Is 50 db reduction good?

Well, I got to do my first tests last night in the new studio in preparations for my first recordingsessions to start this weekend . I have put my customers off so long they threaten to go elsewhere. I was testing the effectiveness of the the sound attenuation of the drum booth walls and was a little dissapointed, but I may be expecting too much. I have built the drum booth with the double walls, mass-air-mass, 2 layers of 5/8ths drywall , 3 inch airgap, insulated walls , acoustic sealant, etc. Hopefully all the things I have read is good. the glass is only 3/8ths on each side... the double doors are well sealed, but not as heavy in mass as particle board type solid doors (they are masonite filled). Anyway, I took my sound meter into the room with my son playing and measured 110 to 115 db (c weighted) and then walked out the booth and shut the doors and read 60 to 65 db, pretty much any where near the booth. Is that (50 db) a good amount of attenuation? Of course I was hoping for more, but before I start tearing into stuff as thought there is a problem, I thought I'd see what you guys estimate I should be getting. When I started the project I estimated I should be getting over 60 dbs attenuation. Thanks Tom

Tom, you have 2 weak point that are effecting you - the first is the window...... you have about 10 1/2 pounds of mass (psf) in the form of drywall - but only 9.84 pounds of glass.... with 2 layers of drywall my recommendation is one side 1/2" - the other side 3/4". Annealed is fine. But - if you took and changed even one piece of glass to 1/2" you would probably get a decent benifit from the difference in coincidence. The next problem is your door......... you simply do not have enough mass......... when they test doors for stc ratings - you cn't improve on that with the best gaskets in the world.......... because they tests doors without any hardware. They fix the door in an opening - seal all 4 sides with caulk - and then test. a Standard solid core door is only good for about 25 to 35 stc........ in order to fix this yo are ging to have to add mass to the door......... I hope that helped.. Rod

Rod, Thanks! I figured the glass and doors would probably be my limit. One question though. If I DID change the glass and add significant mass to the door, how much more stc could I expect. The only reason I ask is if I spent 2000-3000 on it and only could realize a minute amout ( say 5 to 8 db) it may or may not be worth it, but over 15 to 20 db would definately be worth it. Thanks for your input. Tom

tmix wrote:

One question though. If I DID change the glass and add significant mass to the door, how much more stc could I expect.

Tom, That's not an easy question to answer..... not knowing what might be contributed by flanking - how you tied all of this to the existing structure, etc. But in a perfect world.......... you would make it to the value of the wal itself - that would be the limiting factor. And that also depends on what size studs you used- what spacing they are at (for example - you get slightly better LF with 2x6 @ 24" than with 2x4 @ 16"). Rod

Thanks Rod, I understand it is a nearly impossible call. Too many variables. My walls are 2 x 4 (16 oc) 2 layers of 5/8ths on one side taped, bedded, caulked at perimeter and holes with acoustic caulk. Each wall is insulated with typical Owens Corning R-13 (I believe) There is a 3 to 3.5 inch air gap between the walls. Presently there is not anything bridging the the inner walls with the outer walls except the insulation swells out enough to meet in the middle, and at door openings there is double layer of felt to block the insulation from being seen. The ceilings in each room are independant of each other, meaning the ceiling joists are not continuous from one room to another.Ceilings are 2x8 (2 ft oc) double 5/8ths like the walls I did not know at the time this was a no -no to not seal the tops of these spaces between the 2 walls, I was only thinking of minimal flanking. Unfortunately, even after showing this to my building inspector he said (at that time) he had no problem with it. I seriously did not know it was wrong... I guess my inspector didn't know either. Any way.. I wonder what the maximum potential of that sort of wall structure is capable of in a perfect world? Tom

You were expecting 60 dB at what frequency? Your 50dB figure is pretty good, and a representative test of how you plan on using the room. You can improve on it a little by beefing up your windows/doors, but so consider the effective area involved in those weaker components. I doubt that you'll see an overall 10dB improvement by making a small windor and small door beefier. Also, if drums are your source, you may wish to consider the speed of your meter..if it's fast responding, were you comparing peak spl or average? Cheers Kris

In a perfect world you should expect this: TestID TL-93-269 STC 67 Frequency……………..TL Value 50 Hz ……………………..24.7 63 Hz………………………25.4 80 Hz………………………32.2 100 Hz…………………… .39.1 125 Hz……………………..42.8 160 Hz …………………….49.6 200 Hz……………………. 55.3 250 Hz……………………. 61.1 315 Hz…………………… .65.9 400 Hz…………………… .69.1 500 Hz…………………… .72.4 630 Hz……………………..75.1 800 Hz…………………… .77.9 1000 Hz…………………. .80.7 1250 Hz……………………83. 1600 Hz……………………83.9 2000 Hz……………………74.6 2500 Hz……………………75.5 3150 Hz……………………82.4 4000 Hz……………………88.4 5000 Hz……………………92.7 6300 Hz……………………91.0

Kris, I was naively expecting the 60 db attenuation across the board with the exception of below maybe 150 hz. I have learned a lot over the 2 year building project. Part of that learning has been the physical realm, part in the understanding and expectation realm. It has taken a recording drummer with building experiance and taught him many things about acoustics and absorbtion that I never had a clue about. I hope it makes me a better recordist. Tom

Hi Tom,

I did not know at the time this was a no -no to not seal the tops of these spaces between the 2 walls, I was only thinking of minimal flanking.

So what you're saying is the tops of the walls HAVE to be sealed ?? How does one do that ?? Cheers N Y M O

nymo wrote:

Hi Tom,

I did not know at the time this was a no -no to not seal the tops of these spaces between the 2 walls, I was only thinking of minimal flanking.

So what you're saying is the tops of the walls HAVE to be sealed ??

nymo, I believe he's talking about the fire blocking that should be installed to stop the passage of fire and hot gasses from going from the wall cavity to above the ceiling. with room within a room construction - you have to do something to stop the passag of fire - otherwise a fire will engulf the rooms faster than you would believe. You should also have draft stops installed vertically every 10' or so.......... Just a stuffing of rockwool installed to fill the cavities front to back is fine, Rod

Rod, Perhaps that is why the inspector did not have issues. I did have the walls fully stuffed with the insulation (where it softly touched all the way up) and in addition stuffed insulation at the perimeter of tops of walls and door / window openings. That makes me feel better, I did not want an unsafe situation. Tom

tmix wrote:

Rod, Perhaps that is why the inspector did not have issues. I did have the walls fully stuffed with the insulation (where it softly touched all the way up) and in addition stuffed insulation at the perimeter of tops of walls and door / window openings. That makes me feel better, I did not want an unsafe situation. Tom

Tom, I hate to tell you this - but the fluffy stuff touching doesn't get it done. The fluffy stuff is not a fire safing. For floor to floor: It's the rock wool that is manufactured for fire safing tht stops the heat - and then fire caulk - which stops the hot gasses. the building code also recognizes nominal 2x lumber - 2 layers of 3/4" plywood with lapped joints - 5/8" type X gypsum, The vertical jints are not firestops - they are (rather) draft stops nd can be 1/2 gyp - or anything the officlas sees as ok. It's not as much the vertical ones I worry about - it's the top of the wall - the direction the fire wants to head naturally. Sincerely, Rod

There's quite a bit of information on this issue in my project thread, including details of my meeting with my local building department officials on my plan to address this issue. It's sad that inspectors often miss these things... :roll: ...Going through the permitting and inspection process is theoretically supposed to help protect you from making these potentialky dangerous mistakes. Ultimately, though, it's the owner's responsibility to meet code regardless of whether or not the plan checker and inspector misses something. I shudder at the thought of how you'd be able to fix this. It would seem that anything less than a complete removal of ceiling drywall and insertion of mineral wool insulation (e.g., Roxul AFB or thermafiber SAFB) would be insufficient. Perhaps if you'd be willing to do that, you could take it to the next level by redoing the drywall on the walls to and use Green Glue between the layers. A layer of plywood added to the door with Green Glue would help a lot with the door. Too bad there's not a way to "beef up" glass... I'm with Rod on the "replace with thicker" advice. Laminated glass is the way to go there. --Keith :mrgreen: You have my condolences.

Actually, I still have access to a large percentage of the wall tops through the attic. It looks like I will be adding either the fire rock or the 2 by lumber where I can reach.

tmix wrote:

Actually, I still have access to a large percentage of the wall tops through the attic. It looks like I will be adding either the fire rock or the 2 by lumber where I can reach.

Tom, the lumber would have t be attached to both frame - which would create a flanking path between them - you would be much better off with the thermafiber.

I'm doing my fire stops right now so all of talk about them is interesting. So can I use rockwool instead of gypsum? That would save me a lot of time. Do you fill the entire cavity between the walls or just gap at the top of the walls? Do you fire caulk rockwool?

You don't have to use fire caulk because (Rod, correct me if I'm wrong!) you're not building a fire wall... You do have to make sure the mineral wool insulation is secured and not subject to shifting. If you have a three inch gap between frames, you'll want to try to get 6" batts, either 16" or 24" wide, slice the batts in half lengthwise, and stuff the sliced batt between your frames' top plates. But you say you can't reach all areas via the attic...? :roll: --Keith :mrgreen:

Thanks Rod I'll look into it.

sharward wrote:

You don't have to use fire caulk because (Rod, correct me if I'm wrong!) you're not building a fire wall...

Actually Keith, you're creating a 1 hour, floor to floor, fire seperation, and (as I think I mentioned before) for floor to floor seperation - if you go to UL (underwriters Lab.) you would find that thermafiber (rockwool) alone is not an acceptable 1 hour assembly. It always includes the use of some sort of fire caulk. Thermafiber only stops the passage of the fire itself - it does not stop the passage of hot gasses - and hot gasses can begin combustion without open flame once they clear the thermafiber. Sincerely, Rod