[JohninCR]

I'm about to convert an office that is separated from my house by a terrace into a combination Office/HT/Listening room.  I plan to dismantle the current almost nothing walls to start from scratch using only the existing roof and 4 corner support columns.  With the equipment and drivers I already have, I have the capacity for extreme output all the way down to the low teens.  I also watch a lot of movies late at night, so I don't want to wake the kids or disturb the neighbors, making my emphasis the prevention of sound transmission.

Everywhere I read that mass is the only way to contain bass, however, down here in Costa Rica drywall isn't an option and I really don't want fool with the expense mess of concrete based sheets which are available.  Instead I want to use a layered construction to turn all of the walls into panel absorbers that dissipate the bass within the walls, so I kill 2 birds with one stone, bass treatment and sound transmission.  Cheap materials that are available for use include plywood, styrofoam sheets, foam rubber sheets, and polyfill batting.

Bass traps don't require mass to dissipate bass, so I think my plan will work.  I understand that the lowest frequencies will probably require sealed panels the size of a full sheet 3/4" plywood.  I may even have to add some mass to the middle to tune it to the 20hz range.

Any recommendations or tuning tips are greatly appreciated.  If I'm missing the boat please let me know.

[bpape]

First of all, in order for the kind of sealed traps you're discussing to work, they need to be mounted to something very rigid so that the front panel is the only thing resonating.  With what you're describing, you won't have that.  IMO - what you're describing won't work.

Now, you could certainly build MANY leaves of varying density and thickness to help dissipate the sound leaving the space but that would require a LARGE amount of space - like maybe 3-4 foot thick walls.  Even then, the multiple leaves will reinforce some bass frequencies.

Use the mass coupled with an isolated type of construction (like a room floating inside another room).  That's the other thing that works to stop sound.  Physical contact isolation.

Sorry - you just can't change the laws of physics.  Flimsy walls are GOING to let sound out.

[ctviggen]

There's a difference between trapping bass (such as having a plywood sheet rigidly connected to a frame) and stopping transmission of bass.  The Handbook of Acoustics discusses traps made of plywood (thicker = lower frequency absorption), which means that the plywood resonates and reduces the amount of bass in the room.  But I'm not sure that this means that less bass is transmitted through the trap itself -- no data is given to that effect.  Bass trapping is performed to reduce room modes and associated ringing, but the bass is still in the room, just the particular modes are lower.  If you want to stop transmission of bass to other rooms, that's going to entail dual walls, air spaces, etc.

[JohninCR]

Thanks guys, but I'm still not sold.  I'm not trying to get around the laws of physics at all.  Essentially what I want to do is take acoustic energy and turn it into heat and we're talking about at most a few hundred watts of actual energy to be dissipated.  I don't believe mass is the only option because tube traps can be very effective with relatively little mass.  Panel traps will naturally have more mass, but it's not the mass doing the work, it's the construction making the mass more efficient.  My net result will be panel traps plus a room within a room construction which will act like another much larger panel trap.

The roofline currently extends 18" past the existing 5" thick walls, so I figure that I have 15" to work with.   I'll also take care to prevent any sound shortcuts where the walls and roof meet.


Here's what I had in mind, starting from the inside:

Interior wall (material covering at the end)-
1/2" plywood anchored to roof rafter and runner on the concrete floor.  2 panels on each side of the room would have studs on 48" centers and I'll attach some extra mass to the center of one panel on each side to lower it's resonant frequency.  The other studs will be on 24" and 16" centers.  I'll seal all of the joints because each stud space needs to be sealed.

I'll fill the space between the studs hopefully with rolled insulation if I can find it, otherwise with 1-2" foam rubber attached to the backside of the interior wall panels to absorb and dissipate the resonances of the panels.

Chipboard or cheap plywood to cover the outside of the studs using caulking to seal the union with each stud.

Next is 1/2" foam rubber and then 2" thick styrofoam with foam rubber strips between adjoining sections of styrofoam to ensure no styrofoam rubbing noise.  I'm unsure how best to attach this layer to the wall.

At this point I should have a good soundproof wall with at least some absorption of bass frequencies, but I'm not finished.  

Next is a 4-6" airspace then the exterior wall with absorbant damping on the interior side.  I plan to seal the airspace, but I'm uncertain of how to best attach that wall to the structure and whether or not it is best to isolate it to the extent possible from the structure.  

I'm also unsure of what material to use for the exterior wall.  If the concensus is that I still need mass, then I'll bite the bullet and go with the concrete based 1/2" thick sheets, but I think one of the cheaper composite sheets for exterior use should be sufficient since it's really forming another LF bass trap.

Thanks so far, the more feedback the merrier.

John

[ScottMayo]

I'm about to convert an office that is separated from my house by a terrace into a combination Office/HT/Listening room.  I plan to dismantle the current almost nothing walls to start from scratch using only the existing roof and 4 corner support columns.  With the equipment and drivers I already have, I have the capacity for extreme output all the way down to the low teens.  I also watch a lot of movies late at night, so I don't want to wake the kids or disturb the neighbors...

I just went through this exercise. The drywall/homesote/drywall on one set of studs, insulation, a few inches of space, and drywall on a separate set of staggered studs, did a very good job of containing and eating sound. Unfortunately, the *door* in the wall passes quite a lot of sound...

The other hassle was that I wanted a room without windows, but local code forbids that. Windows pass a lot of sound.

So my advice is, don't sweat the wall design until you have priced out soundproofed doors and solved the window problem. I suspect my walls easily stop 50db of sound and maybe 60: but the door barely stops 15. That's where I should have spent the money.

At any rate, to absorb sound, you need something to convert mechanical movement to heat. Something has to deform, flex or rub to do that. Insulation works by having the glass strands rub; springs and rubber mounts do it by flexing. In order to give those things a firm base to work against, you need mass. It's hard to see an alternative; without mass, the thing that should flex or rub will simply shake as a single mass, accomplishing nothing.

Styrofoam is worse than useless. Being rigid and light, it's an almost perfect carrier of sound.

[JohninCR]

Thanks Scott,

I'm in Costa Rica, so I don't have to worry about any code, so no windows for me.  My door will be pretty close to the fullest dipole null as discussed in the other thread about alt to bass treatment and I'm building it myself, so while the door won't have the same sound deadening capacity as the wall, it will be no slouch and the other side of it is an enclosed room offering further isolation from the house and neighbors.

Drywall isn't used in construction down here, so it's not available.  I'm not sure it would be super effective unless it's in a tuned panel trap when you're talking about  100db+ waves from 30hz down to the teens.

You may be right about styrofoam.  I was under the impression that it was very effective in a layered construction so it's high resonant frequency resulting from being rigid and lightweight could be damped.  Maybe I should swap it out for 2 extra layers of chipboard and foam rubber for about the same price.

[John Casler]

You may be right about styrofoam. I was under the impression that it was very effective in a layered construction so it's high resonant frequency resulting from being rigid and lightweight could be damped. Maybe I should swap it out for 2 extra layers of chipboard and foam rubber for about the same price ...

I would use alternating layers of carpet pad 1/2" and thick plastic sheeting with thin chip board layers.

The chipboard will flex slightly and absorb the energy,  The combo should work quite well.

Layered thusly:

Chip board>carpet pad>plastic sheet>carpet pad>chip board>repeat

[JohninCR]

Thanks John,

Ah, the things we take for granted stateside.  I remember looking for carpet padding once here and it was more expensive than carpet, since only one place had it.  Wall to wall carpet is very rare here.  On the other hand material stores are everywhere here and there are lot's of furniture makers, so sheets of open cell foam are easy to find.

I guess the plastic is to keep the rubber layers functioning as independent layers instead of 1 thicker layer?

What about attaching the layers of chipboard and rubber?  I'm thinking that staggering the joints of the different chipboard layer is a good idea with the plastic sheeting being one uninterrupted piece covering the whole wall.  Then use screws only through the next chipboard layer and only around the perimeter of the one being attached.  Maybe just very few screws or just contact cement, enough to hold it up as I add layers, then add a framework over the last layer to make sure it can't fall apart.

[jules]

this is purely intuitive but how would it be filling a wallspace between say an inner and outer layer of ply, with sand?

jules

[jules]

the same treatment might work for a door too ... certainly cheaper than many other options

Jules

[John Ashman]

What about good old-fashioned mud aka adobe?  Or concrete blocks or brick.  Very effective.

[JohninCR]

It rains too much in my part of Costa Rica for adobe.

I'm too cheap and lazy for cinderblocks, plus building a block wall under an existing roof might be tricky, however, there is a sheeting here that I thought was sheetrock from afar.  Up close showed some kind of 1/2" or 5/8" concrete based, steel reinforced material.  I haven't priced it yet, but it looks like a nightmare to handle even with a helper.

Laying bricks properly takes experience, but guess what, no bricks here either, at least not at any construction supply house I've been to.

$500-600 and a weekend doing stuff I already know how to do and I'm ready to trim out the electrical.  Mass is much more expensive here, even MDF. If I can absorb and dissipate what tries to escape with lighter weight layers of cheap material then I get the added benefit of bass treatment along with an easy job.

[bpape]

Now you're looking at the right kinds of things.  The all involve mass and isolation.

The tube traps do a good job of absorbing inside the room but still pass a lot through them.  

My comments were purely based on your initial wall construction thoughts.  If you can use that concrete board, isolate the connections as much as possible to the outer structure, and seal up the room, you'll go a long way toward stopping sound in both directions.

[ScottMayo]

this is purely intuitive but how would it be filling a wallspace between say an inner and outer layer of ply, with sand?
jules

Sand has a good reputation. Of course it makes for a heavy wall and needs some pretty serious construction technique, and you *really* want clean fill in your walls, which in some places is pricey. But if I'd had a braver GC and a bigger budget, I'd have tried it.

Without knowing what's available on Costa Rica, I can't make suggestions. If it's like Tortola, the price of an item relates to shipping cost, so heavy things cost more.

I'm going to raise an eyebrow at 100db+ volumes below 25Hz. It's your room and do as you please, but having spent way too many minutes with a subwoofer at +90db (more in the modal points) playing tones from 100Hz down (searching for room rattles), I can tell you that you're going to damage your ears. My ears would ring, with a nasty buzzing noise, for several minutes, after a test. It's not as groovy as it sounds.

Keep in mind that 100+db at 20Hz will cheerfully go though pretty much anything, including a 6" thick concrete wall. Since bass frequencies are perceived to fall off faster with distance, the best way to solve this problem is to locate about a mile from the people you don't want to bother, and plant a forest between. Seriously, if you want to go all the way, find an airport designer - they deal with problems like this.

[JohninCR]

Thanks Scott,

I appreciate the info about high levels, but it would only be an occasional peak in the LFE's.  100db at 20hz is below what is needed for THX certification, so I don't think it's unrealistic at all, although dissipating it may very well be.

On the other hand, I don't think it's unrealistic to dissipate this relatively small amount of actual energy within a layered wall.  I'm starting to think that airspace (or fiberglass insulation if I can find it) between each layer, along with different materials used for each layer is the way to go.  The initial section is a series of differently tuned panel absorbers.  Then that entire wall will act as a panel trap at the wall's much lower resonant frequency and successive layers will have different resonant frequencies if I use different materials.  While the shallow airspaces will be less than optimum, I have a strong suspicion that 12-15" containing 4 different levels of isolated panel absorption will be even more effective than concrete and remain very economical.

[ScottMayo]

Thanks Scott,

On the other hand, I don't think it's unrealistic to dissipate this relatively small amount of actual energy within a layered wall.  I'm starting to think that airspace (or fiberglass insulation if I can find it) between each layer, ...

What worked well for me was a double wall:
5/8th sheetrock
homasote/celotex
5/8th sheetrock
2x4 studs, with rockwool insulation
8" air gap
studs, staggered, with fiberglass insulation
5/8th inch sheetrock.

The 2 frames were tied to different joists (I had to slap my contractor upside the head to get this - they originally bridged them to the same joist, and they vibrated in unison when hit.)

Screws were arranged so that they only penetrated 2 layers at most; sheetrock was laid so seams weren't near each other. This worked well. I also used a spring loaded ceiling, which worked less well; I think the GC miscalculated the weight and the springs are probably overloaded.

As noted, the door is the weak spot. I need to save up for a 50db door - which, sadly, is a whole lot of money.

[nathanm]

I always thought having one of these would be cool.  A "portable" room.  Kind of like an audiophile trailer home.