Room Acoustics Questions: Please share theories and experiences

David Matz · Dec 25, 2008

Happy Holidays!

A few question for our acoustic experts or anyone who can share his experiences. If the answer is “depends” please provide example scenarios how things may work in different ways. Please feel free to answer any or all of these. Thanks!
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What’s the difference acoustics-wise/ sonically between mineral wool vs. 703 or 705 fiberglass vs acoustic cotton?

How does one deal with windows/ glass? Does bass escape through the windows? What happens to the higher frequencies? What material can be used to deal with the higher frequencies, while letting the bass get out?
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Is there something scientific to golden ratio room size, or is myth/ wishful thinking?

Why is a panel on the corner much more effective than hanging on the wall?
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Is a panel hanging on the wall more effective than one standing on the floor?
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How much space between the wall and the panel is optimal? If the panel is out more than optimal, is it less effective?
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When a panel is straddling a corner, does the empty space reduce the panel’s effectiveness?

If speakers are 12 feet into the room in a large room, does one "need" to absorb the back wave? What is the rule of thumb for moving the speaker into the room so the back wave will be less of a factor?
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What size is a large room? What size is a small room?
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Is there a room size where acoustic treatments are not necessary? Some people in a big room (27 x25 x10, for example) say they do not need room treatments. Could this be true?
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Can a well treated small room ever sound as good as a well treated big room?
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Are open doors or openings into other rooms an effective way to deal with bass issues? How big of an opening starts messing up the soundstage?
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Do blankets, curtains, or cloths help reduce high frequencies or is a myth?
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What does an equalizer do vs. acoustic panels? Do equalizers add value? What needs to be in place for them to make the biggest difference?
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What is the difference between the different products, say Rives PARC vs. Audissey?
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If one has Bill Gates money, how does one make the ultimate listening room?
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If one does not quite have Bill Gates money, but still has a good budget to create a room, what can be done to a room? Is it using acoustic panels on all surfaces instead of floors, ceilings and drywall, or is it more than that?

Ethan Winer · Dec 26, 2008

David Matz said:
What’s the difference acoustics-wise/ sonically between mineral wool vs. 703 or 705 fiberglass vs acoustic cotton?

705 is a little better at bass frequencies. See this:

Density Report

Does bass escape through the windows?

A little, depending on how thick the glass is. But higher bass frequencies are reflected unless the glass is really thin.

What happens to the higher frequencies? What material can be used to deal with the higher frequencies, while letting the bass get out?

Panels made from rigid fiberglass absorb highs and pass lows.

Is there something scientific to golden ratio room size, or is myth/ wishful thinking?

Yes, see this:

RealTraps Mode Calculator

Why is a panel on the corner much more effective than hanging on the wall?

Because bass collects in corners.

Is a panel hanging on the wall more effective than one standing on the floor?

Probably.

How much space between the wall and the panel is optimal?

The ideal air gap is equal to the panel thickness. See this:

http://www.ethanwiner.com/acoustics.html#air gap

If the panel is out more than optimal, is it less effective?

Yes, by definition.

When a panel is straddling a corner, does the empty space reduce the panel’s effectiveness?

The empty space behind? No, it helps.

If speakers are 12 feet into the room in a large room, does one "need" to absorb the back wave?

Maybe. Depends on the room size. 12 feet seems way too far into the room for a normal size room.

What is the rule of thumb for moving the speaker into the room so the back wave will be less of a factor?

The "rule of thumb" is to measure the LF response at high resolution as you experiment with placement. Anything else is just guessing.

What size is a large room? What size is a small room?

I'd say the dividing line is somewhere around 25 by 15 feet.

Is there a room size where acoustic treatments are not necessary?

Probably not.

Some people in a big room (27 x25 x10, for example) say they do not need room treatments. Could this be true?

No, in fact a room with those dimensions needs more than the usual amount of bass trapping because it's nearly square.

Can a well treated small room ever sound as good as a well treated big room?

Probably, but it takes a fair of treatment. Especially bass traps.

Are open doors or openings into other rooms an effective way to deal with bass issues?

It depends on what's on the other side of the opening.

How big of an opening starts messing up the soundstage?

It depends on where the openings are in relation to the speakers and reflection points.

Do blankets, curtains, or cloths help reduce high frequencies

Yes, but proper treatment must deal with all frequencies, not just 2 KHz and above.

What does an equalizer do vs. acoustic panels? Do equalizers add value? What needs to be in place for them to make the biggest difference?

EQ is not a substitute for bass traps and other acoustic treatment. More here:

Audyssey Report

If one has Bill Gates money, how does one make the ultimate listening room?

Easy - hire an expert, or buy from a knowledgeable company that can tell you exactly what treatment products to get and where to put them.

If one does not quite have Bill Gates money, but still has a good budget to create a room, what can be done to a room? Is it using acoustic panels on all surfaces instead of floors, ceilings and drywall, or is it more than that?

You're already way ahead of most people by asking a lot of excellent questions. Start with the articles I linked above. There are many more articles and videos on my company's web site too.

--Ethan
RealTraps

C.A.P · Dec 26, 2008

Now that is a great post Ethan ! Man I just learned a few things I will be doing!

C.A.P · Dec 26, 2008

Now that is a great post Ethan ! Man I just learned a few things I will be doing!

twich54 · Dec 26, 2008

Dave, thanks for the thread start !

Ethan, thanks for the insightfull answers !

JonFo · Dec 27, 2008

Since Ethan already answered most of the questions, I’ll address a few where I feel my direct experience with ML’s in a treated room with room correction systems adds some insights as well.

David Matz said:
…
If speakers are 12 feet into the room in a large room, does one "need" to absorb the back wave? What is the rule of thumb for moving the speaker into the room so the back wave will be less of a factor?

There are two issues: Room treatments to manage bass modes, and there, it doesn’t matter where you put the speakers, the room modes are defined by the dimensions (yet affected by where the spekars are), so yes, you still need room treatments.
Secondly, at 12 feet from the front wall, a dipole speaker is out of the ‘short window’ for rear reflections. That is, the path of the rear wave reflections relative to the front wavefront is going to be well above 15ms long, which puts them in a uniquely localizable category (they are now identified as separate reflections, not mixed up as part of the direct sound). Although it will still cause some mid-bass cancelation and obvious comb-filtering of the direct signal.
Many 2 ch only users will find this type of positioning to sound ‘good’.
Me, I’d still recommend some treatments to absorb the mid-bass range (60 to 500hz) as well as attenuate the highs some. So I’d still line the front with MiniTrap HF’s.
That would allow the speakers to be placed 5’ from the wall and still sound great.

David Matz said:
…
What size is a large room? What size is a small room?

I agree with Ethan, the dividing line is roughly at 15x25, or in cubic feet at around 3,000 or so. But ratios are important.

David Matz said:
What does an equalizer do vs. acoustic panels? Do equalizers add value? What needs to be in place for them to make the biggest difference?

Acoustic panels deal with the fundamental room mode resonances in the room. This benefits ALL seating locations.
Acoustic treatments that balance both bass trapping with HF management (either diffusion and/or absorption) are the most effective at providing a listening space that is free from pesky resonances, yet retains enough ambience to not sound like a coffin.

Now, once you’ve take care of the fundamental room issues, I find that Active room correction adds huge value.
Even if just using some basic parametric EQ in the low frequencies, it can dramatically reduce certain room modes from being over-energized at the main listening locations.
But that’s the rub of EQ, it’s generally overdone for a single location.
My experience shows that one should not strive for ‘flat’ at a given position, but rather sample many points where listeners sit, and look for obvious room resonance frequencies, and treat those as an averaged set. So while a measurement at the prime listening position might indicate a cut of 4dB is required at 63hz, looking at all the other positions revels that a cut of 3dB at 60hz would be much more beneficial across the board.

In my system I use parametric EQ on the sub and Monolith woofers to correct for the worst room induced errors. These were arrived at after looking at over 45 separate measurements.

Once fundamental room modes are dealt with by using physical treatments and targeted EQ, I find that a room correction system like Audyssey can add an amazing benefit.

However, it’s not a piece of cake. The stock MultEQ XT process in most receivers and processors is decent, but requires extreme care in how the measurement process is done to get satisfactory results. Not being one that’s happy with satisfactory (I want perfect), I use the Audyssey Pro system with its calibrated mic and much more powerful PC based convolution computation.
However, once a good multi-position measurement is done (resulting in over 130 individual measurements) I find the results are astounding.
Since Audyssey does FIR based convolution filtering, it is time-coherent and extremely accurate in the frequency domain. Not that the result is a flat FR curve, the room still has an influence, but since it’s already treated, the results are about as good as one could ever expect in a finite space.

The result is incredible soundstages that extend beyond the speakers in all dimensions. Completely intelligible dialog on movies, and crystal clear vocals on music.

No bass modes mean that bass drums are ‘tight’ and double bas sounds like it’s in the room.

So it’s not either / or with respect to treatments vs EQ, it takes accurate applications of both to reach accurate reproduction.

See my system and many of my other posts for more on what’s in place.

David Matz said:
What is the difference between the different products, say Rives PARC vs. Audissey?

Most EQ systems are based on parametric IIR (infinite Impulse Response) that can correct frequency domain response, but introduces varying amounts of time-domain error (some bad implementations even have ringing).
These are OK for correcting the most egregious low-frequency room (or speaker induced errors, but should be avoided in the higher frequencies.
Rives, and DBX DriveRacks are IIR parametric systems.

There is a family of room correction and EQ systems based on FIR (Finite Impulse Response) that are phase-linear and manage to be time coherent while correcting frequency domain issues.
Dolby Lake processors are an example of that.

A further refinement is the Audyssey system, wich is at its heart, an FIR convolution based corrector.
Using their sophisticated (yet tricky) measurement system, they use averaged results and look for ‘worst offender’ resonances and attack those so that the corrections are generally beneficial across the listening area.
I find the results from Audyssey Pro calibration to be supremely beneficial.

The fact that Audyssey also now supports Dynamic EQ on top of their room correction a huge benefit, as I can now listen to music (like I am now) at lower levels and still perceive all the impact of a louder level, as DynamicEQ adjusts the frequency balance for the set SPL level.

David Matz said:
If one has Bill Gates money, how does one make the ultimate listening room?

Copy me? Seriously, I have designs for a room that would be absolutely killer. It involves 5 CLX in Infinite Baffle configs couple with a phased bass array setup (also IB). Next home…

I'll give this one a shot in a another post.

David Matz said:
If one does not quite have Bill Gates money, but still has a good budget to create a room, what can be done to a room? Is it using acoustic panels on all surfaces instead of floors, ceilings and drywall, or is it more than that?

There is a science to it. Some more reading on acoustics, like the new Toole book would provide insights. But frankly the best bet is to send Ethan a diagram of your room and ask for his input.

Also, posting room measurements and questions here will get guys like me to make comments, like it or not

Again, I point to my own room, where I started with optimized dimensions based on my ML speaker set, and still had to put in massive amounts of treatments, layer in EQ and Audyssey Pro on top of it all to get close to ‘Audio Nirvana’.

How much? Well, I’m at $15K and counting just for treatments …

Rich · Dec 27, 2008

Great post, and great replies. One thing that I don't think Ethan quite understood was your question regarding treating the back wave. Ethan's answer referred to measuring low frequency, but what you are referring to is the back wave from the ESL panel, which starts at about 250 hz. or so and goes up from there to 20 khz. Most folks seem to think that positioning ML's such that the panel is about five foot from the wall behind it works best if you aren't treating the back wall. I still recommend full-frequency absorption behind the panels, but some prefer diffusion.

twich54 · Dec 28, 2008

Rich said:
Most folks seem to think that positioning ML's such that the panel is about five foot from the wall behind it works best if you aren't treating the back wall. I still recommend full-frequency absorption behind the panels, but some prefer diffusion.

Ineresting point Rich, one which I feel best suited to absorption and diffussion.

Couple of factors at play here, again, IMO.......

1) the greater the distance....more diffussion less absorption.

2) 'Rake angle' of ESL panel...... the more upright one has their speakers the more absorption on wall behind. It's the 'angle of incidence equals the angle of reflection' thing, all relative the higher one goes in freq.

In the end it's all a matter of experimentation, personal likes / dislikes , etc.

C.A.P · Dec 28, 2008

twich54 said:
In the end it's all a matter of experimentation, personal likes / dislikes , etc.

Exactly !.....................

eknuds01 · Dec 31, 2008

Ethan and Jon:

Regarding room treatments - do you advocate measuring the room response, and then selecting which treatments to use?

I noticed in of Ethan's posts a room mode calculator. For argument's sake, let's say there's a resonance at 50 Hz. What should one do to compensate for this?

Or is all just a matter of trial and error? I know very little about this topic, but it seems to be interesting to say the least. I might play with room treatments, but I don't want to shell out tons of $$$ on a hunch. Rather, I'd want to maybe measure the response some way and then purchase the appropriate room treatment(s). For instance, I know a lot of people advocate the use of bass traps. What is the reason for this?

thanks,
Erik

Ethan Winer · Dec 31, 2008

eknuds01 said:
do you advocate measuring the room response, and then selecting which treatments to use?

Not necessarily because no matter what you measure, the solution is pretty much the same. All rooms need:

* Broadband (not tuned) bass traps straddling as many corners as you can manage, including the wall-ceiling corners. More bass traps on the rear wall behind helps even further. You simply cannot have too much bass trapping. Real bass trapping, that is - thin foam and thin fiberglass don't work to a low enough frequency.

* Mid/high frequency absorption at the first reflection points on the side walls and ceiling.

* Some additional amount of mid/high absorption and/or diffusion on any large areas of bare parallel surfaces, such as opposing walls or the ceiling if the floor is reflective. Diffusion on the rear wall behind you is also useful in larger rooms.

The main reason I suggest people measure their room is so they can see exactly how bad an untreated room really is. I am serious! Most people have no idea how badly skewed the bass response is in their room.

I noticed in of Ethan's posts a room mode calculator. For argument's sake, let's say there's a resonance at 50 Hz. What should one do to compensate for this?

A mode calculator is only a predictor of bass response. In practice, the peaks and nulls will likely be at slightly different frequencies due to wall thickness and other factors. So to know the true response requires actually measuring. I've seen mode calculator predictions vary more than 20 percent from what was actually measured!

50 Hz is pretty low, so that requires a fair number of bass traps to tame. It's also low enough that it can be helped with EQ. I'm not a fan of EQ as a substitute for bass traps, but it's an acceptable supplement for very low frequencies. However, EQ can help peaks only, not nulls.

I know a lot of people advocate the use of bass traps. What is the reason for this?

Ah, you're one of those who needs to see how bad a room is in order to understand why bass traps are needed.

The graph below is typical for a smallish untreated room. In this case the room is 16 by 11 by 8 feet high. Not a pretty story, eh?

--Ethan

eknuds01 · Dec 31, 2008

Ethan Winer said:
Not necessarily because no matter what you measure, the solution is pretty much the same. All rooms need:

* Broadband (not tuned) bass traps straddling as many corners as you can manage, including the wall-ceiling corners. More bass traps on the rear wall behind helps even further. You simply cannot have too much bass trapping. Real bass trapping, that is - thin foam and thin fiberglass don't work to a low enough frequency.

By rear wall - you mean the wall behind the listener, correct?

* Mid/high frequency absorption at the first reflection points on the side walls and ceiling.

Where are these reflection points normally located, or do you normally just hang large acoustic tiles on the side walls hoping to cover the area in question?

* Some additional amount of mid/high absorption and/or diffusion on any large areas of bare parallel surfaces, such as opposing walls or the ceiling if the floor is reflective. Diffusion on the rear wall behind you is also useful in larger rooms.

As of this writing one of the parallel walls in my room is a sliding glass door and I'd rather not cover that. It's possible to treat the wall that is opposite the sliding glass door. The floor is carpeted.

The main reason I suggest people measure their room is so they can see exactly how bad an untreated room really is. I am serious! Most people have no idea how badly skewed the bass response is in their room.

A mode calculator is only a predictor of bass response. In practice, the peaks and nulls will likely be at slightly different frequencies due to wall thickness and other factors. So to know the true response requires actually measuring. I've seen mode calculator predictions vary more than 20 percent from what was actually measured!

50 Hz is pretty low, so that requires a fair number of bass traps to tame. It's also low enough that it can be helped with EQ. I'm not a fan of EQ as a substitute for bass traps, but it's an acceptable supplement for very low frequencies. However, EQ can help peaks only, not nulls.

I was just using the 50 Hz number for argument's sake. I'd have to run the numbers and see what the modes are for my size room.

Ah, you're one of those who needs to see how bad a room is in order to understand why bass traps are needed.

The graph below is typical for a smallish untreated room. In this case the room is 16 by 11 by 8 feet high. Not a pretty story, eh?

--Ethan

Thanks, Ethan. I am starting to learn!! These posts really help.

Erik

churchill · Dec 31, 2008

Agreed! This is great stuff!:rocker: I'm in process of treating my room and I'm looking at bass traps to help smooth out my bass response.

David Matz · Jan 2, 2009

Happy New Year!

Thanks to Ethan, Jonathan, and every else who has responded to this thread. I have a few more questions. Thanks!

<link rel="File-List" href="file:///C:%5CDOCUME%7E1%5CDAVIDM%7E1%5CLOCALS%7E1%5CTemp%5Cmsohtml1%5C01%5Cclip_filelist.xml"><style>  </style> Why does it seem like there is more air and space when the same system is played in a larger room vs. a small room? Is it the un-trapped bass waves “congesting” the sound?
<o></o>
If the bass is not right, why is there a perceived degradation in other frequencies and overall sound?
<o></o>
How much does the bass in the upper part of the room affect the sound on the bottom? How important is it to spread the treatments vertically in the room? How many should be placed higher vs. lower in the room – 25% - 75%? What if one places treatments to cover only where walls meet (behind the speakers and behind the listener) and where the floor and walls meet vs. covering the ceiling-wall corners? How much missing out on leaving out the wall – ceiling intersections?
<o></o>
If building a room from scratch. Why not just attach the 705 or mineral wool to studs and put a fabric over it for aesthetics, instead of using drywall? Would this be the “Bill Gates” (cost no object) option, or is there something else additional that Bill would add to create the ultimate room?
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Would the “Bill Gates” option have flat response or is a flat response a theoretical ideal?
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If there are treatments behind the speakers to help absorb the back waves, what percentage of the waves is absorbed? 25%, 75%? Why not put some really thick treatments behind the speakers to absorb as much as possible?
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Assuming the rear wave from the Martin Logan is trapped, why is treating the wall behind the listening chair more important than treating the wall in front of the speakers?
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Ideally should the entire back wall be covered before treating the front wall?
How should the treatments be split - 2/3 back vs. 1/3 front, 100% back wall 50-50%, if one has limited number of treatments?
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How long does it take for the waves to die down (so one can't hear it or its effects) on their own?
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Are side wall reflections an important issue for Martin Logans if speakers are not toed in? Are side wall reflections more damaging in regards to bass or other bands also?
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How close should the speakers be placed to sidewalls? Why not place the speakers right up against the wall, since there are supposedly few side reflections for planar speakers?
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What is the best way to deal with closets? Leave them open to suck in the bass or close them to let the bass be absorbed by the traps in the room?
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Is there another material other than mineral wool, 703 - 705 fiberglass, or acoustic cotton that may be more effective but not used because of cost?
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Some guys use 2” panels. Why bother with that, unless spouse/ physical constraints prevail. If they don’t why not have panels 12 or 18” thick?
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If a planar speaker is not curved (i.e. Sanders, Sound Lab, etc.), how do any of the answers above change?

Bernard · Jan 2, 2009

Does ASC have a patent on their Tube Traps? I find their products to be by far the best-looking (and practical for me), but also the most expensive. It would be great to find cheaper alternatives that look similar, short of building them yourself.

Burke · Jan 2, 2009

David Matz said:
If building a room from scratch. Why not just attach the 705 or mineral wool to studs and put a fabric over it for aesthetics, instead of using drywall? Would this be the “Bill Gates” (cost no object) option, or is there something else additional that Bill would add to create the ultimate room?

This is what I am doing later this month. Come by anytime and check it out.

JonFo · Jan 2, 2009

David Matz said:
Happy New Year!

Thanks to Ethan, Jonathan, and every else who has responded to this thread. I have a few more questions. Thanks!

Ok, this is getting to seem more like a ploy to keep me from doing anything useful around the house today

At least the music is good :music:

And Happy new year to everyone as well

Why does it seem like there is more air and space when the same system is played in a larger room vs. a small room? Is it the un-trapped bass waves “congesting” the sound?
<o></o>

Part of it is fewer and or less harmful room resonances, but any ‘home’ room (unless you live in a castle) will have significant modes.

Mostly what you ‘hear’ as a difference in the two sizes of rooms are the effects of early reflections. These are the problem that hurt the mid to high-frequencies the most.

A ‘live’ room (i.e. few treatments and lots of flat surfaces) will be easily over-energized by dipole line arrays in the highs. Massive comb-filtering and ringing will destroy the quality of sound ML’s are capable of.

JonFo · Jan 2, 2009

If the bass is not right, why is there a perceived degradation in other frequencies and overall sound?
<o></o>

Several reasons, most having to do with the psychoacoustic perception of tonal balance. If there is a huge bass room mode, it will over-stimulate the ear, and will ‘tilt’ the spectral balance towards the bass. But that bass is itself unbalanced (one small frequency band and its harmonics dominating).

Also, remember that it’s not just about frequency response, time-domain impacts are important as well.

A room resonance ‘sustains’ a note for as much as 100ms at loud levels and still takes another 200ms to decay. See some room resonance waterfall plots for examples (believe I have posted a few).

This topic has had books written on it; see The Physics and Psychophysics of Music by Juan G. Roederer.

JonFo · Jan 2, 2009

How much does the bass in the upper part of the room affect the sound on the bottom? How important is it to spread the treatments vertically in the room? How many should be placed higher vs. lower in the room – 25% - 75%? What if one places treatments to cover only where walls meet (behind the speakers and behind the listener) and where the floor and walls meet vs. covering the ceiling-wall corners? How much missing out on leaving out the wall – ceiling intersections?
<o></o>

Depends on what frequency were talking about. For frequencies in the <100hz range, I believe it is pretty lenient as to whether to treat the ceiling/wall corner vs the wall/floor corner. Either will trap bass at that longitudinal dimension of the room. Remember, bass modes are primarily pressure based.

The recommendations are usually to start in the corners, as those are both high-pressure zones and easy to place large bass traps in.
In my estimation, wall/ceiling intersections are great places to put traps. I have four Mondo traps along my ceiling /wall intersections

Bottom line, ceiling edges, corners and even spots above the speakers are great places for traps. They are out of the way and if your ceiling is black (as it should be if a dedicated theater), then they just disappear).

JonFo · Jan 2, 2009

If building a room from scratch. Why not just attach the 705 or mineral wool to studs and put a fabric over it for aesthetics, instead of using drywall? Would this be the “Bill Gates” (cost no object) option, or is there something else additional that Bill would add to create the ultimate room?
<o></o>

That’s a perfectly valid approach, and many a home theater has been designed and built that way. There are many fabric track tensioning systems that allow one to cover large areas with acoustically transparent fabric. I used some Fabricmate tracks in my side-wall custom treatments. It’s great stuff.

Now, you still need to have some reflectivity and not having any drywall surface in the room actually means you lose some very low frequency absorption (as drywall flexes a bit and absorbs VLF). But if the wall behind the OC705 has some reasonable ‘flex’ that might not be a big deal.

My next room will have about 60% of the side wall surfaces done this way.

Room Acoustics Questions: Please share theories and experiences

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