Acoustical Treatments Master Thread - Page 72

Glenn,

> I was wondering how to effectively handle and work with this opening and the opposing window? <<br />
From an acoustics standpoint you can probably just ignore the window and the opening. This assumes neither is at a reflection point, which may or may not be the case.

> Should I install some wooden blinds over the opening into the kitchen? Would this help contain some of the High/ Mid frequency sound and help in room acoustics? <<br />
Are you asking about sound isolation between rooms, or sound quality inside the room?

--Ethan

Ethan,

Sound isolation between the two rooms is not an issue at all, I am speaking strictly in an acoustical sense.

I was curious as to any possible acoustical anomolies that might be created such as additional ringing, slap echo or a general break down in the acoustic bubble and or envelopment issues particularly on the left side of the room where the opening is into the kitchen.

My thinking was that maybe some wooden blinds installed over the opening might help contain and or allow some of the sound to bounce around the room a little better. If that strategy would help and in the interest of balance/symmetry I thought that maybe I should also install similar blinds over the cellular blackout shade treated window which is directly across the room on the opposing right side wall.

Remember, the 3 ft. X 5 ft. opening is on the left side of the room and the 3 ft. X 5 ft. window is directly opposite the opening on the right side wall like mirrors of each other... I hope that makes sense.

The total room length is 21 feet and the opening and window are about centered on the side walls. My 3 abreast theater seats are just within the back third of the room and when sitting upright my head is about 6 to 6.5 feet from the back wall.

A cursory examination appears to place the opening and window just behind the first side wall reflection point and I might be able to adjust that somewhat via seating and speaker placement changes.

I am probably reading to much into this, and I know of course that I should set everything up and listen and evaluate the sound before passing any real judgements! I am sure however that I will be installing room treatments in the form of first reflection/ broadband absorption and bass absorption as I do have some slap echo that I can hear and the room will definitely require some taming.


...Glenn

Glenn,

> My thinking was that maybe some wooden blinds installed over the opening might help contain and or allow some of the sound to bounce around the room a little better. <<br />
With acoustics, what matters is the big stuff. If you have a window and you cover it with wood blinds, that won't have much affect whether the blinds are open or closed. Maybe if the slats were really wide and you opened them partway. But mostly, a large reflecting surface is a large reflecting surface.

> A cursory examination appears to place the opening and window just behind the first side wall reflection point and I might be able to adjust that somewhat via seating and speaker placement changes. <<br />
There you go.

--Ethan

For an HT/MC Music Room, would it be a good idea to cover the entire front wall with 4" 703? Would this make a good bass trap? Or, would it be better to place the traps in the corner and use a thinner material, (1" or 2") on the rest of the wall? Which would provide more bass trapping?

Thanks.

Craig

The latter. Put the thicker stuff in corners and at room boundaries (straddling corners and places where floor/ceiling and walls meet), with thinner 2" panels at specific points arond the room. Don't cover every wall.

Agreed mostly.

How thick you go on certain parts of the front wall will depend on what you're needing to do in terms of SBIR control. For pure reflection control, 1-2" is fine.

Bryan

Should acoustical measurements be taken before the 2" - 4" decision is made? The front wall in question here is behind a false wall, so no aesthetics are harmed by doing whatever there. It was my recommendation to use 4" instead of 2" thinking that it may reduce the need for chunky-style traps in the listening/viewing are by reaching lower in frequencies absorbed. Is my thinking wrong? If so, why?

pepar:


Please define 'low', in hz range, that you mean. (To me, when I write 'low', what I mean is the typical-HT-room-volume's modal problems range, which would be in the 15hz to 115hz.)

Then define the % increase in normal incidence absorbtion at 125hz (and lower) to 1khz, and % increase in diffuse absorption at 125hz to 1khz, of going from 2" to 4" mounted on the wall.

Then cost justify it, including mounting technique differences. This is all from a 'how good is it as an absorber' point of view, not from a is it needed for some-specific-room point of view.

(also remember the ITU/EBU rule "Early reflections are defined as reflections from boundary surfaces or other surfaces in the room which reach the listening area within the first 15 ms after the arrival of the direct sound. The levels of these reflections should be at least 10 dB below the level of the direct sound for all frequencies in the range 1 kHz to 8 kHz.")

His room sounded tubby and to me that's 125Hz to 250Hz, maybe 300Hz. Note that I said "lower in frequency" not "low frequency."


Incidence absorption? Is that different from plain ol' absorption? Is is a straight calculation, e.g. 4" 703, plain, has ~5x the absorption (.84 vs .17) at 125Hz of 2" 703, plain?


I know what I paid for plain 2" 703, but I've never priced 4". If it was much more, then couldn't one simply use two layers of 2"? On your last sentence - huh?


Wrong it may have been, but my thinking was that he'd get more absorption in the range that sounded, to me, like it was a problem.

pepar:

I'm just muttering in general terms -- not about any specific room.

'normal incidence' is an angle, in this case straight into the face. There aren't many published results about these. But there's a lot of theory, and calculation tools. ChrisW's spreadsheet for example.

'diffuse absorption' is sound coming from everywhere, and is the style for all the results at this page: http://www.bobgolds.com/AbsorptionCoefficients.htm


You're not nessessarily wrong -- you may be 100% right. I was just hoping you'd make me some graphs of the expected difference between the two absorbers you're considering to see what you think the actual difference is between them.
I figure moving from 2" to 4" of 703 gives a
a) diffuse: 400% increase in diffuse absorption at 125hz, and a 20% increase in diffuse absorption at 250hz, and no increase above that frequency. (see bottom of graph)
b) normal: well, I'll do the graphs I was hoping for




Yep, I see a reasonable change in the 125hz to 300hz range moving from 2" to 4" in the normal and grazing and diffuse coefficients. (16000rayls/m may not represent 703, and neither may a computer simulation)

So, how many square feet of this do you think this specific room will need to make a perceptable difference? Where do you want to put it, and what side effects will it have?

Roger that. craig john and I have a specific room in mind.


Roger that as well. (That's you, right, you're da Bob in bobgolds?)

Thanks, and that's exactly what I had in mind. The "where" is behind his false wall. Can you point me to information and/or calculations for determining in advance how much it would take to make a perceptible difference and what "side effect" it would have? (I suppose I should be looking for negative side effects?)

Perceptable
a) changing any reflection at some frequency band by 6db
b) halving RT60 at some frequency band
c) increasing the absorption in some frequency band by 30%

There's lots of side effects. One, from an RT60 perspective would be that if the room sounds good now with little absorption (no draps, no cloth couch, no carpet), and you cover an otherwise bare front wall with absorption that does a much larger job of absorbing 500hz to 8khz, than it does from 15 hz to 300 hz, you might create more problems than cures.

Right now, it does not sound good. There's carpet and a large comfy sectional, but otherwise it is plaster walls and ceiling.

Here is a pic of my J-M Linacoustic-lined false wall cavity - http://www.peparsplace.com/assets/images/HT_009.jpg. My room sounds good, but I need to add traps to control the 125Hz to 250Hz range. I would have lost no more mids and highs by using 4" in my false wall cavity instead of the 2" I did use and I'd have less of a problem - or maybe NO problem - in that range. That is what's at the root of my thinking.

"Should acoustical measurements be taken before the 2" - 4" decision is made? The front wall in question here is behind a false wall, so no aesthetics are harmed by doing whatever there. It was my recommendation to use 4" instead of 2" thinking that it may reduce the need for chunky-style traps in the listening/viewing are by reaching lower in frequencies absorbed. Is my thinking wrong? If so, why?"

Yes. IMO we set up speakers and tweak location to get the smoothest response. Then based on what's left and what's being reinforced by SBIR, we deal with it accordingly. This is more of an iterative way to go about it but it works well in real rooms where there isn't always enough space to do what we'd like.

If we could get by with just 4" on the front wall and that was all that was needed, you'd be correct - assuming that you're not making the SBIR issues worse. You may in fact not WANT to deal with the really lower frequencies as they're already relatively flat in which case we'd go thinner. Also, the corners provide better efficiency over a number of different axial, tangential, and oblique modes where the front wall is going to deal with less of those and less efficiently.

Bryan

pepar:

One should define 'sounds good' a bit better.

For example
I had my room like this

and when I played the end credits to We Were Soldiers DVD, with the men's choir, I was in acoustic heaven. Room support was fantastic. I litterally stopped what I was doing and couldn't stop listening. I played that bit over and over, turned the volume up, showed that bit to all of my friends and relatives. It was fantastic. Easilly the best sound I've ever heard, anywhere.

But that layout wasn't what I wanted.
And the imaging was off and speach inteligibility seemed low.

I tried different speaker positions and treatment layouts.







and although the last one above sounded really great,
what I ended up building was a little different:



That's 4" thick rockwool, with a 4" airgap behind it.
And I can tell you without a shadow of a doubt, it's not that heavenly room support I had before. It's dull and lifeless.

(for more about this and some more detailed acoustic observations regarding each picture please see
http://www.bobgolds.com/LivingRoomPl...ction/home.htm
at 2006/12/01 about 60% of the way down)

Speach inteligibility is pretty good though.

I had some imaging problems, pod race from Star Wars I couldn't tell left from right. Room geometry takes care of left speaker off the left wall, and right speaker off the right wall. But the problem turned ot to be left speaker off right wall, which a right side wall absorber took care of and solved the imaging. Pod race is better with good imaging.

I've been meaning to do some ETF tests and tweaks, but I haven't gotten around to it.

That room will never be optimal acoustically due to other constraints -- and I haven't tried more or less surface area, nor more or less absorbtive material, in that front wall yet. Although I built it so I could fiddle.


A year ago I had the same speakers in the basement (five concrete surfaces walls and floor) and even with two subwoofers never had any bass. Upstairs in the living room (above pictures) I've got bass to burn. I'm hearing things I've never heard before and really enjoying them.


My point is 'sounds good' can actually be a bad thing.

With my lifeless room now, I've tried a bunch of the DSP modes on my receiver (DSP pretend to be a church, DSP pretend to be a concert hall, DSP pretend to be a rock studio, ...) and I've found that while I can get some of that heavenly sound with that particular spot in We Were Soldiers (although not nearly the stop dead in your tracks as if you've fallen under a spell like the first time you fall in love and think that all's right in the world that I had with the two other layouts), if I leave that DSP setting on there's some DVDs that I can't understand speach with -- but I turn off the DSP modes (output 'STRAIGHT') and it clears up immediately.

OK, that gets my attention; thicker on a wall isn't as effective as corners. So that is a point in favor of saving money on 2" on the front wall and spending it on more effective corner treatments. In fact, that is probably the tipping point in favor of that.

I have nearfield listening at all six listening positions. Once I tame the under 300Hz range, I will be a happy camper. So, actually, I misspoke. The room doesn't sound good, the system does, and the room is not in the way.


Not sure what you mean by "room support." And in light of poor imaging and low intelligibility, it's obvious your room's support was interfering. Perhaps the room's signature was complementary to a choir, but not other sounds. This is exactly why I prefer nearfield listening.


To the extent that a room has a noticeable "sound" at all, yes. See my response above.


DSP, IMO, is worse than worthless. Artists, engineers and producers go to great length to create an ambiance. Monkeying with that can only degrade what the creators intended. If a particular DSP setting, or room environment, enhances a particular recording - We Were Soldiers for example - but nothing else, then it is likely that We Were Solders is not "right" either.

Dennis, Ehtan, and anyone else

I am doing a Ht room and i line around Memphis, TN. I have the room sheetrocked and want to get the acoustics right. My room is small 10 (width)x16 (length)x8.5(height). My room "T''s out in the back to 25 foot wide for about 4 feet.

I will sit around 12 feet from the front and thus 4 feet from the back.

Here are my thoughts:

1) Acoustic panels behind the front left and right placed on the sheetrock.
2) Acoustic panels atthe first reflective points (using mirror suggestion)
3) Basss traps in the corners in the front and in the back corners (the room is shaped in a "T" shape. SHould I put bass traps in the corbers created by the "T" as well.
4) Should i consider diffusors on the back walls or acoustic shields for absorbtion.

I mentioned where i live in case you could recommend someone locally I could have come and help me.

It seems that acoustic shields behind the front speakers is very important as well as shields at the first reflective points in relation to where the listener is sitting.

thanks for the help!

Dax

Dax,

> Here are my thoughts: <<br />
That all sounds about right. You can't have too many bass traps, so plan on plenty for best results.

> SHould I put bass traps in the corbers created by the "T" as well. <<br />
Maybe. If you stand back there in the "wings" do you hear bass build up in the corners? If so, traps there might help.

> 4) Should i consider diffusors on the back walls or acoustic shields for absorbtion. <<br />
Good diffusors cost more than good absorbers, whether DIY or commercial, so it comes down to how much you're willing to spend.

> I mentioned where i live in case you could recommend someone locally I could have come and help me. <<br />
It's not clear if you're looking for DIY advice or a commercial solution. I help customers all over the world by email and photos etc.

--Ethan

Re: cost of diffusers, you can make inexpensive polycylindrical diffusers out of bent sheets of practically anything hard and relatively flexible. They are as good as the modern "well-type" diffuser in nearly all situations.

Both types of diffusers have the drawback of needing a lot of depth -- much more than an absorber of equivalent frequency effectiveness.

- Terry

Dennis, you sound pretty firm on not using styrofoam. Is there a valid reason, if it is fire rated.

Terry,

Can you offer any pointers on building "bent-sheet" diffusers such as what radius of curvature is needed to be effective and what the orientation of the arc should be?

I have my theater room sheetrocked and painted. Almost my whole front wall will be covered with the screen.

I keep hearing that shielding the front wall is big but what about my situation?

Should I treat all the areas around the screen or go ahead and treat behind the screen as well?

What should I use? Anything I can get at Lowes or Home Depot? Can I paint it to match the other walls in my theater room (brown).

I was goign to put acoustic shields right behind the front, left, and center channels. Would this not be enough treatment for my front wall.

The dimension are 9.5'wideX16'long.

Thanks for your advice

Start by reading this whole thread. This question has been asked and answered severall times.

Short version:

What about your situation?
Depends on if your screen is perforated.
OC 703 or similar. No. No.

i have read all the thread but i still am confused on how I am going to treat my front wall that has a non-perforated screen that takes up most of that front wall.

Should I go ahead and put OC 703 on the whole wall ebfore my screen goes up even though the scren will cover msot of it up??

No, the screen will reflect high frequencies and make the absorption ineffective and narrowband.

You have a lot of flexibility here.

A larger arc will diffuse down to lower frequencies. To a first approximation, a radius of curvature r will be effective at diffusing frequencies whose wavelengths are less than 2 pi r. This is not a sharp boundary -- you approach the maximum diffusion at maybe 3 or 4 times this frequency. To get the frequency for a particular wavelength in feet, just divide it into 1130 feet per second, the speed of sound. How much of a circular arc you use will depend on how much depth you are willing to allow. Varying the spacing and radius of curvature for multiple diffusers is a good idea.

One nice thing about polycylindrical diffusers is that, unlike the now-common discrete varying depth type of diffuser, they have no high frequency limit on the sounds they will effectively diffuse. As for orientation, it depends on the specific purpose of the diffusion. If it is to maintain a broad sound stage, then vertical orientation is often preferable. On the other hand, horizontal orientation will improve the uniformity of reverberation decay if you have a very uneven balance of horizontal and vertical absorption -- "dead" walls and "live" ceiling and floor, for example.

- Terry

Terry,

> polycylindrical diffusers ... are as good as the modern "well-type" diffuser in nearly all situations. <<br />
That's wrong on so many levels I'm not even sure where to start. But I'll try.

The main advantage of QRD diffusors is they actually diffuse, versus curved plywood that merely deflects. There is a difference, and that difference affects how well each device reduces comb filtering.

But don't take my word for it. Here's a recent quote from Jeff Szymanski, who I'm sure you know of:


And this is from recording studio designer Wes Lachot, another big proponent of QRD diffusors over curved plywood:


--Ethan

Anyone know of any good sites dealing a DIY 2D Quad Diffuser? I found this page, with a link to the infamous BBC paper that looks helpful: http://www.mhsoft.nl/DiffusorCalculator.html

Some people have used wood, but that seems aweful heavy. ANyone have ideas for a hard foam that would reflect well, hold it's shappe, weigh little, cost less, and come in nice, long, square lengths?

Sorry, but this distinction is incorrect. It is wrong on so many levels I don't know where to start.

Since I tried to get you to read some of the acoustics literature and failed, let's look at some commercial diffusers. RPG makes diffusers, right? And D'Antonio and Cox know a thing or three about such things, right?

Have a look at these RPG diffuser products:

http://www.rpginc.com/products/monoradial/index.htm
http://www.rpginc.com/products/opticurve/index.htm
http://www.rpginc.com/products/chaos/index.htm

Either these curved pieces of plywood are diffusers, or RPG has some of the greatest scam artists in the world.

- Terry