# Sandbagged Soundroom?



## sandbag (Aug 2, 2009)

I'm retrofitting a room sharing two interior walls, the ceiling and the slab floor with my residence for use as a HT and listening room. Dimensions are 15' X 16' X 9,' finished with 5/8 gypsum. The ceiling is drywall hung from 2X12 rafters, 16"O/C at the ceiling (and form the floor of master bedroom 3/4" plywood covered w/ closed cell foam/MLV rollgoods, carpet padding and wall to wall carpet). There is only one mechanical connection, through a F/A duct (flex w/ metal boots) running between 2 of the joists, and venting into the bedroom floor (see my proposed fix for this duct). Sidewalls are 5/8" on 2X4 at 16" centers - front wall (video display) shares wall with guest bedroom and closet. Two walls are exterior coated with stucco, each having a 3'X5' window for great ventilation. Fourth wall is shared with the large and unoccupied furnace, waterheater room.

The major problem is low frequency transmission to the master bedroom above and into the guest bedroom sharing a wall with the listening room.

Plans: 
Ceiling - Remove ceiling drywall. Isolate F/A by packing insulation around the duct, covering the metal boots with hmv+ closed cell foam, then adding drywall and MDF to add mass and rigidity to the inside and outside of this 14.5" space. I'm contemplating coating the exposed ceiling plywood with drywall - resilient adhesive, as well as the newly exposed joist surfaces. I plan to leave this currently enclosed space clear to serve as an air pressure plenum linking the airspace around the four sidewalls and ceiling of a new room within a room structure (to be about 13 X 15 X 8.5' interior dimension). 

RWR - My plan is to build the entire room within a room and new ceiling with sandbags, stabilized against tipover with a custom support assembly. The new ceiling will be corrugated steel supported by wooden beams and posts. The sandbags will be stacked on the steel in overlapping layers, but will not contact any of the joists or ceiling existing ceiling components. I expect the look to moderately improve upon the interior of a typical Khe San bunker. Since sandbags have some degree of efficacy in muffling mortar rounds, I expect them to at least help reduce the dBs from my sub in the adjacent rooms.

I am not satisfied that any conventional treatments will adequately address the 120 Hz and below sound that I want in my theater, but not outside of it. High frequency transmission is not particularly offensive now, but I expect to substantially reduce and control it with my RWR "airlock" approaches and direct to outside ventilation plans. No HVAC will be shared with the residence, other than structurally into the existing F/A line mentioned above. 

SO - I am looking for (1) any one with direct experience in sandbagging (for low frequency attenuation), (2) comments or questions regarding the rationality of pursuing this objective under the stated conditions, including - why not bag the sandbag project and spend the money on a free standing A/V room having better "large room" interior dimensions and proportions?

I'd like to try the sandbag approach because I think it should be significantly more effective than adding layers of MLV, drywall/green glue/drywall, battens, mechanical isolators, Helmholtz resonators, or more exotic approaches such as charging interior surfaces to transduce sound energy back to electromagnetic, or others not yet contemplated. However, I have not yet found data or documentation on the use of sandbagging against sound, other than the anecdotal suggestions of sound physicists. The irregular surface of bagged sand may well be too "dead", but I am hopeful that this will make the walls sound further away (larger than drywall). The ceiling will be reflective, but well-damped by the suspended mass, suggesting that a simple "cloud" type surface treatment may suffice. Further, the corrugation should "raise" the ceiling somewhat, compared to a more nearly flat surface.

If necessary, I assume I can liven up the walls by hanging simple drywall reflectors - sort of the reverse of typical "add absorbers" approaches to rooms that are too live and generate unwanted/difficult nodes.

Waiting to hear from the pros, or anyone who has tried my proposed approach, even if only in theory.


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## bpape (Sep 14, 2006)

Realistically, EVERY joist is a shared mechanical connection to the rest of the house. You must fill the joist cavities with insulation (don't pack it) and decouple with clips, channel and drywall. Double with Green Glue will help considerably. 

Yes - building a free-standing room that's totally decoupled is the best solution and you seem to have the room to do it. 

Bryan


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## patchesj (Jun 17, 2009)

Sandbags.. Wow, that is one way to add mass. This might sound silly, but what about precast/tensioned concrete slab?


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## sandbag (Aug 2, 2009)

Too reflective, I think. Sand has the advantage that each bit of silicon containing crystal resonates according to its own characteristics. Concrete tends to resonate as a single mass on account of the mechanical coupling of the individual aggregate particles through solid bonds. That's why one prisoner can tap morse messages to another. While sand particles are in mechanical contact, they are more free to vibrate individually. Larger aggregate particles, like 1/2 inch gravel may be superior to sand since there is more air to be activated. Can't seem to find good data to tell, though.


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## patchesj (Jun 17, 2009)

So I need to build my theater room at the beach.. :bigsmile:


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## sandbag (Aug 2, 2009)

Insulation - I remain unconvinced that thermal insulation is the best solution to sound insulation. I don't think that much benefit is gained by stuffing the space with batts. To avoid hard surface to hard surface transmission, would not open celled foam on facing leaves permit more air movement while reducing leaf to leaf propagation? 

Decoupling - I would think that a 1/4" layer of closed cell foam between layers is a better resilient decoupler than two tubes of green glue, but would sacrifice stiffness and mass. Perhaps an 1./8" or 1/16th" closed cell layer would compare better with 2 - 3 tube green glue between drywall sheets. Sandbags or lead shot have long been the dampener of choice for bass drum shells.


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## sandbag (Aug 2, 2009)

Or dig it into a sand dune and figure out how to keep it out of your listening area, since sand has a 45 degree angle of repose, resulting in an inverted pyramid shaped listening room.

Sand is cheaper than drywall, and bags only cost about 20 cents each.


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## patchesj (Jun 17, 2009)

what "depth" of sand are we talking about on the ceiling? I would think this is going to require serious support.


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## patchesj (Jun 17, 2009)

Another crazy idea... What about water bladders?


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## ScruffyHT (Sep 23, 2008)

sandbag said:


> Insulation - I remain unconvinced that thermal insulation is the best solution to sound insulation. I don't think that much benefit is gained by stuffing the space with batts. To avoid hard surface to hard surface transmission, would not open celled foam on facing leaves permit more air movement while reducing leaf to leaf propagation?


Any kind of spray foam will recouple the cavity with the rooms outside the theatre ... the function of loose batt insulation is just to prevent the cavity from acting like a drum ... cheap R13 is fine for this purpose

If you want to decouple then using clips/channel, staggered studs or a seperate wall altogether is the desired approach - it all depends on how much space you have to work with



sandbag said:


> Decoupling - I would think that a 1/4" layer of closed cell foam between layers is a better resilient decoupler than two tubes of green glue, but would sacrifice stiffness and mass. Perhaps an 1./8" or 1/16th" closed cell layer would compare better with 2 - 3 tube green glue between drywall sheets. Sandbags or lead shot have long been the dampener of choice for bass drum shells.


Green glue is not used to decouple ... it is designed to dampen the bass energy from being transferred from one layer of drywall to the other. It does so by converting sound vibration to heat energy before it has a chance to get to the next layer of drywall.


I gave a lengthy reply in your other thread here ... http://www.hometheatershack.com/for...325-reflections-reflections-live-vs-dead.html


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## ScruffyHT (Sep 23, 2008)

If you do have the space to go with a "room within a room" then this is how I would approach it

build the outside walls at least 1" away from the inside walls - make sure that you dont have the original walls with anything but bare studs and standard batt insulation

use dc04 clips to attach new walls to roof joists

add DD 5/8 drywall with green glue to walls - you can use 1 tube of GG per sheet and still get 70% efficiency from the green glue

you will be hanging new roof joists that will sit on top of the new interior walls - add DD 5/8 plus green glue here as well

for hvac use mufflers and dead vents 

for electrical use backer boxes on all plugs/lights

for the door - a solid core door properly weatherstripped with a good threshold


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## patchesj (Jun 17, 2009)

Why do you need dc09 clips?? For room in room, do the walls need to be attached to the "old" ceiling joists at all? What would you recommend to isolate the new walls from the existing floor (if it's a concrete floor, anything at all?)?


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## ScruffyHT (Sep 23, 2008)

If you can manage to keep the walls perfectly vertical on all 4 sides while attaching the new ceiling joists then go for it  ... however using the dc04 clips they are spaced about 4 feet apart so there will be minimal contact with the original roof joists and will ensure perfectly vertical & decoupled interior walls

new interior walls will be 1/2" shorter than the original walls


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## patchesj (Jun 17, 2009)

So back to the sand concept... How much sand are you planning on putting up there? How much would be required to have an impact on <100Hz? And the crazy question of the day, how much weight could a double 5/8th drywall ceiling hold between the joists??


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## bpape (Sep 14, 2006)

No way would I do sand in the ceiling. Just too much weight, moisture issues, etc. Drywall will never hold it long term.

Closed cell foam on the faces of joists will be compressed and recouple everything as soon as you screw things together. Not a good solution.

DC-04 clips stabilize the structure while providing decoupling and allowing some flex to the wall which will help reduce transmission.

Clips, Green Glue, Insulated cavities, and double drywall are proven methods that have been measured and the results are quantifiable. Sand is used ONLY to damp cavities inside the room that are sitting on the floor to keep them from vibrating and resonating. There's so much weight there that even doing it on a normal floor with joists and wood - it would be recommended to consult a structural engineer before doing it. Drywall - not gonna happen.

Bryan


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## sandbag (Aug 2, 2009)

How much sand - about 4" - 5" in two layers of bags resting on the new ceiling membrane comprising corrugated sheet steel. The steel would be laid over the top of 4"x 8" rafters on 42" centers. Each rafter would be supported by two 6"x6" posts about 10' apart. The sidewalls are less of an issue as they could be conventional since they would not be load bearing. I like the sand idea, as it is sort of an internally damped gypsum board thereby lacking the drumskin type resonances propogated by traditional sheet goods.


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## bpape (Sep 14, 2006)

However, what you're doing is actually creating a 3rd 'leaf' which is not good. In some cases, a triple leaf can actually transmit MORE bass through the structure.

Bryan


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## sandbag (Aug 2, 2009)

Eschewing sand for the moment -- the existing walls are all integral support walls engineered to resist earthquake shear forces -- welcome to California! Hence there is a layer of glued and screwed 5/8 plywood between the 5/8 drywall coating and the 16" o/c studs. I understand that one should avoid hard surfaces facing one another. Given the engineering constraints, I have considered securing 3.5" fiberglass to the support wall surfaces and building my new wall 2" out from the old. 

My impression of the purpose in avoiding internal leaves is that by eliminating sheet on the internal surfaces of the walls allows the studs freedom to vibrate torsionally, radiating sound energy from the unsecured edges. The insulation then absorbs some of this energy, thereby resulting in greater attenuation due to the significant energies lost in vibrating the stud edges. I further suspect that fire stops tend to reduce this vibration and therefore would contribute to less sound attenuation than unblocked studs. I appreciate conventional wisdom, but I need data to be convinced of anything concerning sound attenuation. I tend to ignore STC's since they are nearly meaningless at the frequencies with which I am concerned. As I mentioned, my current walls are not bad at mid and higher frequencies. Another possibility as to why exposed stud edge spaced apart from exposed stud edge doublewall construction works is that the long waves are being broken up by the Helmholtz effect. If this is true, then wouldn't a series of Helmoltz resonators inside the walls be even more effective over a wider range of low frequencies. The point in all of this is to lose sound energy by requiring it to pass through "unfriendly territory. So, to me, the unanswered question is how to create the unfriendliest territory between the listening room and the adjacent residential space, assuming that you've done everything you can to eliminate mechanical transmission problems by spacing the surfaces and members apart. 

Do you have experience with Quietwall TM board? Their THX board is essentially a series of membranes adhered with a green glue type of adhesive under pressure. They even include a layer of metal foil!! Does this stuff work against low frequencies? Their data is confined to STCs which, as noted above, don't help me with the big waves. 

I've not read the old government report generated for multifamily housing sound treatments, is there anything in that report I should consider?


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## bpape (Sep 14, 2006)

Their thickest stuff is pretty good - if you have 3 guys to lift every sheet and a hefty pocket book. For the same money or less, you can do 3 layers of drywall and 2 layers of green glue that will definitely outperform it.

For data, look at the Green Glue web site for the test data. You're correct that STC is meaningless in the low frequencies. Their tests show various structures and not only the resultant STC, but also the loss response plots.

Bryan


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## Ted White (May 4, 2009)

Bpape and ScruffyHT have covered it nicely. Foam insulation is nearly useless, both closed and open cell. Not nearly open enough for required interraction with soundwaves. All you will accomplish is a reduction in air cavity depth and an increase in the LF resonance point. I cannot recall a single professional installation that ever included it.

The only way to deal with low frequencies is to lower the natural resonance point of your partitions. You can only hope to deal with frequencies 1.5X the LF resonance point. So if your interior wall has a natural resonance point of 100Hz, you will start to see a performance drop as you approach 150Hz, with performance plummeting until the 100Hz point is reached. After that, it is a mass law defined system. No further gains from decoupling, absorption, or damping will be realized. 

So we try and march that LF resonance point down as low as we can. Assuming the walls or ceiling are decoupled, you can further lower the LF resonance point by adding more mass, adding insulation for absorption and increasing the cavity depth. All very tried and true. I've tested such assemblies for many years.


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## Ted White (May 4, 2009)

sandbag said:


> Insulation - I remain unconvinced that thermal insulation is the best solution to sound insulation. I don't think that much benefit is gained by stuffing the space with batts.
> 
> This is more correct in a coupled system. In a decoupled system the absorption has increased benefit.
> 
> ...


Not so. The foam is doing nothing. No mass, no damping capacity, no absorption and doesn't decouple. 

The sand and lead shot function as mass and damping. The kinetic energy is converted to thermal via friction. Similar to what fiber insulation does and foam does not. Sand and lead are not used due to the impracticality, as well as health hazards associated with both.


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## tonyvdb (Sep 5, 2007)

Sand is a great sound absorber. My cousin and myself helped build a recording studio at our church (room within a room) and the interior door was a slider that we built it was 6" thick and was filled with sand and weighed over 200lbs. When that door and the exterior door was closed not one ounce of sound came out of it and believe me we tested it, The monitors we used were big EV studio monitors that had an amazing frequency response going down to 22Hz with over 400 watts per channel driving them.:bigsmile:


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## Ted White (May 4, 2009)

tonyvdb said:


> Sand is a great sound absorber.


It really is. If Sandbag could incorporate it into his room theme he'd be pretty isolated.


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