Hi all -- thanks in advance for any advice you are able to provide.

My theater will be 14' x 22', with a soffit along one of the long walls for a main HVAC trunk. I'll be stuffing the space around the duct full of insulation and double sheetrocking over it, yielding a 9" x 17" (h x w) soffit. I was planning to build matching soffits on the three remaining walls, but thought perhaps it would be better to just sheetrock the room as is, and build bass trap soffits on the remaining three walls. I will be using linacoustic on the bottom half of the walls.

Based on the reading I have done, I think I have three DIY options, each consisting of a wood frame skeleton and covering with GOM. As far as what goes in the soffit, please help me determine which would be most favorable:

A) Stuff full of pink fluffy insulation
B) 2-4" of OC703 on sides and bottom of soffit
C) Completely fill with OC703 (OC705?)

I don't typically see linacoustic mentioned in bass trap construction, best to skip that as a candidate?

A couple of related questions:

1. For such a small span, would the GOM be held taut enough by only the frame, and need not contact the insulation material (I think this would simplify the design)?
2. I'm sure there are pictures of soffit bass traps in members' galleries, any good frame construction pics you could point me to?

Please note that I can't go any deeper than the 9" with the soffits, as head room is already an issue. I could possibly go wider than 17" if necessary, but would prefer to minimize this dimension for aesthetics. Also, please consider budget as an important parameter in my decision, so any price/performance insight would be appreciated as well.

Thanks,
Ned

I'd drywall the other soffit on the other long wall to keep left to right symmetry in the room. You can certainly, however, use the soffit on the rear wall for a bass absorber.

With a 9" depth, maybe consider a few inches of 703 at the face and then fill in behind with 'fluffy stuff'.

bpape,

Thanks for your thoughts on this and many other threads in which you have contributed.

I'd drywall the other soffit on the other long wall to keep left to right symmetry in the room. You can certainly, however, use the soffit on the rear wall for a bass absorber.

Interesting. I did not realize that bass trap symmetry was critical -- thinking instead that the extra long trap would benefit the overall sound. Guess I'll have to rethink that one! Also, are you recommending I use only the rear soffit as a trap, or both front and rear?

With a 9" depth, maybe consider a few inches of 703 at the face and then fill in behind with 'fluffy stuff'.

Interesting thought again. What are the roles of the different absorbers in this construction? Also, when you say "face" do you mean the bottom, side, or both?

Thanks again for your insight!

Ned

Hi Ned,

No doubt Bryan is correct in being concerned about symmetry. However, there is obviously a trade-off in eliminating the majority of your planned bass trapping.

One way around this uncertainty would be to measure the room response before you install your bass traps. Then install the framing and fluffy insulation inside, but don't close in the soffits. Then remeasure the room at multiple locations and see if the improvement in bass response is better or worse than without the traps. If its better, close in the soffits with GoM, if its worse close in the soffits with double drywall. In either case you are going to need to fill the soffit with insulation to prevent them from resonating like drums, so the insulation is not wasted.

Just a thought.

Larry

Larry,

So now you're trying to bamboozle me into getting some software to measure room response? ;) It's actually not something I had considered before. I understand I would be able to measure bass eveness across the room, but would I also be able to measure any "uneveness" caused by the asymmetry of the room?

Anyway great idea to do the test run. I was planning to build the soffit with 2x2's if I go with GOM and 2x4 for drywall, so I guess I'd probably have to do the 2x4's or put in more 2x2's than I was thinking to go your route.

Ned

Larry,

So now you're trying to bamboozle me into getting some software to measure room response? ;) It's actually not something I had considered before. I understand I would be able to measure bass eveness across the room, but would I also be able to measure any "uneveness" caused by the asymmetry of the room?

Anyway great idea to do the test run. I was planning to build the soffit with 2x2's if I go with GOM and 2x4 for drywall, so I guess I'd probably have to do the 2x4's or put in more 2x2's than I was thinking to go your route.

Ned

Hi Ned,

Yeah, that's me the big bamboozler. ;)

You will find that if you measure your room before completing your bass traps that bass response, i.e. the frequency response below say 500 hz, will differ significantly from seat to seat. Adding Bass traps will smooth out the bass response so that the peaks and dips at every seat is less pronounced.

Ideally you would want to apply absorption uniformly throughout the room, however, I'm guessing that what ever "bass unevenness" is introduced by the asymmetric placement of insulation will still be smoother at every seat than your original measurements. If so, does it matter that some seats are less smooth than an others, if they are all smoother than the original condition?

I think that if you only install bass traps across the back you may find little, if any, measureable difference, before and after. However, as I said, you are going to need the insulation for the long wall anyway, so why not measure it and and see if the additional insulation helps or hurts, thereby removing all uncertainty.

Larry

Ned,

As promised in my reply to your PM, I'm glad to toss in my two cents.

> My theater will be 14' x 22', with a soffit along one of the long walls for a main HVAC trunk. I'll be stuffing the space around the duct full of insulation and double sheetrocking over it, yielding a 9" x 17" (h x w) soffit. <

You already got all the right answers. I'll just point out that as soon as you put sheet rock over the cavities you have lost any potential for bass trapping. If you're concerned about losing symmetry at mid and high frequencies from having absorption on only oine side, you could cover the cavity with cardboard instead of sheet rock. That will not give the isolation sheet rock gives, of course, but it will reflect mids and highs similarly.

--Ethan

Cardboard is a fire hazard. Consider something with similar properties that won't burn.

(I once asked if I could build a column cover out of paper mache -- the answer was 'no')

Ethan, thanks for the advice -- good to know I'm on the right track.

Bob, thanks for the note of caution regarding carboard.

If I went with OC703 face with fluffy filling (ala bpape above), how would that perform in the reflection of highs and mids?

Ned

OC703 will absorb highs and mids.
http://www.bobgolds.com/AbsorptionCoefficients.htm

Thanks Bob, this link was very instructive. Is this your webpage?

One confusion I had was that the page starts off by saying "1.00 = 100% absorbtion." It goes on to say that these coefficients "are not percentages." I can only presume that the numbers on your page are not percentages, since they sometimes exceed 1.00. If this is your page, perhaps you could clarify? Maybe I'm just being dense....

Ned

billfold:

Yes it's my website.

Take the quote by Jeff about them not being percentages as the correct one.
Think of the "1.00 = 100% absorption" quote as a helpful approximation.

The number values refer to the results of the lab tests -- and are reported as computed, which is the correct thing to do. There are a variety of ways that the absorption coefficients can go over 1. I know a few of the ways, but I don't know them all.

Back about a century, a fellow named Wallace Sabine ("Sabin" is a unit of absorption -- named after him) defined that to be something like the absorption you'd get from an open window of 1' x 1'. i.e. an open window of 1'x1' gives 1 sabin of absrption. An open window being a 100% absorber. Hence the "1.00 = 100% absorption". Wallace Sabine was instrimental in the building of the reverberation chamber at Riverbank.

http://en.wikipedia.org/wiki/Wallace_Clement_Sabine

Terry Montlick's recent post quoting Leo Beranek about the open window max absorption coefficient of 1 is just a convenient fiction (http://www.avsforum.com/avs-vb/showthread.php?p=7786267&&#post7786267)

OK, the room is finally ready for the bass trap install. I'm wondering, for the surface of the "fluffy pink" insulation that is behind the GOM on the sides of the soffit, would it be preferable to spray paint the insulation black? I'm thinking this would help for flash photography and perhaps also room air quality. Any acoustic downsides to doing this? I.E., would this diminsh the bass trap-ability of the insulation?

Wow. Search and you shall find :). I have the same issue as billfold, except my HVAC trunks almost extends to the middle of the room (for about 2/3 of the length from the screen) . If I build a matching soffit on the opposite wall it will leave about 20 inches between them.

My idea was to construct the soffit with drywall and fill with safe n' sound insulation. To put the insulation to use I was going to cut ports in the drywall and use speaker fabric to cover them. I thought this way I could maintain room symmetry (both aesthetically and acoustically) and still have some bass absorption but I really don't know.
All comments very appreciated.

Someone recently posted that they didn't like the puffyness of insulation pushing out on their GoM. Try to find some way to keep it from touching.

Thanks Bob, I plan to frame the soffit so that the insulation is about 1" away from the GOM, so bulging shouldn't be an issue. Would anybody think a layer of paint on the insulation would affect the bass trapping?

Alternatively, if the insulation is not touching the GOM, would it not show up in flash photography? Does the untreated insulation behind GOM pose an air quality issue? If these answers are both "no" I'd skip the paint, of course.

Thanks,
Ned

Hi billfold,
I would love to see pictures of the construction. I may attempt to do the same thing. My guess that the paint would not affect the low- frequency absorption. I read that a carpet cover doesn't affect the low frequency absorbtion so I can't imagine a thin layer of paint doing so.

Anyone have any comments on the drywall soffit with port-holes idea?

I think I read somewhere about using pegboard as a covering for a bass trap. The article/page did go into hole spacing in detail for the correct frequency etc., but I's think the off the shelf stuff would be OK. You could then cover it with your favorite AT fabric. I was thinking of this for 3 of my 4 soffit corners, as like you, the 4th hides the duct work. Vertical traps in the corners of my smallish room will be a tough fit, and wouldn't be symetrical either, because of the entry door.

Good luck!
Chip

ChipWV:
I think I read somewhere about using pegboard as a covering for a bass trap. The article/page did go into hole spacing in detail for the correct frequency etc., but I's think the off the shelf stuff would be OK. There are too many holes in pegboard to turn it into a bass trap in the helmholtz sense. Or more accurately, the ratio of hole area to surface area is too large.

Quote from: ethanwiner*com/acoustics*html

"BETTER BASS TRAPS

Yet another type of bass trap is the Helmholtz resonator. Unlike foam, fiberglass, and tubes fitted with fiberglass, a Helmholtz resonator can be designed to absorb very low frequencies. This type of trap works on the principle of a tuned cavity and is often very efficient over a narrow range of frequencies. Think of a glass soda bottle that resonates when you blow across its opening, and you have the general idea. Although a Helmholtz design can be very efficient, the downside is that it works over a fairly narrow range and needs to be rather large to absorb very low frequencies. The range can be widened by filling the cavity with fiberglass, or by creating several openings having different sizes. One common design uses a box filled with fiberglass with its front opening partially covered by a series of thin wood boards separated by air spaces. This is called a slat resonator. Another also uses a box filled with fiberglass but has a cover made of pegboard containing many small holes. Although there is no denying that a Helmholtz trap can be very effective, the fact that it works over a narrow range of frequencies limits its usefulness. While it can be sized to absorb the dominant resonant frequencies in a particular room, it cannot absorb all the other low frequencies. And broadband absorption is needed to prevent acoustic interference that skews the frequency response throughout the entire bass range. "

End Quote

While I can't begin to speak as an authority on the application of this information, it makes sense to me. It's already an area that should be filled with the fluffy stuff, why not let some sound into it and let it get absorbed. The pegboard would provide some reflection and some absorbtion and may help with the symetry, as the opposite soffit would be all reflective. Maybe Ethan Winer and/or bpape will chime in here to confirm this application.

Please feel free to tear apart my assumptions :p

Chip

ChipWV:

You can do the math yourself. Look up the formulas for "Perforated Panel Absorbers".

But to save you the time I see your Ethan quote and I raise you acoustician and builder/tester of such devices F. Alton Everest's "Master Handbook Of Acoustics, forth edition" pg 221:

In commonly available perforated materials, such as pegboard, the holes are so numerous that resonances at only the higher frequencies can be obtained with practical air spaces. To obtain much needed low frequency absorption, the holes can be drilled by hand. Drilling 7/32" holes 6" on centers gives a perforation percentage of about 1.0%

Area of a circle is Pi r^2.
So if a perforated panel uses 1/4" holes every 1", then each hole is 0.049 square inches. There are 144 holes in a square foot, or about 7 square inches out of 144 square inches in a square foot of pegboard, or 4.9% hole to panel surface. With a 4" air gap that would give it a resonance frequency of about 700hz. With 2" air grap that would give it a resonance frequency of about 900hz.

Page 223 gives instructions to build 110hz, 89hz, 74hz and 63hz devices.
e.g. 63hz
Depth of airspace: 5 5/8"
Hole diameter: 1/4"
Hole Spacing: 4.43"

http://www.bobgolds.com/MasterHandbookOfAcoustics_pg227_SingleModeTamedByResonantAbs orber.JPG

BasementBob,

That's why I'm here... a little knowledge, or information in this case, is a dangerous thing. I've been doing lots of reading around here, and lots more to do!

Thanks for the education
Chip

That's why I'm here... a little knowledge, or information in this case, is a dangerous thing.
Indeed. A main problem with just a little knowledge is that intuition often betrays us. For instance I would never guess that a 5 5/8" deep space with just 1/4" holes space 4.43" apart could possibly affect sound all the way down to 63 Hz. Intuition tells me that a sound wave so large wouldn't be able to travel through the holes. I really gotta read through that book.

What is the principle that the resonator is based on? Does the sound pressure generated by the flexing of the front panel cause the holes to emmit another sound wave that cancels out the original one refelcting off the front panel?

Either way thanks for posting Chip. I just learned something new and I may very well incorporate this into my room in some way.

Either way thanks for posting Chip. I just learned something new and I may very well incorporate this into my room in some way.
Glad to sacrafice myself in the persuit of knowledge. Even though it's not a Helmholtz trap (not sure I was even infurring it was, just wanted to give credit from where I got the idea), it still seems like a good way to hold in the insulation, allow the sound in to trap it, and maintain or add some structural integrety to the soffit. So would this method/material be benefitial as a "Soffit Trap"? I see this as an alternative for those of us with hard to treat rooms.

I was thinking of the pegboard on the bottom horizontal surface covered with black speaker cloth and drywall on the vertical surface. Crown moulding with light rope at the bottom corner.

My father often quotes a saying from my great grandfather "Papier ist geduldig" translated from German "Paper is Patient" In other words, we can all design to perfection on paper, but in the 3D world we have to make sacrafices. I think all of us DIYer's here can relate to that!

Again, please feel free to tear apart my assumptions.
Chip

ChipWV:
I'm not an acoustician. I've just been surfing for three years longer than you have. I have a reasonable little acoustics library of a few classics.
I believe everything I write -- but that's no reason why you should. :)

"Perforated Panel Absorbers" are one kind of Helmholtz device.

The symmetry I was concerned about was more mid and highs. For the bottom, you can slide with a bit less symmetry. However, an entire side of the room being completely different than the other is not something I would consider nor recommend.

While it may be reducing some of your bass control plans, IMO, that would be preferable to creating a much more distracting problem. The issue with pegboard as Bob pointed out is that it's not going to really work as a bass absorber (tuned) and the little bit that it may will be unpredictable and very weak. However, you will be allowing mids and highs in through those holes that you are not on the other side.

Now, if you want some bass control and IF you have the height, you can build a matching soffit on the other side and then build a sealed panel membrane absorber below it. Somthing around 3.5" with the appropriate front face will get down close to subwoofer range.

Bryan

I am far from an expert but maybe you could attach some cheap black felt from Joanne Fabrics on the outside of the frame before the GOM. You can pull that pretty tight to double support the insulation.

or


at HD you can by that cheap plastic porch screen to staple onto the front like this

http://www.sausage-stuffer.com/images/47379_non_stick_drying_sheet_jerky.jpg

I would think that would be safe...

The symmetry I was concerned about was more mid and highs. For the bottom, you can slide with a bit less symmetry. However, an entire side of the room being completely different than the other is not something I would consider nor recommend.

That strikes me as a very reasonable concern. If accepted, then the design goal becomes finding a way to build matching soffits all around.

As to stuffings, you will achieve excellent results if you can get a 4" layer of 703 on both legs of the soffit. Filling the entire void with 703 [a solid block] would be best practice, but would not be high efficiency. To get an idea of how a 4" panels in a corner performs as a broadband absorber versus a full block, you might look at the THIS (http://forum.studiotips.com/viewtopic.php?t=536) thread and compare the SCA versus the SuperChunk.

Adding some pink fluffy to a hollow center is a great way to eat up all the insulation scraps on the job... but I question whether the benefits / results would worth the cost of additional "new" batts.

If you need to achieve HF rejection in the device then add a layer of +/- 6mm poly film to the 703.

If you have room and aethetics allow, you might consider a simple panel/frame across the diagonal of the ceiling/wall corner - versus a box soffit - this would much easier to build and cover with full 703 panels [and would leave a 17x24x17 triangular void for the HVAC trunk]. If that approach is acceptable then you could effect the treatment with a standardized panel system [either DIY or factory made].

If you have room and aethetics allow, you might consider a simple panel/frame across the diagonal of the ceiling/wall corner - versus a box soffit - this would much easier to build and cover with full 703 panels [and would leave a 17x24x17 triangular void for the HVAC trunk]. If that approach is acceptable then you could effect the treatment with a standardized panel system [either DIY or factory made].

Scott,

That is a fantastic idea. Unfortunately, I don't have enough headroom to implement.

Based on all your great feedback, I have decided to do the following:

1) Drywall soffit around HVAC pipe.
2) Linacoustic on vertical face of soffit (plain drywall on bottom for headroom)
3) GOM over soffit
4) Matching soffits on other three walls with OC703 and insulation backfill.
5) GOM over the false soffits.

Recognizing the L/R will still not be symmetrical, at least the linacoustic will help tame the high and mid reflections over the 9" surface on the "real" soffit. I think this may represent the best compromise between symmetry and bass trapping.

Thanks again,
Ned

I am trying to work on the particulars for the soffit. This thread seems to have some good info in it (not all of it understood), so probably better to add to this than start a new one.

Assuming that trapping is desireable, and using the soffit for some trapping is also desireable, is it feasable if the soffit also contains significant lighting and cable runs?

Assuming you need the insulation exposed to the room, not entombed in sheetrock/MDF/OSB (whatever), do you need both the bottom and the sides open or just one or the other?

Here is a rough layout showing the lighting desired. Assume 9 4" cans, not the 11 pictured. The front of the room will be a false wall, on a stage, with an AT screen. Can use some of that space for treatment also.

http://i167.photobucket.com/albums/u129/dc_pilgrim/Drawing6.jpg
A small bit of ductwork will need to be encased:
http://i167.photobucket.com/albums/u129/dc_pilgrim/to_be_soffitted.jpg

Assuming that trapping is desireable, and using the soffit for some trapping is also desireable, is it feasable if the soffit also contains significant lighting and cable runs?

Assuming you need the insulation exposed to the room, not entombed in sheetrock/MDF/OSB (whatever), do you need both the bottom and the sides open or just one or the other?


This thread had good info and I have these same two questions above. Was there any answers?

Having both sides open is always better.

--Ethan