AVS Forum banner
Status
Not open for further replies.
1 - 20 of 64 Posts

·
Registered
Joined
·
252 Posts
Discussion Starter · #1 ·
By the word "broadband bass", I'm referring to the frequencies from 0-to maybe 300-500 Hz and not a specific frequency like 57Hz where a tuned absorber is needed for maximum absorption.


In the building of my dedicated HT mostly for action movies, I'll be using an appropriate mix of probably 1" of Owens Corning 703 below and polyester batting above on the walls for mid and high frequency absorption. What I'm pondering about is building a porous soffit, with nothing inside the soffit, acting as a broadband bass trap for my smallish 12w x 16l x 10h HT, about 2000 cu ft.


1. To start the talk process, how about a 12" x 12" soffit all around the room and covered with 5/8" drywall. Without instruments to test, does this sound like a reasonable size also considering asthetics?


2. I read somewhere that 1" of rigid OC 703 is about equal to 6" of fluffy pink fiberglass. Whatever the ratio, if I use OC 703 inside the soffit, I can get lots of 703 in it. Is there a diminishing return? I know it's posible, but is it reasonable for the entire 12" x 12". or whatever size, to be filled with 703 on the theory that since the porous soffit bass trap may be my only attempt at bass absorption, make it as absorbing as possible?


3. Without instruments to test, is it possible to have too much bass absorption by building the soffit beyond 12" x 12" and stuffed full of 703?


4. What size and filled with how much 703 would be a reasonable compromise of all the variables, including aesthetics?


5. Assume after testing that the broadband bass absorption is not enough, should this soffit arrangement be turned vertically for the room's corners? This might be asthetically difficult in my smallish dedicated HT.


Take Aim
 

·
Registered
Joined
·
5,002 Posts
What else is in the room?

How many couch seats, and are they leather (8 berklines)?

What's the carpet like (padding, thickness)?

What are the walls like (e.g. two layers drywall on both sides of double studs)?

What's the floor made of (e.g. basement slab)?

Is there a riser, and if so what's the top made of (3 layers of 3/4" plywood)? Does it have holes/ports?
 

·
Registered
Joined
·
252 Posts
Discussion Starter · #3 ·
The dedicated HT is under construction at glacial speed, mostly because I actively read the many areas of AVScience and ask a bunch of questions. I was a novice at HT. However, after untold hours of study and visiting the HT's of Larry Chanin and Jerry Pease, I've learned a lot.


However, I am constantly amazed at the knowledge base available and how helpful people are in the various forums. Hopefully, some stuff I bought along the way is the right stuff. The HT will never be "perfect" but my goal is an above average HT in video, sound, asthetics, and comfort. Many questions remain and more that I haven't even thought of will pop up. At the moment, and subject to change, here it is.


A 14" riser holds a Lay-z-Boy 2-seat sofa in a medium-to-dark red leather connected to a one seater, all powered and with memory foam. The front row at ground level has two individual Lay-z-Boy rockers that recline and fully swivel separated by a small table. My wife says I now need three seats in the front row. All bought and being held by the dealer.


The riser was contructed with one step and rope lighting. The first level has 2x8 sides and one layer of 3/4" plywood on top. The top level has 2x8 sides and 2 layers of plywood on top. The riser is isolated from the concrete with something like 2" x 4" by 1" thick heavy rubber "isolators." I know about cutting holes/ports in the top but I don't have any details about it. The riser is nailed to two walls.


The carpet is Karastan Andrea, 100% nylon, number 73208/3761. It's medium brown with flecks of greens and blues. The pad is 1/2" of a uniform rubber-type that is solid in construction. I will find out more details.


The Florida home was just built by US Home. It's one story built over a concrete slab with no basement. They agreed to not build a closet in one of the bedrooms. I permanently blocked out the window with plywood and insulation. Since it's not a bedroom, because of no closet, there is no egress required thru a window.


I've ordered a 2nd door that is an outswing version. It will isolate the HT after walking thru the usual bedroom door and a 3'x3' bedroom hallway entrance. In between the doors and facing the hallway is the Equipment Room, 33" x 39" x 10'. It holds all the equipment and has a USTec central wiring panel for cable TV, DirecTV, telephone, LAN, etc and cables that feed the other rooms. The house was pre-wired by Advanced Audio of Sarasota and has tons of wiring everywhere.


I'm looking into a Russound CAV6.6 with doorbell and intercom attachments for whole house audio/video.


The ER has a dedicated 80 amp sub-panel with 4 20 amp circuits to minimize ground loops. The ceiling has a Broan Ultra Silent vent fan cooling the Middle Atlantic Slim 5 rack on wheels that pulls out into the small hallway for easy access inside the ER and to the back of the rack.


The walls/ceiling now have one layer of 5/8" drywall attached with many screws. I have on hand three cases of Green Glue to be applied when the 2nd layer of 5/8" drywall goes up on the walls and ceiling. However, no RISC clips to isolate the room or double studs. I have a case of USG Acoustic Sealant in 29 oz tubes waiting to seal up the place.


All walls in the house have QuietSeal fiberglass batts. R-30 in the attic.


The walls will probably get a combination of 1" OC 703 and polyester batting using the customary installation of OC from floor to ceiling at the first reflection point. Thereafter, 703 from floor to ear level and batting to ceiling. I like the alternating vertical panels of black GOM 701 #408 and #549 Chrome Green, similar to Larry's HT. I'll probably attach it with Fabricmate. This allows me to acoustically test the HT and remove the GOM to alter the combination of 703/batting if needed.


I may put 703 on the front wall behind whatever I finally decide for the front.


I called RPG about maybe using their Flatffusor dispersion for the back wall, but didn't get the call-back. It comes in 2'x2' sheets and is kind of like using their Skyline or Hemiffusor but without the 4" protrusion into the room.


I'll also be using the same sconce that Larry has, except the frame is flat black. I have four from the Justice Design Group, #7200 with fire glass.


I'm going to get the ETF software and also the mixer and mike they offer. This should give me an acoustic summary once the project is completed to see how I did.


As I think about it, my HT will be a summary of the ideas of others, the two theaters I saw, and applying this to my situation. The HT won't be perfect, but I believe it will be quite good. The room should be reasonably isolated from the rest of the house so I can enjoy my action movies in 7.1. However, I might get into surround music since in years gone by I played the viola, piano, and church organs.


Well, I'm worn out from all the typing and thinking above. However, ask any questions. I never had a detailed plan from inception, only some fuzzy ideas. It's more like on-the-job learning while construction moves along. But my questions today are about soffit construction for broadband bass absorption.

Take Aim
 

·
Registered
Joined
·
6,697 Posts
Quote:
Originally Posted by takeaim
I may put 703 on the front wall behind whatever I finally decide for the front.
Hi,


Jim (takeaim) neglected to say that so far he is planning on building cabinetry around his screen. The bottom section of cabinetry will be mainly for storage. His LCR in-wall speakers will be mounted in the front panels of the top section of cabinets.


Assuming the top section of the cabinetry is only used to mount his speakers, would it be best to line the insides of the cabinets with 1" of fiberglass, or fill the cavities with batt insulation? Or none of the above?


As I said, he plans on using the bottom cabinets for storage and to also house a subwoofer. Assuming the cabinets have solid doors on the sections not concealing the subwoofer, would it make sense to line the cabinets, and the back of the doors with rigid fiberglass?


Larry
 

·
Registered
Joined
·
252 Posts
Discussion Starter · #5 ·
To reply to my own questions, I'm leaning toward a 12" x 12" soffit running around the HT on three sides.


The front of the room might get a soffit of 20"w x 12"h. I haven't figured out the front wall yet, but there are at least two arrangements. I would start with 20" deep cabinets about 36-40" off the floor for storage and my subwoofer. From the ceiling hangs the soffit. That leaves the middle which is about 68" in the 10' high HT. Might do drawers and doors on both sides of the screen (or TV) or just a frame a few inches off the wall to hide the in-wall Triad speakers.


The soffit will most likely have OC 703. I'm figuring about 3-4" thick both vertically and horizontally within the soffit. That much 703 corresponds to maybe 18-24" of fluffy fiberglass, using a ratio of 6:1 that I read about.

Take Aim
 

·
Registered
Joined
·
5,002 Posts
takeaim:


(Don't forget, I'm not an acoustician -- I just web surf. So take this with a healty degree of skepticism)


There are better ways of predicting a room than RT60, but I as far as I know it's a good starting point, and it's certainly simple to do.


I prefer engineering to mere hoping for the best.



So here we go


from http://www.bobgolds.com/Mode/RoomModes.htm

we get recommended Sabins/RT60, as well as frequencies of concern.


Room Dimensions: Length=16 ft, Width=12 ft, Height=10 ft

Room Ratio: 1 : 1.2 : 1.6

R. Walker BBC 1996:

-- 1.1w / h
-- l
-- no integer multiple within 5%: Pass

Nearest Known Ratio:

-- "14) Derived from C. P. Boner (stylized ratio)" 1 : 1.25 : 1.6
RT60 (ITU Control Room Recommended): 204 ms
Absorbtion to achieve RT60: 461 sabins

Volume: 1920 ft^3

Surface Area Total: 944 ft^2

Surface Area Floor: 192 ft^2

Surface Area Ceiling+Floor: 384 ft^2

Surface Area Front Wall: 120 ft^2

Surface Area Front and Rear Wall: 240 ft^2

Surface Area Left Wall: 160 ft^2

Surface Area Left and Right Wall: 320 ft^2

Surface Area 4 Walls: 560 ft^2

Surface Area 4 Walls + floor: 752 ft^2

(sabins - front wall - carpet) / Left+Right+Rear wall: 33 %

(sabins - front wall) / Left+Right+Rear wall: 77 %

Schroeder Fc: 115hz

Frequency Regions:

- No modal boost: 1hz to 35hz

- Room Modes dominate: 35hz to 115hz

- Diffraction and Diffusion dominate: 115hz to 460hz

- Specular reflections and ray accoustics prevail: 460hz to 20000hz

Count (35.3-206hz) : Axials=12, Tangentials=47, Obliques=60


and, below, we have the frequencies of concern:


35.3 hz 32'0", 16'0", 8'0" (1,0,0 Axial)

47.1 hz 23'12", 11'12", 5'12" (0,1,0 Axial)

56.5 hz 20'0", 10'0", 5'0" (0,0,1 Axial)

58.9 hz 19'2", 9'7", 4'10" (1,1,0 Tangential)

66.6 hz 16'12", 8'6", 4'3" (1,0,1 Tangential)

70.6 hz 16'0", 8'0", 4'0" (2,0,0 Axial)

73.5 hz 15'4", 7'8", 3'10" (0,1,1 Tangential)

81.6 hz 13'10", 6'11", 3'6" (1,1,1 Oblique)

84.9 hz 13'4", 6'8", 3'4" (2,1,0 Tangential)

90.4 hz 12'6", 6'3", 3'2" (2,0,1 Tangential)

94.2 hz 11'12", 5'12", 2'12" (0,2,0 Axial)


Then comes an RT60 calc based on what you have in the room. This is the amount of Sabins of absorbtion you're likely to get per frequency band: 125hz, 250hz, 500hz, 1000hz, 2000hz, 4000hz. RT60 and Sabins in each of these should be roughly level, and around the recommended levels above. It's a target, a guess, not a plan.

Also it's garbage in is garbage out -- so if my surface area dimensions or absorbtion coeficients are wrong for any reason (such as proximity to each other or other mounting issues or other variations from a diffuse field), well, so's the prediction.

Nevertheless,

http://www.bobgolds.com/RT60_takeaim.gif

Excel file containing above


So it looks to me that you're over absorbed in the high frequencies already (500 hz to 4000hz). I was hoping, based on the ITU spec above, for 204ms, and we got 150ms to 130ms.


So putting 703 based porous soffits up there wouldn't be my first choice, although it'll get rid of some of the surface area of the polyester batting which would help, it's still not a low freqency treatment, and doesn't balance out the rest of the issues.


First you need to get rid of some of your high frequency absorbtion (500hz to 4khz).


Only then should you consider how to get more low frequency absorbtion (30hz to 100hz). But I'm guessing it'll be some sort of resonance absorber. Possibly a port helmholtz in your riser, and perforated soffets. RPG's "BADâ„¢ Panel provides absorption below about 1kHz, and diffusion above." and is an interesting example of a perforated helmholtz absorber. http://www.rpginc.com/products/badpa...comparison.htm

If you build a soffet like that, with slightly smaller holes, and fewer of them, and 8" to 12" of fiberglass behind it, it'll be a bass absorber more tuned to your room. Although a John Sayers style slat absorber is probably easier to build.


Once you figure out how much, then you can figure out where to put it (SBIR/Haas etc).



BTW, just FYI, not actually relevant to either the topic or your room, but have you ever been here: http://www.rpginc.com/products/cinemusic/index.htm
 

·
Registered
Joined
·
252 Posts
Discussion Starter · #7 ·
BasementBob and others:

I ask a few questions and get bombarded with detail. That's just what I want and need. Thanks for your help. However, answers can lead to more questions.


1. Where did you get the Sabin absorption figures for different frequencies using different things, such as carpet, wall treatment, etc, that are in the spreadsheet?


2. Did you collect data from various websites and use it to manually build a spreadsheet just for my HT? Is there a standard spreadsheet somewhere waiting for figures to be included?


3. Near the bottom of the excel spreadsheet is an unnamed line. The numbers along the line seem to mostly be a fixed 279.18 that adds into the total Sabins per frequency. Please explain. Even without adding the 279.18, it certainly looks like there is still far too much absorption. So I will rethink the % of 703/batting.


4. The Karastan carpet with 1/2" pad adds a lot of absorption as the frequency increases. The pad is just to soften the walking surface because the house is built over an unpainted concrete pad. Should the pad be removed or a thinner pad installed. Should I seal the concrete? What might sealing the concrete actually do or benefit? I think carpet is supposed to be installed over a pad because the bottom of the carpet should not touch concrete. Maybe the moisture from the concrete can hurt the carpet.


5. I did call RPG last Friday about the Flatffusor, which looks to be the same kind of idea found in their BAD panels. My thought was to use some of that on the back wall in place of the batting. RPG referred me to two Florida dealers who both said they would get back to me. What I really wanted is just the diffusor with the many holes in it. I could put the Flatffusor or BAD panels on the entire rear wall, which would remove both the 703 and batting. That would lower the Sabins and increase diffusion.


6. If the soffit is an exoskeleton design covered with 5/8" drywall, why does the soffit need holes? Doesn't bass pass right through drywall and some gets absorbed by the 703/fiberglass and dead air space inside the soffit? Don't mid and high frequencies mostly bounce off the drywall enclosed soffit? I believe I'm after a broadband bass absorbing soffit system, not necessarily one tuned to a specific frequency.


7. If OC 703 is so dense, why not use it in place of the fluffy pink? Based on density per cu ft, the 703 is maybe 6 times more dense. If that is approximately so, 1" of 703 = 6" of pink. I can put lots of 703 in a 12" x 12" soffit running around the room. I might guess that 4", or whatever, of 703 both vertically and horizontally followed by 8" of dead air space in a soffit could offer broadband bass absorption. Isn't this design somewhat like that of John Sayers who hangs a blanket in the corner 6 to 8 inches away from the wall creating a dead air space? I've seen DIY corner traps about 24" diagonally that are filled with 703 and there are similarly designed commercial products using various materials.


My carpenter is due Tuesday night to build me a soffit while I'm still pondering its details. Oh well.

Take Aim
 

·
Registered
Joined
·
6,697 Posts
Quote:
Originally Posted by takeaim
3. Near the bottom of the excel spreadsheet is an unnamed line. The numbers along the line seem to mostly be a fixed 279.18 that adds into the total Sabins per frequency. Please explain. Even without adding the 279.18, it certainly looks like there is still far too much absorption. So I will rethink the % of 703/batting.
Hi,


I'd be interested in where this line came from as well. Without it the absorption would be almost perfect. For example, at 5000 hz the absorption would be 449 sabins which would yield a Reverberation Time of 210ms. Almost the same as the ITU recommendation.


Larry
 

·
Registered
Joined
·
5,002 Posts
takeaim:
Quote:
3. Near the bottom of the excel spreadsheet is an unnamed line. The numbers along the line seem to mostly be a fixed 279.18 that adds into the total Sabins per frequency. Please explain.
Ooops.

I'll get rid of it.

It was a WideBand/BroadBand absorber from someone else's room.

I had removed the description to remind me to get rid of the line ...


done.

Hit refresh in your browser to see it above.

http://www.bobgolds.com/RT60_takeaim_chart.gif
 

·
Registered
Joined
·
5,002 Posts
takeaim

Quote:
1. Where did you get the Sabin absorption figures for different frequencies using different things, such as carpet, wall treatment, etc, that are in the spreadsheet?
I collect them.
http://www.bobgolds.com/Sabin.htm

start with "Sabin Data Text File" (note a lot of the 63hz data in that is simply copied from the 125hz data)



BTW, I expect you to edit the spreadsheet, and post GIF of the spreadsheet allong with corresponding descriptions of your new acoustics plan.


Quote:
2. Did you collect data from various websites and use it to manually build a spreadsheet just for my HT? Is there a standard spreadsheet somewhere waiting for figures to be included?
I've done this before. So I just edit the spreadsheet.

Quote:
4. The Karastan carpet with 1/2" pad adds a lot of absorption as the frequency increases. The pad is just to soften the walking surface because the house is built over an unpainted concrete pad. Should the pad be removed or a thinner pad installed. Should I seal the concrete? What might sealing the concrete actually do or benefit? I think carpet is supposed to be installed over a pad because the bottom of the carpet should not touch concrete. Maybe the moisture from the concrete can hurt the carpet.
Carpet always absorbs a lot of HF, and very little LF. Pads can have an effect on that, increasing the MF absorbtion a bit, but not much. The thicker the carpet, and the thicker the pad, the better for absorbtion -- but you're realistically limited by what you can walk on, and the fact that as you increase the pad depth the carpet bends more, decreasing it's wear life. The concrete slab, sealed or not, is a relector -- and relative to the carpet it can be ignored acoustically.

Quote:
5. I did call RPG last Friday about the Flatffusor, which looks to be the same kind of idea found in their BAD panels. My thought was to use some of that on the back wall in place of the batting. RPG referred me to two Florida dealers who both said they would get back to me. What I really wanted is just the diffusor with the many holes in it. I could put the Flatffusor or BAD panels on the entire rear wall, which would remove both the 703 and batting. That would lower the Sabins and increase diffusion.
Personally I'm more of a DIY guy. My plan is to build a room such that I can tweak the acoustics after it's built.

Quote:
6. If the soffit is an exoskeleton design covered with 5/8" drywall, why does the soffit need holes? Doesn't bass pass right through drywall and some gets absorbed by the 703/fiberglass and dead air space inside the soffit? Don't mid and high frequencies mostly bounce off the drywall enclosed soffit? I believe I'm after a broadband bass absorbing soffit system, not necessarily one tuned to a specific frequency.
I agree you need more bass absorbtion, all the way upto 500hz.

Quote:
7. If OC 703 is so dense, why not use it in place of the fluffy pink? Based on density per cu ft, the 703 is maybe 6 times more dense. If that is approximately so, 1" of 703 = 6" of pink. I can put lots of 703 in a 12" x 12" soffit running around the room.
Well, now we're out of my depth, but I'll be happy to spout on. :)

But from what I understand it's a tradeoff between flow resistivity and reflectivity and cost.

Certainly fluffy fiberglass pink is cheaper and easier to buy.

Fluffy fiberglass pink has no structural strength -- hold it up by one end in your hand and the rest flops over -- so structure has to be provided.

Fluffy fiberglass pink has a lower flow resistivity (rayls) and a lower reflectivity (good). But this lower flow resistivity is much too low to be used in a 1" to 4" deep product, but is a good thing in a 24" deep product. Superchunks are 17" deep, and 701 is recommended instead of 703.

http://www.bobgolds.com/NormalVsGrazing.GIF


These are pretty much what can be made.

This is just how I think of them, not actual coefficients based on something.

For example corner chunks have a different curve than low bass.

And resonant absorbers can be made with a peak lower than the lowest shown.

etc.
http://www.bobgolds.com/BroadbandAbs...s_BassTrap.GIF

Quote:
I might guess that 4", or whatever, of 703 both vertically and horizontally followed by 8" of dead air space in a soffit could offer broadband bass absorption. Isn't this design somewhat like that of John Sayers who hangs a blanket in the corner 6 to 8 inches away from the wall creating a dead air space? I've seen DIY corner traps about 24" diagonally that are filled with 703 and there are similarly designed commercial products using various materials
The best you can get with porous absorbtion is the thick broadband line above.


There are several types of absorbers

a) thin porous most of wall

b) thick porous entire ceiling full

c) corner porous panel (corner chunk)

d) corner porous full (superchunk)

e) parallel out from surface porous (e.g. 4" out, acoustic ceiling tiles)

f) covered porous (FRK, leather couch)

g) constant slat/slot helmholtz (traditional formuals)

h) variable slat/slot helmholtz (John Sayers example)

i) port helmholtz (e.g. helmholtz riser)

j) constant perforated/microperforated helmholtz

k) variable perforated helmholtz

l) membrane, panel, diaphramatic (cupboards, bad risers)

m) polycylindrical {membrane,slat,port,perforated} absorbers

n) active noise suppression


porous includes

1) fiberglass (fluffy pink, rigid)

2) rockwool (semi rigid SafeNSound, rigid)

3) cotton

4) polyester batting (WalMart fabric department ish)

5) foam (often available in shapes like wedges)


Any absorber may be placed in a checkerboard or separated baffles.
 

·
Registered
Joined
·
252 Posts
Discussion Starter · #11 ·
Let me revise the excel spreadsheet by subtracting the 279.18 figure.


125 Hz 250 500 1,000 2,000 4,000 Hz

Sabins 106.46 189.21 345.69 426.69 434.58 447.02


If the Sabins should be about 461 across the spectrum, then my HT is probably reasonable for mid and high absorption, but too little at the bass frequencies.


What are the new figures for RT60? Is there a formula?

Take Aim
 

·
Registered
Joined
·
5,002 Posts
takeaim:


You should download the spreadsheet again.

Quote:
Let me revise the excel spreadsheet by subtracting the 279.18 figure.
Already done for you.

Quote:
What are the new figures for RT60?
See above gif (after hitting refresh in your browser), above excel spreadsheet, and above graph. (no changes since 1:20pm today my time)

Quote:
Is there a formula?
Yes. It's in the excel spreadsheet.
 

·
Registered
Joined
·
6,697 Posts
Quote:
Originally Posted by BasementBob
Quote:
Originally Posted by takeaim
6. If the soffit is an exoskeleton design covered with 5/8" drywall, why does the soffit need holes? Doesn't bass pass right through drywall and some gets absorbed by the 703/fiberglass and dead air space inside the soffit? Don't mid and high frequencies mostly bounce off the drywall enclosed soffit? I believe I'm after a broadband bass absorbing soffit system, not necessarily one tuned to a specific frequency.
I agree you need more bass absorbtion, all the way upto 500hz.
Hi Bob,


Great thread, but you're making it waay too easy for Jim! Just kidding Jim. ;)


If you don't mind I'd like to redirect your attention to Jim's question above. If he built a drywall enclosed soffit filled with fiberglass, wouldn't it serve as a bass trap versus a broadband absorber? As he points out, wouldn't the higher frequencies be reflected by the drywall with the bass passing through to be absorbed by the foot thick aborber? Or do you think that the bottom of the soffit should be constructed out of something else rather than drywall?


Thanks for taking the effort.


Larry
 

·
Registered
Joined
·
5,002 Posts
LarryChanin:


I'm trying to watch ConAir right now. :) Almost over.

Quote:
6. If the soffit is an exoskeleton design covered with 5/8" drywall, why does the soffit need holes? Doesn't bass pass right through drywall and some gets absorbed by the 703/fiberglass and dead air space inside the soffit? Don't mid and high frequencies mostly bounce off the drywall enclosed soffit? I believe I'm after a broadband bass absorbing soffit system, not necessarily one tuned to a specific frequency.
I don't know.

My guess is that as a resonant membrane, the stiffness of the exoskeleton structure would defeat any reasonable amount of absorbtion.

That leaves the drywall itself, which tends to reflect a lot of energy. I don't really see it being any different than the walls themselves, except for the stiffness (walls on 16 inch centers, soffet on 12 inch centers), so my guess is that it would absorb less than the existing walls. So what's the point of building it that way?


The walls are thicker (2 layers of drywall is more mass, which lowers the resonance frequency if any) and thinner between leafs (which raises the resonance frequency if any). Like I said -- I don't know.
 

·
Registered
Joined
·
6,697 Posts
Quote:
Originally Posted by takeaim
7. If OC 703 is so dense, why not use it in place of the fluffy pink? Based on density per cu ft, the 703 is maybe 6 times more dense. If that is approximately so, 1" of 703 = 6" of pink. I can put lots of 703 in a 12" x 12" soffit running around the room. I might guess that 4", or whatever, of 703 both vertically and horizontally followed by 8" of dead air space in a soffit could offer broadband bass absorption. Isn't this design somewhat like that of John Sayers who hangs a blanket in the corner 6 to 8 inches away from the wall creating a dead air space? I've seen DIY corner traps about 24" diagonally that are filled with 703 and there are similarly designed commercial products using various materials.
To Bob and others,


I know that in a lot of Dennis Erskine designed theaters folks build soffits out of MDF and completely stuff it with fiberglass batts. I believe the objective is to reduce the likelihood of the soffit cavities resonating at the cavity's natural resonant frequency. If this is so, it would seem that we wouldn't want to leave air space. If filling the space is paramount, it's obviously cheaper to use fluffy pink instead of the more expensive high density fiberglass.


Larry
 

·
Registered
Joined
·
5,002 Posts
LarryChanin:


ConAir status - airplane just went through a guitar.

Quote:
I know that in a lot of Dennis Erskine designed theaters folks build soffits out of MDF and completely stuff it with fiberglass batts.
Can you describe in detail the construction? Spacing of 2x4's/2x2's, thickness of MDF, dimensions of soffet, etc.
 

·
Registered
Joined
·
6,697 Posts
Quote:
Originally Posted by BasementBob
LarryChanin:

Quote:
Originally Posted by LarryChanin
I know that in a lot of Dennis Erskine designed theaters folks build soffits out of MDF and completely stuff it with fiberglass batts.
Can you describe in detail the construction? Spacing of 2x4's/2x2's, thickness of MDF, dimensions of soffet, etc.
Hi Bob,


Before I attempt to answer your question, I would like to clarify that I wasn't suggesting that the primary purpose of the soffits in a Dennis Erskine design is to serve as a bass trap. In fact, as I understand it, Dennis likes to use soffits in conjunction with columns to serve as a sort of conduit for HVAC and electrical. The idea being that the room is sealed with drywall, then ideally it is penetrated at a single location for HVAC and electrical and resealed with acoustic caulk. Then the soffit/column system is used to permit routing multiple vents, returns, electric outlets, boxes for sconces, etc., throughout the room without creating additional penetrations of the room seal. Being located in a corner, obviously Dennis wouldn't miss an opportunity to cover the bottom of the soffit with 1" of fiberglass, so there is an acoustic benefit as well. However, I don't think the soffits in general are intended to be bass traps. Nevertheless, whenever possible he seems to recommend filling the solid walled cavities with fiberglass.


In answer to your question, here's a photo of Ted White's soffits in his DE designed theater.
http://www.tedwhite.homestead.com/fi..._conduit_1.jpg


Here's a link to the archives on the subject:
Adding "soffit" to floating ceiling


Here's Ted's comments in that thread regarding some of your questions:
Quote:
You're doing a single layer of MDF over the 2x2. This would be supported every 16" or 24" or however far you have the RC spaced. I suppose that tie in to the RC every 16" would be OK.
His comments were made in the context of attaching the MDF soffit to Resilient Channel, but I believe it still accurately describes a standard DE design without RC. The MDF is 3/4" thick.


Since the soffits are frequently housing ductwork, etc., I believe the size of the soffit may vary significantly from room to room. You can see in the thread referenced that one forum member with a DE designed theater has a soffit 11" D x 14" W.


Here's a photo of Dan Hanson's MDF soffit, with integral light tray.
http://members.shaw.ca/danhanson/The...Traydetail.jpg

I think it's a DE design.


Here's a photo of Jeff's (thebland) DE designed soffit. http://www.winglake.com/images/home_...r/IMG_0484.jpg

One side houses 6 HVAC supply vents, the other side houses 6 return vents.


Larry
 

·
Registered
Joined
·
5,002 Posts
LarryChanin:



Ted White's soffit's, assuming they resonates like a membrane

3/4 MDF is about 3.1875 pcf.

F = 170 / sqrt(M * D),

F = 170 / sqrt(3.1875 * 11)

F = 29hz

I wonder how efficient it is, or if it works in some other way.



TheBland's soffit's might be a ported helmholtz. The ports being the area of the vents, and the volume being the volume of the ductwork inside the soffit.

at http://www.mhsoft.nl/Helmholtzabsorber.asp

I typed in

Height: 600cm

Width: 40cm

Depth: 30cm

theoreticly Port Length: 60cm (6 vents you said)

Port Width: 25cm

and got calculated responant frequency: 22 hz
 

·
Registered
Joined
·
6,697 Posts
Quote:
Originally Posted by BasementBob
LarryChanin:



Ted White's soffit's, assuming they resonates like a membrane

3/4 MDF is about 3.1875 pcf.

F = 170 / sqrt(M * D),

F = 170 / sqrt(3.1875 * 11)

F = 29hz

I wonder how efficient it is, or if it works in some other way.



TheBland's soffit's might be a ported helmholtz. The ports being the area of the vents, and the volume being the volume of the ductwork inside the soffit.

at http://www.mhsoft.nl/Helmholtzabsorber.asp

I typed in

Height: 600cm

Width: 40cm

Depth: 30cm

theoreticly Port Length: 60cm (6 vents you said)

Port Width: 25cm

and got calculated responant frequency: 22 hz
Hi Bob,


No it's a lot simplier than that.


I didn't assume that the ports were HVAC ducts by looking at the photo. My comment was based on a statement that Jeff made regarding his soffits.


Larry
 
1 - 20 of 64 Posts
Status
Not open for further replies.
Top