Acoustical Treatments Master Thread - Page 112

Well thank you Terry. Ask and yea shall receive a prompt answer.
Well, I have read many times that 2 inch is best for first reflection points because it takes care of the highs and mid range frequencies better than 1 inch.
I was not expecting anyone to tell me that 1" would be sufficient. So would my knauf be sufficient to use as a first reflection point absorber for my center channel?
Would I get better performance if I doubled it?

Thank!!!

Not for specific control of early reflections. It is hard to know whether your room would benefit from the extended lower frequency absorption of 2" material without acoustical measurements and analysis.

Regards,
Terry

No, it does not show through at all. 100% opaque.

Terry,

Can I get a bit of clarification, please?

I noticed that earlier in this thread you suggested, when using this method, you prefer 2" of semi-rigid fiberglass on the front wall as opposed to 1". In the post above, I believe you have also said this many times before, you tell us that 1" inch is sufficient for early reflection points.

Can you help me better understand why the front wall needs to be "deader" than the first reflection points along the side walls? I know that this helps imaging but I'm not quite understanding how that happens. How much improvement will the extra inch of semi-rigid fiberglass offer? I have some extra and wonder if I should go ahead put it up since I'm still in the building phase.

I have already built and installed the "chunk style" or "wedge" bass traps (24x17x17) in the front-side corners and have 1" Linacoustic covering the entire front wall. The side walls (at the moment) have 1" Linacoustic from the floor to 49". The rear wall is bare. The room is 13x21.2x7.5. The floor is not carpeted yet but that will go down over the heart pine as soon as the A/C is complete next week.

My room images very well, as it is now, and I am certain the sound will change quite a bit once the carpet is down. I suspect that I may need to reduce the amount of linacoustic on the side walls. Any suggestions you might have are greatly appreciated.

Thanks,
Dan

Hi again Jeff,

Specifically for first reflections, you need only 1" of fiberglass or equivalent absorber. But with only 5 feet to your back wall, you have other problems.

You are likely to have some low frequency notches starting pretty far down. Use as much absorption depth, and/or space the absorber as far from the wall as humanly possible, or as WAF will allow!

Regards,
Terry

Thanks for the response Terry.

With the 5' issue...would I be better off violating the 38% guideline and moving the couch further up? I would also consider building free standing frames for the panels and putting them just behind the listener, but it may get in the way of the A/V rack.

Here's a pic of my layout roughly.
http://picasaweb.google.com/jrames/T...82471775025394

The speaker layout is approximate, and is going to change as I test / add absorbers. I don't have all of the relative distances in there, but the listeners ears are just about 5' from that rear wall as I stated. The main theater area is 18x21 with 81" ceilings (yes, its like a hobbit cave). Most of the remaining area is enclosed with drywall, except for that hallway just to the bottom right of the theater area and the nook leading into the stairway.

Jeff

Hi Jeff,

You realize that acoustical consulting is my sole source of income (I don't sell any treatment products), right?

I am more than happy to provide general treatment guidlines, tips, etc. But at some point, one gets into more detailed design, everything interacts with everything else, and one cannot isolate just one piece of a design and provide a "sound" recommendation. For that I need much more information and would have to devote significant time resources to analyze it. It is like an MD giving diagnosis and treatment over the web. This is not a good idea, and to some degree, constitutes malpractice because a detailed medical exam has not been performed.

Feel free to take the advice of others. But it is really hard for me to weigh in any further as a profession without quite possibly being off base, and compromising my standards. Hope you understand.

Regards,
Terry

Terry,

One inch absorption falls off dramatically under 1k compared to 2". Shouldn't first reflection point absorbers go at least to 500Hz? And don't most spaces used for home theaters benefit, generally, from the additional absorption down to ~250Hz of the 2"?

- Jeff

No problem. I was actually posting the layout and specifics for everyone to comment on. The only reason I asked for some specific clarification was because you'd given me some specific advice. Point taken...

I have found that in practice, 1" thick rigid or semi-rigid fiberglass is completely adequate to sufficiently attenuate early reflections as measured by an energy energy time curve.

Regards,
Terry

...also, some current research suggests that diffusion can be appropriate for early reflections. This decision would hinge upon the quality of the off axis response of the specific speaker being used.

Terry, Bryan, Ethan,

Need your input on the post below. I have a dywaller coming soon, and I need to decide if I should drywall, or treat the walls immediatly adjacent to my screen.

My screen wall will be treated with 6" of insulation (possibly with 2" of OC if you reccomend) and covered with 1" linacoustic, and GOM.

The vertical front corners will include floor to ceiling Basstraps, 17x17x24.
I will also be making panel(s) to cover my first reflection points about 7' away from the screen.

Not sure with my semi-open floorplan >> if there is much advantage to covering the first 3~5 feet of sidewall adjacent to my screen...?

Thanks In Advance,
Craig

Yes, I would build it out as much as possible. Symmetry at the front of the room can make a big difference in the evenness of sound from your front speakers.

Regards,
Terry

I agree with Terry that symmetry is important, especially in the front part of the room. Also, the side walls reflection points probably extend further back in the room, so be sure to treat back far enough. Likewise, the ceiling reflection points. The rear wall can be a source of "early" reflections if it's closer than 10 feet from any seats. The rear wall is also a prime source of peaks and nulls at low frequencies.

--Ethan

I don't think "or" is the right question. Drywall and then treat.

Sorry I missed your question earlier, Dan.

Basically, early reflection image problems happen at medium to high frequency. At these frequencies, your speakers are mostly radiating forward, not omnidirectionally. So early reflections travelling backwards from speakers to the front wall are not an issue. They certainly can be for the side walls, ceiling, and floor, depending on the directivity and positions of your front speakers.

The thicker absorption at the front wall is to help deal with low frequency reflections, which are much more omnidirectional. In fact, at subwoofer frequencies emanating from a typical subwoofer-sized box, the sound radiation is perfectly omnidirectional.

Now, 2" isn't going to get you much into the subwoofer range at all. But it is a good practical compromise for most people. It will help a lot with potentially interfering reflections in the 3 figures of hertz, which will be fairly omnidirectional.

Regards,
Terry

Terry,

Thanks for the answers. That helps clear it up somewhat.

Given the limited information I have already provided, do you feel my room would benefit from the extra inch of linacoustic on the front wall or are the "wedge" bass traps and 1" sufficient? If the extra inch will make a difference, what difference(s) should I expect and/or notice?

Thanks,
Dan

I'd recommend using an extra inch on the front wall, if you have the fiberglass. This will not affect imaging, but can improve the response (reduce coloration) in above-subwoofer frequency bass. I cannot tell you what you will personally experience or notice. But I would expect there to be some measurable improvement in the steady-state frequency response.

Regards,
Terry

Terry,

Thanks again! I have plenty of spare linacoustic so I'll put an another inch up tomorrow while the A/C guys are installing the ductwork.

Thanks,
Dan

Re 1" or 2":

from: EBU Tech. 3276 - 2nd edition

Thanks for bringing up this reference, Bob. At 1 kHz, 1-inch of rigid or semi-rigid fiberglass will absorb about 90+% of the energy incident upon it. This corresponds to a drop in reflected energy of 10 dB or more. Since the position of the early reflection surfaces are off-axis, they will get somewhat less than the full on-axis response, and the reflection will be even weaker. Hence 1-inch fiberglass is more than enough to sufficiently reduce early reflections.

Or just try it. It works.

Regards,
Terry

For a room with all wall, floor and ceiling surfaces of 8"-20" of concrete, how does 2" front insulation (AT screen), 4" ceiling, 6" rear, 2" sides up to 48" and two 24"x24"x34" super chunks in the front corners sound? The local supplier has Rockwool RHT 40:

RHT 40 2" (51mm) 3.5 pcf (56 kg/m3) 0.26 0.68 1.14 1.13 1.06 1.07 1.00
RHT 40 4" (100mm) 3.5 pcf (56 kg/m3) 1.07 1.01 1.07 1.06 1.07 1.16 1.05

To get 4" and 6" I'll be layering 2" 4'x8' sheets.

The walls have eight 20" wide columns made of 1" MDF (about 20%-30% of the side and back wall surface area is MDF with no treatment) and 1' wide soffit around the room's ceiling, made of MDF with 7" of crown made of the ultralight decorative molding (so really only 5" of flat face on the soffit is exposed). Between the columns will be thin fabric (to be picked) to let most sound through to get some reflections above ear level.

Because of water problems, all insulation and columns are held 1" off the wall to allow water to flow behind it to drains and there is no framing or drywall except near the entry into the room. Carpet is outdoor carpet/commercial like that can get wet and not very thick.

I've bought a feedback pro, test microphone, mixer, and all the other equipment to do testing but before I buy all this insulation I wanted to make sure I was fairly close on my choices.

I will be getting some paper to adhere to possibly the back wall or ceiling insulation to reflect highs. The main reason for all the insulation is try to kill the massive amounts of bass that the cement walls will reflect. That's why I'm looking at 2" min on walls, so I can get something with over a 1.x noise reduction in the bass range and evenly kill everything rather than taking too much highs but not bass.

For audio equipment it will likely be the Denon AVR-4308CI with 140Watts per channel in a 7.1 configuration and RBH or Energy speakers. Two subs, yet to be picked.


(Column will be covered in Dazian celtic cloth so they look full height and surround speakers will be placed in some columns)

I just got a price quote for the insulation for my room and it was well over a grand... :O I can get linacoustic RC for nearly a forth of the price and other threads say linacoustic is fairly close to 703 even though linacoustic is slightly less dense? How about a rear wall with three 2" linacoustic layers to get the 6"?, ceiling done with linacoustic (or atleast one of two layers with it) and everything but the bass traps in 2" linacoustic, also maybe do 17x17x24 rather than 24x24x34 bass traps?

Terry,

I placed the extra inch of linacoustic on the front wall this morning and had a chance to check out the sound briefly. The A/C guys took an hour for lunch so I didn't have a lot of time. It definitely improved the quality of the mids and high bass. The room sounds "deader" overall. I guess I was getting a bit of resonance from the screen or something but whatever it was, that is gone. Is this the "coloration" you mentioned?

Anyway, THANKS!!!!! The room sounds better.

as in, the greater off axis speaker response, the more applicable diffusion types of treatment might be? or would it be the converse, i.e., absorption more often applicable?

I believe Dennis is referring to Toole's recent work. If so, it's as in the flatter the off-axis response, the more diffusion may be better than absorption. Or no treatment at all. This applies only to lateral reflections.

Not exactly the flatter the off axis response ...

You'd expect response to roll off as you go off axis...good off axis response would mean the shape of the response curves would be the same except for the roll off.

Let me put my question another way, in hopes of eliciting a response that even my feeble mind can grasp. ...

To put it more simply, the better your speakers are at providing good sound outside of the "sweet spot" (or at least, similar sound to what you hear in the sweet spot), the more applicable the diffusion or 'no treatment at all' tactics might be for those particular speakers?

For example, I've been considering the purchase of some Ascend Sierra speakers, which have a design priority for just that, to sound just as good or very nearly as good from more than just the primary seating location, as opposed to some other speaker designs which are designed to sound great in the "sweet spot", but fall down a bit more when the listener is positioned in other secondary seating locations.

I think the off axis we're talking about with regards to treating the left and right front first reflection points is much further off axis than a few seats left and right of a sweet spot. Beyond that, I don't think it's knowable in advance of installing the Ascends in your theater whether they will sound better with absorbers, diffusors or nothing at those points.

Just my feeble $.02.

IMO, very very few speakers have the kind of smooth, identical off-axis response to qualify. When you consider how many seats we're talking about and the severe off axis waves that are reflected to seating, IMO, absorbtion is still appropriate 99% of the time.

As for the front wall, while there are no mid and high frequency issues from the mains, thicker absorption can absolutely be used to tame SBIR issues - or - NOT used to deliberately allow SBIR to be USEFUL in compensating for anomolies caused by other issues that may not be able to be dealt with any other way.

This is where what Terry was saying earlier comes into play. It's great to get general recommendations but to really get into the details takes a lot of information and a lot of work doing an analysis.

Bryan