Sounds Attenuation in Practice Area

[wesg]

I'm starting a new thread because I'm don't want to pollute that other fellow's reverb thread any more than I have.

 

I have a practice area directly below the master bedroom. The room has 1/2" fiberglass ceiling tiles in a T-bar ceiling, a ~4" gap, 2x6" joists on 16" centers, a plywood floor and hardwood flooring. I'm hoping to drop the level in the bedroom by 12 dB or so. My goal isn't quiet (I suspect that is not achievable) but quieter would be helpful. I imagine I won't be able to do much about frequencies under 100Hz.

 

Don says I'm not going to get any mileage at all out of Roxul Safe'n'Sound, and other sources concur - I need mass. Guess I will return it, I haven't opened it yet. (So what the heck is the point of Safe'N'Sound?)

 

Is there any way to get what I want with a T-bar ceiling? I don't really want to put drywall up in there. I like being able to run wires under there. Getting that much drywall on the ceiling won't be all that much work, but redoing all the wiring will be a huge PITA. There is a LOT of stuff in the plenum, a fair bit of household electrical was also stapled to the joists instead of being run through them. Also, I would have to consider HVAC, central vac, and so on.

 

So, I have been considering upgrading the tiles in my existing ceiling. The new tiles would need to be safe to use near a wood stove, and ideally cost less than $2 sq ft. And be somewhat effective. I have been looking at ceiling panels, but I have been having difficulty making sense of specifications in CAC or STC, and translating that into amount of attenuation.

 

What's enough for my application? How does STC add up? Is STC 36 twice as quiet as STC 33, or just about the same?

 

http://www.armstrong.com/residential-ceilings/ceiling/random-textured-933-panel/14146-45136?src=ymal says, "it reduces sound by 55 percent for good acoustical performance". Does that mean anything? Does it mean a 3dB drop, assuming nothing leaks around it?

 

Thanks,

Wes

[Craig Vecchione]

Wes, I've got a few thoughts about this. It's a pretty common scenario many of us encounter.

 

First, can you clarify your goal? What would a 12dB drop in the bedroom do for you? Is sleeping there during practice a goal, or just being able to watch tv or read without too much disturbance?

 

Second, the issue of having your cake (lowered sound transmission) and eating it too (still having access to the ceiling infrastructure) makes this a really difficult challenge....depending upon your goal STC.

 

Here's a pretty good reference: http://www.stcratings.com/

 

An STC of 50 means that sound transmission through the wall or floor being tested will be reduced by 50dB, but note that the test is for 16 frequencies between 150Hz-4,000Hz. It also doesn't address flanking sound transmission around the wall or floor when used in an actual room. Don't discount this facet of the problem, and also note that air leaks are a huge source of sound transmission. Sealing every possible air path, especially holes for plumbing, HVAC and electrical that lead to uninsulated wall cavities.

 

In practice Safe 'n Sound works pretty well, definitely a bit better than thermal batts. I think the thumbs-down is because there are so many other factors to reducing sound transmission that folks who are unaware of them will assume one treatment method or material will work wonders.

 

Getting to your case, sealing all air leaks, adding insulation, and possibly finding better ceiling panels will help. You could and should consider adding an additional layer of insulation batting to the 4" air space, perpendicular to the joists. It'll greatly complicate access, but you need every help you can get if you're not going to move to a solid ceiling.

 

Should you decide to go with a solid ceiling, two layers of 5/8" drywall installed on metal z-bar hangers is a common accepted installation. You could also investigate using wallboard sound panels such as CertainTeed's SilentFX and QuietRock products. This is in addition to batts between the joists.

[wesg]

Thanks, Craig;

 

The goal I have in mind is to make it more comfortable to watch TV in the bedroom while we are rehearsing. Right now, quieter parts of the shows are hard to hear even with the TV turned all the way up. Getting the "noise floor" down, especially in the 300-3300Hz band, would make a huge difference. High-frequency sounds (cymbals) are already dampened somewhat well by the existing construction. Low frequency noises aren't as big a deal because they don't hurt intelligibility....although any attenuation there would be welcome to reduce their annoyance.

 

I saw that QuietRock stuff on TV the other day (Holmes on Homes). Looks pretty awesome. Apparently it has the equivalent STC of 8 sheets of 1/2" drywall. But it's 50 bucks a sheet.

 

Thanks for the link and STC translation. So does that also mean than an STC 30 means a 30 dB reduction, with the same caveats and measurement method?

 

You're right about the importance of reducing the air leaks. My wife currently puts the dog's bed on bed on top of the HVAC register in the bedroom. That alone makes gor a massive improvement in sound level in the bedroom. I have been trying to think of a good way to block sound entrance into the basement register that is closely connected to it. I may try and get a big magnet or something there. Maybe glue a piece of drywall to the magnet.

 

This sure is a head-scratchy problem. I know how to FIX the problem (build a room within a room) ...but I can't figure out to just somewhat improve the situation without spending a ton of money. Room within a room is really tricky, aside from the ceiling issues you have to consider -- windows, doors, wood stove, remaining available space, and resale value.

 

Wes

[Craig Vecchione]

Yes, STC 30 means a 30dB reduction within 150-4kHz. In general, expect a "real" measurement in an actual structure to be about 5dB louder than what is scored in testing.

 

You could definitely fit the downstairs part with a tight-fitting cover made of damping material. Put it on for rehearsals and remove it when you're done. The interior could have alternating partial baffles to prevent a direct sound path. Note that this will reduce airflow. If possible, enclose the HVAC main trunk in a plywood box to block as much sound as possible from traveling through the thin sheetmetal.

[agedhorse]

One thing to consider is to use 5/8" drywall based ceiling tiles in your t-bar ceiling, glue the fiberglass tiles to the drywall tiles to get a better (more absorptive) surface in the basement room. Drywall based tiles are used in commercial kitchens, and where additional fire barrier (not always approved depending on application) is required or desired.

 

Sealing cracks/openings is a big help. HVAC duct has almost zero STC, which is why it's such a problem.

[Craig Vecchione]

If you go with drywall ceiling tiles, be sure to at least add a LOT more hanger wires, and consider upgrading to a commercial grid system. The typical Armstrong grid system sold at home centers is not designed for anything as heavy as 5/8" drywall, and a failure can be a real pain in the neck....

[agedhorse]

Good point, everything I work with is 100% commercial duty, siesmicly braced.

[Craig Vecchione]

Compression posts too?

[agedhorse]

The whole enchilada.

[Craig Vecchione]

Good thing. I was in an office when our tiny-by-comparison earthquake hit a few years ago. I really didn't like watching the ceiling come alive above me.

[wesg]

Thanks, guys -- this was very useful. I was not aware of the existence of drywall ceiling panels, and I'm not sure the grid strength issue would have entered my mind or not.

 

That's actually not a bad solution. It might even be cost/time-effective to replace the entire grid system to accommodate a drywall panel system. I'm going to have to do some more research, the staff at the home centers I frequent appear to be completely unaware these exist. It is certainly more practical than doing a traditional drywall ceiling.

 

Glueing the existing tiles to the drywall tiles is brilliant. That means that the room maintains most of its character, I am guaranteed not to make anything sonically worse. I also like the idea of getting additional fire barrier near the wood stove out of the deal. It's unlikely that I will ever screw up and burn the house down, but you never know. There is a smoke detector near the stove that also sounds next to the bedrooms, to hopefully give us enough time to get out should anything ever happen.

 

Adding baffles to the HVAC duct work is an interesting idea. That particular duct is 4" round tin and shaped like the integral symbol. I wonder if adding a horizontal U-shaped pipe about 18" long would be helpful or not. I had originally considered baffling the duct like a motorcycle exhaust, but wasn't too sure how to actually do that.

 

Wes

[dboomer]

My recommendation would be to forget about hanging drywall in the grid. Just pony up and build a ceiling. Two layers of drywall would be even better. But you will have to support it properly.

[Craig Vecchione]

Thanks, guys -- this was very useful. I was not aware of the existence of drywall ceiling panels, and I'm not sure the grid strength issue would have entered my mind or not.

 

That's actually not a bad solution. It might even be cost/time-effective to replace the entire grid system to accommodate a drywall panel system. I'm going to have to do some more research, the staff at the home centers I frequent appear to be completely unaware these exist. It is certainly more practical than doing a traditional drywall ceiling.

 

Glueing the existing tiles to the drywall tiles is brilliant. That means that the room maintains most of its character, I am guaranteed not to make anything sonically worse. I also like the idea of getting additional fire barrier near the wood stove out of the deal. It's unlikely that I will ever screw up and burn the house down, but you never know. There is a smoke detector near the stove that also sounds next to the bedrooms, to hopefully give us enough time to get out should anything ever happen.

 

Adding baffles to the HVAC duct work is an interesting idea. That particular duct is 4" round tin and shaped like the integral symbol. I wonder if adding a horizontal U-shaped pipe about 18" long would be helpful or not. I had originally considered baffling the duct like a motorcycle exhaust, but wasn't too sure how to actually do that.

 

Wes

 

You can't baffle round duct, only square/rectangular. The U-shaped trap would add a lot of resistance/distance, but might reduce some of the sound. But I'd still just go with capping the registers during rehearsal.

 

I'm also with Don in that a real ceiling of doubled 5/8" is way better. Even with all that drywall tiling, there are HUGE air leaks around every tile, and the grid itself has zero STC. You can often reroute wiring and not need access to the ceiling. Consider that most of your house probably has inaccessible stuff in the walls, and is not a problem.

[rpjazzguitar]

If you define the problem as the person upstairs having difficulty watching TV, the logical solution is headphones upstairs.

[dboomer]

First ... The title of this thread should be sound transmission reduction and not attenuation IN practice area There are commercial baffles available but you can achieve the same thing by adding three 90 degree bends in the duct using "flex duct". If the goal is not to disturb the upstairs then you should consider adding padding to the upstairs carpeting or floating the furnature on little shock absorbers. You should also "float" your drum kit and amps on shock absorbing platforms. Take a look at what the home theater guys do on a DIY basis.

[Craig Vecchione]

Three 90's would make a turn. It'll reduce the sound, but also the airflow, as the duct won't go to the register any more!