The rear side has a 3/4" overhang all away around the door so when it is closed it overlaps the door jambs and reduces bleed between the door and jamb.



jhbrandt

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Re: Filling the hollow doors for better sound insulation
« Reply #11 on: February 22, 2014, 04:11:50 AM »

Hi Shane,

MDF is the professionals choice for sound-proofing a door. I have never used gypsum board on a door and wouldn't recommend it, since the door must 'flex'. Flexing drywall will breakup eventually, if not immediately.

MLV is also a very poor choice for adding mass to a door, bang for buck, you're better off with MDF and/or an MDF/sheet lead sandwich. For example; one sheet of 3/4" (18mm) MDF is $12.57 at Home Depot & has a mass of 2.93 lbs/ft2 (14.3 kg/m2). MLV that is about 1/4" thick weighs in at about 2 lbs/ft2 (9.8 kg/m2) and costs $82.35 for the same amount... and you're going to need a layer and a half to match the mass of the MDF.

MLV might do for a sandwich although it is much lighter than lead - but for the equivalent weight, MLV is cheaper. But I would never use MLV as a covering. MDF can be painted or veneered, and will provide a nice finished look.

Mass is what does it.

Don't put rock wool inside a hollow door.

You need mass and seal for a good sound-proof door.

The doors that we have put in hundreds of recording studios world-wide are usually 160 kilogram (350 lb) monsters - often sand-filled.  ;)

We use a combination of Automotive balloon seal (trunk rubber) for our door seals and either a Pemko or Zero International neoprene drop seal mortised into the bottom of the door.

That's how it's done. Any other way and you're just throwing good money after bad. I don't recommend trying to reinvent the wheel here. Go with tried and TESTED methods.

Also, felt is not recommended. It's absorption properties are very similar to very thin carpet which has a peak absorption at around 2 kHz, and pretty much leaves the rest of the audio spectrum untouched. Also, if felt is IN the sealing area of the door/door stop, it will allow air passage - which is synonymous with sound passage. There must be an air-tight seal around the perimeter of the door. Rubber fits the bill here.

Cheers,
John

johnbergstromslc

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Re: Filling the hollow doors for better sound insulation
« Reply #12 on: February 23, 2014, 02:39:44 AM »

I did originally recommend that he replace the door with a solid-core slab, or use 2 sheets of MDF, but it became clear the guy had already made up his mind he was going to use drywall.  I've answered enough posts on this forum to know that when people have made up their minds that their ideas are the best ones, it's pretty futile to try to convince them otherwise...

Also, he said he was from Slovenia, so MLV is NOT going to be readily available, if he can afford it at all.  Adding 3-4 sheets of drywall would probably not be safe (not to mention look ridiculous) and since he was probably just trying to reduce general noise, I told him not to go overboard, since he seemed to recognize the importance of sealing the jamb.  One layer of 5/8" drywall would double the mass of the door and with the addition of good seals he would get 10-12 dB of reduction.  Not fantastic, but a lot better than what he had.  And easily reversible if/when he has to move.

(Kinda sounds like to me that if he just sealed what he already had, he could have gotten a huge improvement.) 

As for the "I personally can't understand why anyone would simply add drywall to their door!!!" comment, let's do the math:

Drywall: 8-10 bucks a sheet
MDF: 30-35 bucks
MLV: $50+ for 4X8 sheet

If budget is a consideration, as it seemed to be in this case, it's a no-brainer.  And if he attached it with foam tape, like I recommended, there will be no destructive 'flexing'.

I don't try to convince people to build recording studios (and put in 350 lb doors) when they are just looking for some extra peace and quiet.  I give practical, cost-effective advice...
     

jhbrandt

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Re: Filling the hollow doors for better sound insulation
« Reply #13 on: February 24, 2014, 03:18:40 AM »

A big +1 to J.B.'s comments  :)

"I don't try to convince people to build recording studios (and put in 350 lb doors) when they are just looking for some extra peace and quiet."
LOL! I agree. The studio door was simply an example that it is the MASS that performs the 'sound-blocking'.

Cheers,
John