Acoustical Treatments Master Thread - Page 3

Sound from the speaker radiates as a sphere. Thus all frequencies radiate in all directions. The speaker cabinet causes defraction but does not "cast a shadow". (One of the reasons the formula for determining acoustic energy decay (SPL) is 1/r^2.)

[If sound at all frequencies did not radiate spherically, you couldn't hear a speaker standing behind it. Although, I do claim I cannot hear what my son is saying when he's looking the other way. ]

I agree with Dennis.

Normally HTs are small, ranging from say 300 to 700 sq ft. The smaller the room, the more need for treatment behind the seats (rear wall and side walls). As to reflection points between the speakers and the seat, many will fall very close together so it's not like you have to treat a lot of spots. For a two row theater with the second row on risers, say 20 ft wide and 25 ft in depth, the side-wall soft treatment need not exceed about 24 sq ft per side.

The wall behind the front speakers needs to be dead. You want imaging. So by deadening the front wall you increase the point source characteristics of the front array. The problem here is that the screen often occupies a significant portion of the front wall and that sucker is reflective.

And yes, horse hair jute, in my many years of 2 channel experience, the best carpet pad for making a room sound good, is no longer commercially available. Too many "utes" were smoking it with bad side effects.

So treatment is a room 13ft by 20ft would need to be more aggressive on the side and rear walls than in a room 20X25ft.

I would say as a generalization, the closer you sit to the rear wall, the more it needs to be treated (with diffusion). Obvioiusly, if the rear is an an infinite distance behind you, there is no need to treat it. Likewise for the side walls. On the other hand, the bigger your room, the further away you will likely sit from the screen, say 15 or 16ft away at the closest instead of say 12 1/2 ft. The further away the more near reflections rather than just a first order you will need to treat, so there really is no clear generalization.

As Dennis says below (the power of an edit ), you really can't get a small room to sound (e.g., decay) like a large room. However, diffusion will help.

As Mark pointed out, this really not a good thing for generalizations. Having said that, as a room gets smaller the time delay between the initial sound and the reflected sound decreases and the relative dB SPL increases. This would suggest different treatments. Part of the difficulty in smaller rooms, is developing the capability to create the reverberent field. As the delay becomes shorter and the difference in dB SPL between direct and reflected sound decreases, you can easily have problems.

My room is very small compared to most here. My finished dimension will be approx. 9'3"W x 21'L with 18' in the length visible since I will have a false screen wall with an acoustically transparent screen.

I am going to cover the front wall as suggested with sound absorbing material, but given the width of my room, would I need to add more insulsheild to the side walls than the standard 4' (or ear level) height? Also, when it's mentioned that the insulsheild is placed below ear level, does this ear level refer to those on the riser(s) or just those in the first row?

Proudx,

> So treatment is a room 13ft by 20ft would need to be more aggressive on the side and rear walls than in a room 20X25ft. <<br />
You need to distinguish the type of treatment. There's low frequency absorption, mid and high frequency absorption, and mid and high frequency diffusion. Smaller rooms need more absorption. Diffusion should never be closer than ten feet from your ears, so that rules out diffusion entirely in very small rooms. In your 13x20 room you should be sitting forward of the half-way point anyway, so diffusion on the rear wall could be useful.

--Ethan

Mancubus:

Haven't seen your room...but, I'd not suggest you use more Insul-Shield. I'd suggest you start six inches higher from the floor and bring the top higher.

Ear level...whether or not it's in the nose bleed seats or orchestra pit.

I disagree with Ethan. The closer you sit toward a rear wall, the more you need diffusion.

Sorry to lower the tone of the conversation, but I'm a little confused about how this stuff is used. Is it a layer that would be placed over sheetrock (and then covered, for example, with GOM), or is it used in place of sheetrock? Or is it something else entirely?

Mark,

> I disagree with Ethan. The closer you sit toward a rear wall, the more you need diffusion. <<br />
On what do you base that? If anything, the closer you are to the rear wall the more you need bass traps because of the acoustic interference issues discussed here in great detail recently.

Dr. Peter D'Antonio of RPG wrote an excellent tutorial on diffusion, which you can download from his company's site. He's sort of the father of modern diffusion theory, and he is very clear that you should never place diffusion on any wall that's closer than ten feet from where you sit.

Here's a direct link to that PDF file:

http://www.rpginc.com/news/library/T...ffCritList.pdf

--Ethan

jschlege
I know how you feel. I stay semi-confused about acoustics all the time. I have some 4x8 Homasote that I bought from Home Depot. I'm going to use it on my front wall only. It's too expensive at nearly $18/sheet. I'll try to find something else for the sides.

This is a question for the experts: I'm placing mine over the sheetrock but shoud I place firring strips over the sheetrock first and leave a space or is it OK to screw it directly to the sheetrock. My screen and speakers are all 30" from the wall. Also, I see lots of HT sites that don't have fabric on the walls. I've also seen some in AV Interiors that really look like paint. I assume that it is not absolutely necessary to cover this stuff with fabric?
Jeff

My room is 14x20 with a drop ceiling at just under 8 feet. I plan on putting Linacoustic on the entire front wall (except where the screen is), from top to bottom on the first 2 feet on the side walls, and from ear-level down on the other three walls. I plan on painting the sheetrock on the upper half of the rest of the walls with some sort of treatment at the first reflection points of the front speakers. I also plan to use a thick carpet pad. Does this sound like effective treatments?

Jeff,

> I'm placing mine [homosote] over the sheetrock but shoud I place firring strips over the sheetrock first and leave a space or is it OK to screw it directly to the sheetrock. <<br />
Homosote and other thin absorbent materials are effective only at higher frequencies. But all rooms need absorption at all frequencies. You can improve the performance of homosote a little by spacing it away from the sheetrock. Much better is to space it six to twelve inches away, and pack the resulting air gap entirely with fiberglass. This way the fiberglass can do some serious absorbing, and the homosote gives an exposed surface that's easier to make look nice.

--Ethan

I guess I'm still a little confused. I was planning on using Insul-Shield. Is that installed on the studs like sheet rock, or over the sheet rock. I appreciate that you could probably set it off from the sheet rock and insulate in between, but that would give me insulation insul. shield over insulation over sheetrock over insulation, and I'm not sure I want to go there. If insul. shield can replace sheetrock, I would prefer just to do that (with the appropriate insulation behind.

Insulshield is placed over the sheetrock. Sheetrock is screwed to the studs.

Do some searches and you will find how all the acoustic treatments come together. The begining of this thread is a good start.

Thanks. The begining of the thread has very useful information on placement/type, but not installation. I just need very basic installation info. for the acoustic treatments and GOM.

All the comments have been very helpful. Although I have one question regarding corner treatment.
I am planning to put some corner bass traps in my HT. But at $500 each, It's not an option for me. Is there anything less expensive yet work as effectively? Thanks for your advice!

JS,

> The begining of the thread has very useful information on placement/type, but not installation. <<br />
See my Acoustics FAQ, tenth in the list on my Articles page:

www.ethanwiner.com/articles.html

It explains these issues in detail, including how to mount rigid fiberglass.

--Ethan

Ethan Winer...Please remove from your signature your link to your site and also please stop directing people to you for business. Please be so kind to re-read the rules of the site for this is not allowed on AVS Forum.

Thank you kindly.

google.com


Cut and paste (many links)!!

DIY will save significant $, and [done properly] give the same result.

Mark,

> I disagree with Ethan. The closer you sit toward a rear wall, the more you need diffusion. <<br />
Some further clarification on this:

Last night I met with Wes Lachot, a top notch recording studio designer, and asked him about this. Wes said that ten feet should not be considered a hard cutoff, and he's seen diffusion be useful even when it's only eight feet from your ears. But he did agree with Dr. D'Antonio's basic premise that diffusion is always better when the round trip distance from head to diffusing boundary is at least 20 milliseconds, and 30 ms or more is even better.

--Ethan

Having a need and being able to use are two different things.

Dr. D's rule of 3 times the lower limt wave length, is, to the most causual of observers, dependent on the lower cut off limit of the diffusor panel. I often use a 1 1/4 inch deep diffusor panel directly mounted onto the rear wall, all covered by Guilford fabric. Dr. D's panels are considerably deeper and correspondingly have a much lower cutoff, thus requiring that the listener be a further distance away from the wall to negate the adverse near field effects of a diffusor.

When a listener is close to a rear wall (the optimum design would indeed be different if there were 8 to 10 ft between the rear wall and the last row listener), the wall needs to be treated-not with heavy absorption and cetainly not with a bass trap (although the room may indeed need bass traping, try thr rear corners with a couple of bass traps (and I do not mean something labeled by some company as a bass trap but something tuned to pick up the room's specific bass resonance). Diffusion is what is indeed needed. But with a close wall position we want shallow depth diffusers or diffusers with a rather high lower limit. Here a diffuser could be such things as a bookshelf full of books, or some half round ASC tube traps with the reflective side out with a few inches of spave between each one (which would offer some absorption below 300 cycles), a rough sand paint wall etc. The point I am making is that you do not want a smooth hard reflective wall but you certainly do not want heavy absorption. You do not want a Dr. D difffuser because you would be sitting to close to it. Just some light diffusion.

Mark and Dennis,

I'm in the process of designing a home theater with dimensions 24'x15'x10'. I want to place two equipment racks on the rear wall along with a little kitchenette with sink, microwave, below-counter refrigerator with a few wall and floor-mounted cabinets.

With the exception of the rear wall, I plan on using the acoustic approach typical of Dennis' designs with fiberglass on the front wall and front sides, and soffit. The rest of the side walls would be treated with fiberglass from ear level to the floor. However, in the rear of the room I've been considering some sort of diffusion since if left untreated the cabinets would be very reflective. I was toying with the idea of fastening half-round bamboo to the cabinets to serve as diffusers. An alternate absorption approach would be to hang heavy retractable drapes in front of all those reflective surfaces.

Short of forgetting about the kitchenette in the first place, do you have any opinions or recommendations regarding these approaches? The rear row of seating will be between 6 and 8 feet from the rear wall.

Thanks.

Larry

Larry:

The bambo cabinet doors would provide diffusion. You could also use something like http://www.rpginc.com/products/topakustik/index.htm (reflector style) for your cabinet faces. With a handy table saw you could like build something similar.

Hi Dennis,

Thanks for the suggestion. If you were using this approach on one of your designs would you first partially complete the room's standard acoustic treatments, then measure the room's response, and finally select the appropriate Topakustik product to fine tune the reverberation time, and other acoustic parameters?

Although these RPG products are listed as diffusor systems, the literature mainly discusses the absorption properties of the perforated styles. Does the reflector style provide significant amounts of diffusion?

For my design the cabinet faces would be 4 to 6 feet from the rear seating positions. Do you share Mark's opinion that as the listener gets closer to the rear wall diffusion makes more sense? Would you mind discussing the pros and cons of rear wall diffusion versus absorption with regard to rear seating location?

Thanks very much.

Larry

Mark,

I still don't follow your logic. If the last row of seats is closer to the rear wall than you'd like, it seems to me the goal should be to make the rear wall seem acoustically farther back. This is exactly what happens when the wall is made absorbent. There's no difference acoustically between a wall that is infinitely absorbent and one that is infinitely far away. Now, you might say the goal is not to make it seem infinitely far away, but just somewhat farther back. Okay, but in that case...

> with a close wall position we want shallow depth diffusers or diffusers with a rather high lower limit. <<br />
When you install a rear wall diffuser that operates only at the highest frequencies, that leaves the wall reflective at all mid and low frequencies. So you still have the original problem of being too close to the rear wall at the majority of frequencies. So listeners in the rear still suffer from comb filtering and echos at many important frequencies. And at low frequencies the 1/4 wave cancellation problems are even worse near a reflective wall.

> although the room may indeed need bass traping, try thr rear corners with a couple of bass traps (and I do not mean something labeled by some company as a bass trap but something tuned to pick up the room's specific bass resonance) <<br />
Yes, most/all rooms need some amount of bass trapping, but I'm not sure what you're getting at. We already settled the issue of broadband versus targetted bass trapping two weeks ago, and broadband emerged the clear winner.

--Ethan

But we're missing some critical data...how high above the floor are the cabinets...you have below counter cabs and above counter cab?


I neither share it or reject it. There's plenty of other factors to be considered...characteristics of the room, distance of the wall from the source, height of the source vs ear height vs wall height, etc., etc.

Again, there are no pros and cons, this is entirely a room dependent matter that would be studied during the design phase of a project.

I cannot say whether your room requires more or less absorption (for RT60 purposes) nor more or less diffusion. However, the product(s) I referenced can provide either absorption/diffusion or diffusion only. I believe the spec sheets can be downloaded from that page.

Once again, I agree with Dennis. Every room is different and that is why, if one is going to do it right, you need the services of an expert acoustician, I am not one! I do know where to go to get one or more, though my favorite is Norman Varney (I am biased, he is an old friend, too).

No, we haven't solved anything, Ethan, Every HT room to be optimized requires some strong absorption. Very broadband. This is required on the front wall, the front corners, and on the side walls (to kill the reflections between the LCR speakers and the listeners). That is every listener should hear these three speakers only directly and not also bounced off both side walls. That is very very clear.

HOWEVER, and note well, the side wall absorption need not go deep into the bass region. The 3db down point of the front speakers should be sufficient for this absorption. Elsewhere along the sides we do not want much if any absorption above 1000 HZ or so. Below 1000 hz we indeed want broadband absorption. You can go as low as you want here but there are practical reasons as to how low you can go. There are limits as to how much absorption you have room for in wall or on wall.

To deal with very low bass absorption by side wall treatment is generally not praticable. Dedicated broad band bass absorbers (say from 300 cycles down (upper mid bass and below). We need to depressurize the bass waves so they do not inhibit the ability of the front speakers to work optimumly. High bass back pressure will really screw the midrange up. Fact. Most rooms will have a bass resonance peak, A flat broadband absorper will tame this but the peak will still be there because of the other bass frequencies attenuation. So measure where the peak is is, or you can easily calculate what it will be from the room dimensions, and then treat it if is severe. Nothing really wrong with broad band bass absorption but it is not the end all.

By no means do we want rhe rear wall to be highly absorptive. That would be a very big mistake. The room will sound overly dead, decay times will go to hell and a handbasket. We want the wall to sound like it is relatively far away but seill there, and not like the listener is in an open field or an anachoic chamber. Diffusion (and many diffusors will absorb as well) is what is called for.

And not that I waqt to give everything away, but on the rear wall my mild diffusion goes on the upper half of the wall and below that I generally treat with broadband diffusion (below 1000 HZ). No reason this broad band can't be as low as you want. The lower rear wall is generally hidden acoustically by the seats so you can make it as dead as you want, Hide you deep bass absorption there if you want. Just do not make the wall above halfway up absorptive. OK you can have some absorption but mostly diffusion

Jmsun,

See this thread,

http://www.avsforum.com/avs-vb/showt...hreadid=278701