Reducing audio "brightness" in room (1 Viewer)

[Aaron Gould]

I'm building a home theatre in my basement, but am concerned about potential "brightness" in the way of audio. All walls are sheetrock, ceiling will almost certainly be drop-ceiling, and a not overly thick carpet will be installed (with underpadding that is advertised to have "sound absorption" qualities).

In the back of the room (right behind the couch) will be my mini-office. The desk along with a couple shelves add more hard and flat surfaces to the area. More sound reflection...

I am thinking of applying cloth of some sort to the bottom half of the walls as I've seem many a HTF member do. Is this effective when directly on top of drywall?

Soundboard also interests me. My drywalls are already installed, along with the electrical outlets and various in-wall audio jacks. Is it too late for soundboard seeing that all my outlets and jacks are flush with the drywall, and can't be extended outward another 1/2 inch to accommodate the soundboard?

Just for visual purposes, here's my theater/office layout:

[C] [/C]

 

[Terry Montlick]

I'm not an expert on installation, but if you can figure out a way to incorporate rigid fiberglass such as Soundboard, you will greatly improve your home theater sound compared to straight sheetrock surfaces. Thickness is the key. An inch of rigid fiberglass is about the minimum to get effective absorption down to around 500 Hz. If you hit the "mirror points" with this stuff, it will do double duty to supress the nasty early reflections that the sheetrock will give you.

But you should have lower frequency absorption as well, ideally to below 100 Hz. Thicker material will extend absorption into lower frequencies. It looks like you have a good amount of space behind the front speakers. This might be a good place to put low frequency absorbers such as ASC tube traps. It also appears that you might have some low frequency absorber depth to spare at the rear wall.

Covering at least 25% of the wall surface should give you significantly improved acoustics. Even medium-priced home theater electronics and speakers give you extremely good sound, so non-ideal acoustics quickly become the bottleneck for quality audio. Without an investment in room acoustics, you can only degrade the sound as it travels from your speakers to your ears.

 

[Aaron Gould]

Terry, thanks for the informative response! Definitely information I was after.

You mention that 25% of the wall surface should be acoustically treated. Could 25% of it mean a strip at ear level around the room, about 2 feet in height, two feet off the floor)? This would get around the need to bring forward the electrical outlets as they're below two feet on the walls.

Or do you mean a few panels (say, 3' x 3' squares for example) placed like pictures every couple feet around the walls?

By the way, my front and surround speakers are Mirage omni-directionals (OM-10 fronts, OM-R2 surrounds). Only the center channel (OM-R2) is directional. Does this change the picture any? Also, my subwoofer will be behind the front-left speaker in the room. I neglected to put that in the image.

 

[gregstaten]

Aaron - all of the walls in my theater are acoustically treated. The lower half of the walls is covered with J&M TheatreShield and the upper half with cotton batting. That's then covered with acoustically transparent cloth. The trick to creating a truly dead room is to have a combination of absorption and diffusion.

-greg

 

[Aaron Gould]

Greg, nice home theater! Just checked out the pics.

I'm wondering about your acoustic treatment -- is the top half thicker than the bottom half? This interests me since I cannot move my electrical outlets forward anymore as they are firmly attached flush with the drywall.

 

[Terry Montlick]

Could 25% of it mean a strip at ear level around the room, about 2 feet in height, two feet off the floor)? ... Or do you mean a few panels (say, 3' x 3' squares for example) placed like pictures every couple feet around the walls?

Both will work.

Greg is right about the need for diffusion. This gives your remaining reverberation a nice constant and clean decay, and keeps the reflected sound field uniform.

If you go with multiple panels, the edges of the panels will automatically create diffusion. But your existing desk and bookshelf should help, as will the little L-shaped recess where your double doors are.

 

[Aaron Gould]

I think I'm going to go with the panels. At this point in the construction of the room, it's by far the easiest option I think. I don't suppose Home Depot carries soundboard do they? Or is this material a specialty item?

Just a couple quick questions on the panel size and placement -- are there any mathematical equations I should be using? Is there a set size for these panels, and a set distance between them? Should these panels be at ear level, or the top of the walls?

Thanks again guys, the help is appreciated!

 

[Terry Montlick]

Home Depot doesn't carry this, I'm afraid. Lots of companies will sell you fabric-wrapped fiberglass panels. Do a web search on "acoustical panels." These are all pretty much equivalent, and use rigid fiberglass board manufactured for industrial use.

I don't recommend buying the fiberglass yourself and wrapping it, because the face and edges must be hardened with resin to make them resistant to compression and dents, and this really isn't practical at home. You should have lots of choices for fabric colors and styles - it's usually Guilford of Maine, a great company with top acoustically transparent fabrics.

There's no formula for dimensions - you're free to go by aesthetics. The total surface area is the thing.

 

[Pam W]

Aaron,

We used JM Linacoustic RC (almost identical acoustical properties to Theatershield)on the lower portion of our walls and covered it with fabric. The common procedure is to take it to the seated ear level of the listener (or therabouts) for absorption, then have the upper portion of the walls reflective; either with batting and fabric or nothing at all. We took ours to the top of our vertical wall (almost 5 feet)which is perfect for the second row seating, then kept our angled walls bare! It also aesthetically made it easier to do. We are still in process, so I don't have finished pics yet.

pam

 

[Aaron Gould]

I was just browsing my local A/V retailer's web site, and it turns out they carry acoustical panels manufactured by "Acoustic Innovations" (http://www.acousticinnovations.com). Is this a reputable company?

Just curious, what's the normal price range for these panels? I don't want to be ripped off if my A/V store is the only one in town with this type of product...

 

[gregstaten]

I'm wondering about your acoustic treatment -- is the top half thicker than the bottom half? This interests me since I cannot move my electrical outlets forward anymore as they are firmly attached flush with the drywall.

Thanks for the kind words, btw. The thickness of the bottom and top sections are identical. I don't have any electrical outlets in the room other than at the center base of the riser. Even if they are flush, though, you can easily add an extender electrical box to move them out from the wall.

I believe the total thickness of the treatment is 1". As Terry mentioned, Guilford of Maine makes a wide assortment of acoustically transparent cloth. You should be able to find anything you like.

-greg

 

[Aaron Gould]

Well, there have been a couple developments in my situation!

As it turns out, a good family friend actually makes fiberglass and fiberglass products himself -- right down to the chemically hardening spray! He created his own boat hull completely out of fiberglass, so rectangular panels shouldn't be a stretch.

So, at only the cost of materials, I would be able to make my own acoustic panels!

My question is about thickness of panels. Terry mentioned that an inch is sufficient to 500Hz. Since I don't want to make these *too* thick (room's only 11'-2"), would 2 inch thickness do significantly more dampening?

Also, should I ensure that there are minimal air pockets (bubbles, etc) in the fiberglass, or are air pockets beneficial?

 

[Terry Montlick]

Yup, 2 inches will absorb into lower frequencies. Of course, there's no sharp cut-off. Here are the absorption coefficients for Owens Corning 705 at the 6 standard octave bands. An absorption coefficient of 1.0 means perfect absorption - essentially no sound in this band gets reflected back.

Hz 125 250 500 1000 2000 4000

1" 0.02 0.27 0.63 0.85 0.93 0.95
2" 0.16 0.71 1.02 1.01 0.99 0.99

So 2" looks roughly like 1" shifted down an octave, for increased low frequency covering. The 2" numbers (especially the ones that go over 1!) are somewhat high. This has to do with the way the absorption is measured. The method Owens Corning apparently used is called the "Reverberant Room" method, which allows coefficients to exceed 1, and gives a little automatic bias toward thicker materials. So you any numbers over 0.9 you can consider about the same.

- Terry

 

[Aaron Gould]

Thanks again Terry.

Depending on cost of the material, and how much I have at the time of producing the panels, I'll aim for 2 inch thickness.

Since these panels don't appear to affect bass frequencies too much, I may consider placing a pair of tube traps in the corners by the front speakers. I won't have room in the back corners for traps, so only the fronts will have to do.

As well, my current plans call for shelving on either side of the TV. Since those shelves will be behind the front speakers and filled with books, A/V equipment and other random items, that will hopefully help diffuse the sound in the front of the room a bit.

 

[Larry Talbot]

It looks like you have found a solution, Aaron, but I thought I'd post a link to my thread dealing with the same issue...I ended up making panels and am very happy with them.


http://www.hometheaterforum.com/htfo...hreadid=146767

 

[Aaron Gould]

Thanks for the link Larry. The more reading the better!