Q about Helmholtz, Greenglue, and soundproofing,oh my

Posted: Tue Sep 30, 2008 2:02 pm

HELMHOLTZ
I was wondering if when constructing a helmholtz resonator if it matters how the front slats are oriented.
Usually I've seen them parallel to the face.
Any there any rules that say you can't slant them slightly? (Like a venetian blind half way closed) This would disperse HF slightly upward instead of directly back into the room.
Good thing? bad thing?

SOUNDPROOFING.
The wall of my studio will be joined to the house. We made that wall with staggered studs. 1/2" drywall on the house side and 5/8" on the studio side. (insulation in between)
I'm finding that may not be enough separation.
I'm under the impression that construction of another wall would create a 3rd leaf and probably not preferred.
So I was thinking about a 2nd 5/8" wall on the studio side attached with Green Glue.
Any other (better) thoughts?
I prefer to not do any work on the inside of home. Only on the studio side.

Thanks in advance.
Thomas Anthony
Natural Sound
www.NaturalSoundRecording.com

Posted: Wed Oct 01, 2008 3:02 am

Thomas,

Welcome to our little corner of the world!

As far as your angling of a Helmholtz... I don't know for sure, but I suppose you could angle the slats.

My understanding of how a Helmholtz works is due primarily to the spacing of the openings and the width of the slats.

Something tells me you need to calculate the area of the opening. e.g. if you desired an opening of 1/2", then to put the slats on a 45 degree angle, you'd have to space the slats something less than about 1/16" or so. But the spacing would be dependent upon the width of the slats.

Either that, or you would need to cut the slats back into the frame to get the face edges parallel. (The probable reality)

Ultimately, my suspicion is that while you could indeed build one this way, it would certainly be a miserable time coming up with the correct formula/measurements... and then building it would be even more painful.

Not sure if I quite follow your party wall construction...

1. Interior Home Space
2. 1/2" Gypsum
3. ................................/Insulation filled Staggared Stud Wall - house side (2x4?)
4. Staggared Stud Wall Plates (plate width and air gap?)
5. ................................\Insulation filled Staggared Stud Wall - studio side (2x4?)
6. 5/8" Gypsum
7. Interior Studio Space

If that's correct, then sure! Add the 2nd layer of 5/8" with green glue!

Posted: Wed Oct 01, 2008 5:05 am

The formulas for the resonant frequency of a Helmholtz resonator are based on the geometry of the opening. Slanting the slats changes that geometry drastically - probably enough so that all of the assumptions and approximations that went into creating the formulas (and there are a bunch of them) are no longer any good. You'd either have to run extensive computer calculations, spend months doing careful experiments or put up a bunch of slats at random and hope that they did some good. Not a good idea. Best to follow the instructions exactly.

Posted: Wed Oct 01, 2008 7:02 am

Thanks all,
Yes Madmax, that is exactly how the wall is set up. Although I'm not sure how the plate width and air gap was obtained.
So Greenglue it is.

I think you're both right about not slanting those slats. My math is not one of my strong points anyway. So unless someone else chimes in with an easy procedure, it's back to the KISS method for sure.

NEXT QUESTION:
the other 3 walls are outside walls.
CBS construction with 8" insulated foam filled block
1" rigid insulation
1/2" gypsum on furring.
There is one window in the room that faces the backyard which backs up to a forest so I'm not sure if I need to be concerned about that.

Any advise to adding more soundproofing to these walls, or is it better to leave well enough alone? (cost Vs appreciable improvement)
Helpful background info:
No live recording except for the occasional vocal or gtr amp. No live bands or drums. Mostly average control room volume. 80 to 85 db

Thanks,
more quiz questions to follow, I'm sure.

Posted: Thu Oct 02, 2008 6:23 am

Thomas,

It sounds like your room addition is actually almost completed.

Not sure if I can actually make the call for ya', as far as adding another layer of gypsum to the exterior walls.... that's gonna have to be your call.

I would suggest getting an amp in there and crankin' it up. If it's a problem, then yes, add another layer. If it's really bad, add another layer of 5/8"gypsum and use Green Glue... otherwise, your window should be the only weak link as far as your walls go...

Another weak link could possibly be your ceiling if it only has 1 layer of gypsum. HOWEVER, I would caution you to evaluate your room first. Adding anything to your ceiling without knowing how it's constructed could be fatal!

Not taking you to task here, but w/o a bit more detail, this might be bad advice, so take it with a possible grain of salt...

In addition to not having enough mass on the common wall, you may be experiencing flanking noise in the house from the floor and/or from the ceiling/roof construction of the addition.

Generally, if a staggared stud wall is used, the next larger lumber is used for the plates. e.g. 2x4 stud walls use 2x6 plates. While it minimizes the transmission from wall to wall, the plate is still common.

If the floor is tied directly to the house, there's a possibility for sound to be passing directly to the house that way. Again, depends on the construction method and the materials used.

The ceiling rests on the walls, as does the roof. Depending on how the roof was attached to the house, you might be getting flanking noise from the studio to the house that way... especially if the two attic spaces are open to each other, that would allow sound to freely travel.

Seeing as you are in Florida, I'm pretty confident that your floor is a monolithic slab for the addition. The question is whether the contractor structurally tied the slab to the house with something like rebar and/or used any type of expansion/isolation joint material between the two structures.

If the slabs are tied (and I would suspect they are) you've got a pretty rough job ahead of you... You'll need to determine whether the cost of having someone in to cut that rebar in the slab is worth it, and/or if it's even feasable to do at this point.

If the attic spaces are open to each other, you'll need to evaluate whether you have adequate venting in the addition, to be able to seal off the two spaces.

If they separated, and you determine the flanking is from the attic area, then you may need to add some mass to the wall in the attic.

If you can maybe get a few pix, or drawings up somewhere, it would probably make this pretty easy to figure out. Otherwise, can you get a bit more detailed on the reason for your statement of;

Quote:

I'm finding that may not be enough separation.

Posted: Thu Oct 02, 2008 8:36 am

Thank you for taking the time to be so thorough. I've ck'd in on your nightmare from time to time, and can't believe you're in your 4th year of this budget sucking adventure. I don't envy you. I did a similar thing back in 1988. 2 years of planning and a year of construction. But things were different back then.

Ok- A little more info.
Yes the slab is all one pour for the entire house.
I wasn't totally sure I was going to use this space for a studio or mother in law suite, but we minimally did the staggered wall just in case. (the house including the studio were built from the ground up in 2005) And we would figure the rest out later if needed. (and so here we are)

Now, there was a conversation with the contractor that the staggered wall should go up through the attic to the roof. I have not yet ck'd on this as we don't live in the house yet, but it's on my to do list the next time we go visit in Nov.

The reason for the 'not enough separation' comment was, because back in my 1988 studio we had built 2 iso rooms, and I was used to the sound of someone speaking from the inside where you could sort of hear them,(room in room construction) but they sounded like they were about 30 feet away. So when we ck'd out the house after it was built, my wife was in the spare bedroom (the attached part) and I could hear her clear as day in the 'studio'. So I started thinking holy crap - what about the other 3 walls?

So yeah, there's the floor issue which I'm hoping will improve once there's carpet and wood in there. And the attic issue, which I'll ck out next trip. But in either case, I won't be able to ( let me rephrase that. Not insane enough to) do much about those now. But adding the extra drywall is within my willing.

I don't mind GG and rocking the entire room, but I've been reading as much as I can that now I'm more confused than ever. But I thought I read a thread at the John Sayers site (which is down at the moment) that extra drywall where a block wall is concerned won't make an appreciable amount of difference. Maybe I need to revisit that thought.

I will try to post a pic of the room in question next.

Thanks again for your time.

Posted: Thu Oct 02, 2008 10:15 am

here's the existing structure
The lower left (clear bluish wall) is the part connected to the house. The room has it's own outside entrance. You can not get to the studio directly from the house.

This is Plan A.
The idea here is that the control room is at least moved as far away from the house as possible. The little performance area next to the house would only occasionally be used for Voc or gtr etc.

Plan B would be to do a double wall construction for the dividing control room wall which adds a 2nd sliding glass door. But that might be more space, work, time, money, vs benefit.
I'm trying to find a sliding door with a higher STC rating than your standard sliding door and just go with the one wall (plan A) with dbl layer on both sides with GG. It's hard to know how far to go with these things.
advice?

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