Treatment to an existing residential door for improved noise control

[EricatNordic]

Are there any treatments that are common to residential construction that can be done to reduce the transmission of sound through old hollow core interior doors?

I've sealed airgaps with no effect.

Normally I'd go buy a new door. But the finish wood in the house is all from the same period, and I would like to keep the matched look and vintage.

In boats we have filled sound boxes with sand to kill transmission. I considered filling the door with sand to see what would happen. But that sand would be heavy. Any better ideas?

-Eric

 

[GregLocock]

Unfortunately most acoustic treatments work by virtue of their weight per unit area. Salesmen will tell you different. There are tuned systems available (heavy sheet on light foam) but they only offer enhanced performance over the same weight of sand for a small frequency range.

Sealing is the most weight efficient method.

So I guess I'm saying that you've covered all the obvious bases.

One lighter trick might be to reinforce one of the two skins with a second skin of equal or slightly greater thickness. This decouples the panel modes of the two skins.

Another option would be constrained layer damping, that is a bitumen layer backed by a thin tin sheet. This damps out the surface waves in the main skin.


The advanatage of these two is that in the worst case scenario they still work as mass loading.




Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[MiketheEngineer]

Would foaming the hollow core help??

Drill some holes in the bottom and inject it??

If you try this - make sure you don't "blow up" the door. There is a new foam - not that yellow stuff - that is white and water soluble. Clean up is easy and the foam does not seem to over expand like the old stuff.

Good luck

 

[Strong]

Eric,

I don't expect that the foam would work very well, but it is certainly an easy method. If you really have your heart set on saving the door, then consider some surgery. Remove door, hinges and latch. Use power saw to slice door edges, so that both panels have a half-thick frame. Attach acoustical barrier material with staples to inside of one door panel. Attach door panels together with construction adhesive and/or fasteners.

Acoustical barrier:

http://www.acoustiblok.com

Walt

 

[EricatNordic]

I've never used the white foam mentioned. Is it also a BASF product like the autofroth? You got me curious, although not really for this application. Closed cell urethane foams tend to be dissapointing as sound insullation. They would almost certainly blow the door apart too.

I considered cutting the door in half as suggested and either lining with sound barrier, or laminating the door skins together with scraps of nidacore from the dumpster at work and then rebanding the edges.

Thats a lot of work though. Before chopping things to bits I wanted to see if there was a pour in filler that the construction industry people knew about and I didn't.

I'm going to give the sand a try first. But I will try a mix sand and foam peanuts to keep the weight down enough that I can still pick the door up by hand.

Thanks everyone.

 

[Strong]

If you mix sand and peanut foam, then over time the sand will settle to the bottom and the foam goes to the top. The result will probably be a partially filled cavity that has a sound short circuit through the peanut foam.

Injecting urethane foam may provide some sound reduction by damping the wood panels and providing cavity sound absorption. The slight increase in overall mass would not increase the transmission loss.

Now if you could uniformly inject sand with the foam, you would end up with a mass-filled limp-mass barrier!! How about filling the cavity with half sand volume, then place door horizontal and shake until the sand is uniformly distributed. The inject foam (air relief holes on opposite side) into the open cavity. Allow foam to cure before moving door to an upright vertical position. It's crazy, but you would now have a combination of foam and sand core with the benefits of both (mass, damping, and absorption)!!

Walt

 

[IRstuff]

What about stiffening the panels in the door?

TTFN

FAQ731-376

 

[EricatNordic]

Nice idea Walt. I will try it. Normally sound foam is open cell though right?

Is there an open cell pour (squirt actually) foam for sale to the general public? So basically we end up with a very heavy open cell foam core door.

The best part about the endeavor is thinking about the look on the next owners face whenn they cut into this thing years later.

BTW I considered stiffening the door, but don't really want to split it in half for fear it might never go back together. That will be the fall back plan.

-Eric

 

[Rob45]

Foaming is unlikely to do any good, as is any other method of stiffening.
In fact, you already have the very opposite of what you want: you have a door that is light and stiff, and what you want is one that is limp and heavy.

As Greg pointed out previously, weight is your single most important barrier characteristic, and "limp" is the most important characteristic of a heavy barrier. Limp so that there's no resonance, and so the coincidence effect is minimized.

Filling the door with sand is not such a bad idea. You might even investigate Lodengraf damping.

- R

 

[Sprocket59]

Eric,

Whichever method you use, if the door becomes significantly heavier, you should consider using longer screws through the hinges and into the framing.

If you use an expanding foam, it may be prudent to clamp both faces of the door with stiff, flat boards and allow the foam to expand through holes at both ends of the door.

 

[EricatNordic]

I had not heard of Lodengraf damping before. Interesting reading. But I haven't got the part about where to buy these materials yet.

I did drill the door and tried the sand. I knew the door was cored with corregated cardboard and used a long steel rod sharpened at one end to punch through the cardboard so the sand would fill all the way to the bottom. What I didn't know is that the door is also cored with 2 solid wood stiffeners 1/3 and 2/3 down the door. I don't have a 6 ft electricians drill bit, so I was only able to fill the top third of the door with sand.

Still the results showed some promise. Judging by ear, there was a noticable improvement in the quality of the sounds heard through the door. I can still hear conversations on the otherside, but the sharp vowell sounds are softer now, which makes the noise less intrusive. Big difference in the feel of the door when we open/close it though. I think I could swing it through the wall if I slammed it.

I'm going to get a longer bit and finish the job soon.

-Eric

 

[MiketheEngineer]

Based on other posts - seems the foam idea wouldn't work but you did ask about it.

It is made by DAP and is available in the insulation aisles. IT is latex based (water clean up)white in color and will not over foam. Seems to have no "power" when it foams. Acts more like soap bubbles - fills the space and stops. I like it because it works and easy clean up. Does take a while to cure.

Someone suggested longer screws -- VERY good idea.

 

[EricatNordic]

Yes, Will use longer screws and a third hinge if I can find one to match. Even just 1/3 full the door feels like its made of concrete.

Not that I would ever need such a thing, but I just imagined the WTF look on a swat teams face when they smash a door down and out comes 500 lbs of beach sand. : ) To bad I'm not that white trash.

I'm very interested in the DAP product for other applications though. I'll check it out.

Thanks everyone!

-Eric

 

[Sprocket59]

Eric,

Instead of buying a longer bit, you could drill through the hinge side of the door to access the middle cavity. Then plug the hole with a short length of dowel and sand flush.

 

[Strong]

Eric,

I realize that the discussion is about redcing sound transmission through the door panel, but typically the greatest fault with conventional doors is the sound leaking around the jamb and threshold. A small open gap will transmit more sound than through the entire panel! Remember "air tight is sound tight". Consider installing jamb and threshold seals if you haven't done this already.

Walt

 

[EricatNordic]

Thanks Walt. That was step one for me. I stuffed towels around the loud side and taped off the inside before ever getting into the sand and foam experiment. I spent a good bit of time with a stethyscope playing different frequences and listening for primary transmission areas. Then I cranked up the volume and looked for harmonic responses in the room.

The sound was definetly coming through the door itself. The only thing besides the door that was noticable resonating was the plumbing.