Acoustical Treatments Master Thread - Page 5

I guess it's back to the mushrooms.

I will be putting velvet curtains on the front wall all around the screen. The generally accepted wisdom is to treat the front wall to absorb as much sound as possible ("dead end").

Is the addition of heavy velvet curtains along the front wall negate the need to put sound panels behind the curtain?

Also, since the velvet is not acoustically transparent what use would putting acoustically transparent ("expensive") fabric over the panels should I choose to treat the wall.

Hi Guys:

I read the entire thread and there was still one question that I think never got answered. And of course it applied to me, too . My room:

Plan View
View toward room front
View toward room back

It's larger than several discussed in the thread and my seating is not against the back wall.

It appears from this thread that I should:
1) Treat the screen wall top to bottom (linacoustic or equiv).
2) Treat some portion of the side walls.

I will have a drop tile ceiling in the area inside the soffit.
I will have a carpeted and padded floor.

My side walls due to construction and foundation issued will have a curtain wall with a shelf-cap instead of chair rail. That is, it's 2-walls thick at the bottom for lack of a picture). I have no need to put fabric on the upper section of the wall if not needed, it woudl be drywall. Is it necesary to use the upper treatment?

I have a lot of flexibility on the side curtain wall's height. Minimum 12" off floor, maximum - ceiling. I planned on about 4' all the way back. Maybe thats too much and I shoudl only do first reflection area?

For my elevated rear sofa, do I need to elevate the sidewall first reflection points, too?

I'm not sure but since my rear wall is relatively far away, should I treat it or not?

As for my soffits, how woudl one treat them? RIgid fiberglass panels covered with fabric on the floor facing part, or up the sides of the soffit's too?

With all this absorpion will the room be too dead?

Thanks,
Scott

FD,

> The generally accepted wisdom is to treat the front wall to absorb as much sound as possible ("dead end"). <<br />
Not necessarily. What matters most are 1) absorbing the first reflections which are at specific places on the side walls and ceiling, and 2) absorbing low frequencies to clean up the inevitable muddiness that exists in all untreated rooms.

> Is the addition of heavy velvet curtains along the front wall negate the need to put sound panels behind the curtain? <<br />
No, because curtains absorb only higher frequencies. You still need bass trapping.

> since the velvet is not acoustically transparent <<br />
It doesn't matter if a curtain absorbs a little extra on its own. Though this assumes you don't plan to put your speakers behind the curtain!

I don't know how deeply you want to get into the fun and exciting world of acoustic treatment. But if you want to learn more see the Acoustics FAQ, second in the list on my Articles page:

www.ethanwiner.com/articles.html

--Ethan

Curtains will do very little below very high frequencies. You still need to treat the front wall. The curtains do so little that it won't matter if they are in front of the treatments.

Actually, heavy curtains with a thick drape, covering 3.25 walls (floor to ceiling, rear wall, two side walls, some of front wall) can look very nice and have a huge effect on sound. Whether that is good or bad depends on the rest of the room. But you're right, even the best of them still absorb more highs than lows. But one can always put more absorbtion (wool and/or membrane) behind the curtains to extend into the low end and still look nice.


Thin curtains with no drape are worse than carpet

Scott,
The polybatting used on the upper half of rooms is usually just to provide a backing to keep the GOM or other fabric treatment even to the base. I don't believe Polyfill makes too much of a practical impact on diffusion/dispersion. Now whether you need treatments in this area (or around the soffits is a separate issue and depends on the RT60 and other features of your room. A couple well placed panels in the drywall areas once the room is built could be done to "tune" the room.

The first reflection points from your rear couch should be treated (theoretically) as that will improve the sound for those sitting there a well.

rear wall again varies. Most use dispersion/diffusion. I needed some increased deadening so I treated th bottom of my wall with soundselect. I do note some boomy base in the 2 back-corner seats and may some day try and find out what frequency and then apply a basstrap in those corners.

Dennis E. had addressed the soffit issue in the past, do a search in the builders area with soffit treatment and dennis's name and you might come up with it. Will try taking a look later if you can't find it.

Good luck,
Adam

Adam:

Thanks, no batting on the upper area is good for me.

I will look into varying the height of my first reflection point treatment to accomodate the 2nd sofa.

I'll plan to leave rear wall alone for now, or roll the dice and cover the same as side walls.

I couldn't find anything helpful about the soffit treatments. If you think you know where to look I'd love to.

With my walls at almost 50% coverage, presmuably with soffits treated, my ceiling with acoustic tile, my floor with carpet, and my front wall at 100% treatment, should I expect the room to be too dead? bpape indicated it woudl go a long ways toward deading, but being new to this I don't know how much is too much.

Thanks,
Scott

OK, I've done quite a bit of reading and feel like I know where in the room to apply the acoustic materials. However, I have a very basic question about the materials themselves. Are some of these more like rigid boards and others more like blankets? If so, it looks like there should be two different categories, with one for boards and the other for blankets, as some people may prefer one over the other. Is the following break-down accurate?

Board-style:
Owens Corning SelectSound Black Acoustic Board
Johns Manville Insul-Shield
Certainteed Certapro Acoustaboard Black
Knauf Duct board EI-475


Blanket-style:
Owens Corning Fiberglas 703 Series duct insulation
Johns Manville Linacoustic Permacote rolls
Knauf Duct liner EM
Owens Corning SelectSound Black Acoustic Blanket

Ethan,

I over generalized in my original post. I've been reading alot of the posts here about acoustics and about LEDE, first, second reflections, etc. I've seen quite a few theaters from posters on this site that put either insul-shield or OC 703 panels or similar material along the front wall (Dead end) and cover that with acoustically transparent fabric. I was planning on starting with the front wall and then move to the side and rear walls as time and money allow. I was intending for the panels along the front wall behind the heavy velvet curtain to absorb high frequencies only. I intend to build bass traps like Jon Risch's to cover the low end.

Regarding putting panels of OC 703 or similar material, What I REALLY want to find out is if I still need to put panels behind the heavy velvet curtains. It looks like I do not need to based on the numbers that Basement Bob posted regarding cotton curtains. I could and probably am wrong.

If I do put panels behind the curtains, would I still need to go through the expense of putting acoutically transparent fabric over the panels? I would think that since the curtains are NOT acoustically transparent that it would be a moot point. Again, I could be wrong.

I willingly bow the massive collective wisdom of the people who frequent this great site for help. Thanks again for the replies!

fdlozano:


Those examples were the best and worst absobtion figures for curtains that I found. And even the best, the heavy cotton ones that are way out from the wall and huge drape, still absorb a lot more in highs than in lows.

OK, let's talk about what's going on here.

We've got modes, flutter, and SBIR (first reflection).

Curtains will do wonders for flutter.
Curtains will do nothing perceptable for modes.
Curtains will do a bit for side reflections (mid and high), but nothing for SBIR reflections off the front wall (which tend to be louder lower).

DE designs seem to have about 1" of linacoustic or InsulSheildBlack for the front wall. ( 0.09 0.29 0.67 0.89 1.03 0.99 ).
If you have lots of couches and thin walls then some of your modal issues may already be handled.

In the stereo control room world there are two designs for front speakers - baffles and thick absorbtion on the front wall on either side of the speakers. Either handle SBIR.

LEDE may not be appropriate for small rooms, unless you're doing a dual purpose room which is also for playing musical instruments.

OK, you want a bit of a dead room from all sides for 7.1
But you don't want to absorb everything because it has three bad effects:
1) it's expensive
2) it requires you to turn up your amplifier possibly to the point of distortion
3) it removes the reflected sound which is needed for spaciousness

So what you're really after is
a) absorb first reflections and SBIR for the best imaging
b) a flat RT60 (even absorbtion at all frequencies)
c) Any reflection path less than 20ms you might want to absorb or diffuse so that it hits the listener area down about 30db or more relative to the direct sound.


You got it. Curtains over absorbers are fine.

Yes. You should still put other absorbtion behind the curtains. When you cover the panels, just make sure it is a somewhat porous, non-shiny finish material. Muslin does wonderfully. The idea of the panels is to absorb. Don't cover them with a cloth that reflects and you'll do fine.

I agree with bpape and based on discussions with Dennis, you STILL will want to treat the front wall. You will also want to treat the first reflection points on the side walls, but it sounds like you plan on doing that anyways.

Ethan, I'm curious as to why you disagree with Dennis and others about treating the front wall. I've seen you state this in another thread---most recently above where you said "Not necessarily." in reference to the generally accepted practice of doing this in a Home Theater enviroment. It appears you disagree with this statement by Dennis from this thread:

"In multi-channel, the entire wall behind the front speakers is treated. You want none of the back reflections to overlay the surround field or the bring the reverberent field forward (your reverberent field and surround field is created by the multi-channel processor or mix, not so much the room as is mandatory for 2-channel). Depending on speaker placement, this treatment is brought forward along the side walls. Wall treatments are floor to slightly above ear level (where exactly is also a function of front speaker heights). While one could argue the sound at their feet is of no concern, often that square footage of treatment is required to bring the room's RT60 down to the lower levels required for multi-channel playback."

Actually after re-reading that thread....I see YOU (Ethan) asked Dennis the front wall treatment question here:

"Dennis,

> recognize you are *not* interested in just the first reflections...it's early reflections which would include reflections from the wall behind the speaker (unless you're treating that surface). <<br />
That's a good point, and brings up a question you can probably answer for me about the importance of absorption on the front wall.

There's no question that loudspeakers radiate low frequencies in all directions. So those will bounce off the front wall and collide with direct sound from the speakers. But since mid and high frequencies are radiated from the front of the speaker, why is mid/high frequency absorption so often used on the front wall behind the speakers?

--Ethan"

And Dennis responded with:

"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. ]"

So again....my question Ethan, Why do you not agree with this?

-Jason

I believe what I said in the other thread was that with carpet, acoustic tile ceilings and 50% wall coverage, the room would likely be too dead in the upper frequencies and still too live at lower ones.

Thank you for the clarification, that helps me understand what the problem might be anyway. If I have to stay with carpet and ceiling, what woudl be a course of action then? Wall treatment only at first reflection points? or just less overall treatment at some percentage of coverage? Then bass traps for lows? Would a drywall ceiling cure or help the overdeadened highs? Or what?

Thanks,
Scott

From the perspective of early reflections, it is generally not necessary to treat the front wall. A commonly cited figure for the sound level at which early reflections cause imaging shift problems is 10 dB below the level of the direct sound. This is the recommendation chosen by a few professional organizations.

Looking at the directivity patterns of typical baffled loudspeakers, the rear sound radiation does not reach -10 dB at medium to high frequencies. Non-directional low frequencies are too broad to contribute early reflection "spikes" to the impulse response. So the possibility of a -10 dB early reflection from the front wall of a home theater is pretty low.*

If you are acoustically treating a home theater room, the front wall is a readily available surface which, if treated, will help lower reverberation time. Also, the same research which showed that an approximately -10 dB reflection is necessary to shift the image away from a speaker also showed that lower levels can be perceived as changing the sound.

Any reflections from the front wall, while not usually causing specific acoustical harm, can do no good. It is not a hard and fast rule to treat the front wall of a home theater. It is generally just a good idea -- a no-brainer if your theater design allows it.

Regards,
Terry

* If you are using an "acoustically transparent" microperforated projection screen in front of speakers, this can bounce significant high frequency energy toward the front wall, which will reverberate between screen and front wall if not absorbed. In this case, front wall treatment is mandatory, IMHO.

SBIR is something different than "early reflections". None-the-less, have speakers (various from speaker to speaker) near a boundary can have unfortunate side effects unless that boundary (or the speaker) has been modified to adjust (or eq is used). It is a great place for absorption to achieve RT60 targets as Terry noted.

Whether or not the reflected sound from the front wall surfaces will be perceived as an echo or 'fused' to the direct sound will depend again, on speaker distance from the boundary. HAAS effects not withstanding, such out of phase early reflections can have a negative impact on intelligibility.

Flutter echo is seldom a problem in a home theater as is isolated to only a few seating locations should it occur. None-the-less, front wall absorption will reduce the opportunity for flutter echo to become an issue.

Terry,

Thanks for your take on the front wall treatment. I guess in my particular case (having bi-polar mains) where the front wall has speakers directed at and bouncing off of it, it becomes even more important to treat the front wall. Regardless of speaker design, you basically said the same thing Dennis said and what I and bpape stated above.....treat the front wall! It still doesn't answer my question as to why Ethan disagrees with this. I can guess that he might think that treating that large of a surface will deaden a room too much and cause even muddier (if that's even a word) bass...but that's what I am trying to find out.

It's helpful (at least for me) to seperate room acoustic treatments in two categories: high and mid treatments (i.e. 703 and the like) and bass treatments/traps. The front wall treatment I am curious about (with regards to Ethan) is the high and mid treatment of the front wall.

-Jason

Drywall ceiling will significantly reduce the amount of high frequency absorbtion. Look at the square footage of the ceiling in relation to the square footage of all the room's surfaces.

If you stay with tiled ceiling, you might do more scattered absorbtion and hit the reflection points. It is a good thing to have absorbtion throughout the room for more effective decay control. Do some calculations on what you think your room will be like. There is a spreadsheet floating around from Eric Desart that has a lot of good stuff in it regarding decay time calculations. Fill in the blanks and see where you fall at given frequencies. You'll see quickly where you still need help.

Threads Merged

Exactly. High frequency, off-axis response is significantly attenuated. Tweeters, typically above 2.5k-3k Hz have dispersion patterns like flashlights. The point where midrange off-axis response is only 6dB down can be as low as 200 Hz.

Don't you mean dipole mains? Dipole mains (open baffle speakers), by design, rely on the contribution of the backwave. That is why proper placement is critical.

Well in my case I actually meant bi-polar (I own Def Tech BP2000's). But in either a bi or di-polar situation my point is still vaild (I think the only main difference between bi and di-polars is the drivers opperate out of phase in a di-polar setup and in phase in a bi-polar setup---but I might be wrong). The point I was making is now you definitely have early reflection points off the front wall and it's still important to treat it (the front wall).

-Jason

But if the speaker was designed for bipolar operation - are you not destroying the speaker designers intended sound by covering up the front wall?

Why use the bipoles then - best go for monopole! In other words - if reflections are bad - then why choose a speaker that is designed for reflections?

Ah, the Def Techs. Some of the top-end Wilsons do this with a tweeter on the back. I might be wrong, but I think the point of this alignment is to improve imaging and "space". I would agree with Kras, they might not be the best for an HT application.

From a acoustical treatment (in regards to the front wall) standpoint, I already asked this question and was provided the following answer by Dennis:

"Bipolar speakers would not be handled significantly different than dipolar other than perhaps placement of the speakers." (see page 6 of this thread)

I realize some people THINK these speakers are not ideal for HT.....I happen to THINK they are. That's why there are so many manufacturers of speakers....so people can pick and choose what they like

-Jason

The point is that that the rear facing tweeter of a bipolar (or dipolar) speaker is useless if the front wall is absorbing those treble frequencies. It essentially becomes a monopole.

If you like the bipolar (or dipolar) sound - then you should NOT treat early reflections. You are wasting money on an unused tweeter if you do treat.

With the Def Tech, you're not just wasting the tweeter, you're pretty much wasting a whole MTM.

Kras...I am well aware of your version of "the point".... however, you are wrong. Treating the front wall with 703 or similar WILL NOT absorb 100% of the sound from the rear facing tweeters and drivers, and warrant them "useless." It will help tame them, which is what I want. I have yet to hear a speaker that sounds as good as the Def's I have. If achieving the sound I like is "wasting my money" oh well, I can live with that. BTW, it's NOT the sound of bi-polars or di-polars that I like....it's the sound of Def Techs.

It's the Holidays....you need to stop being so Anti-Def Tech and live a little!

-Jason

Oh well....I guess I will have to rip them out (the rear facing MTM's) and sell them on Ebay to retrieve some of my wasted money