# In the middle of adding acoustic panels and bass traps.... some questions



## Todd Anderson (Jul 24, 2009)

Hello all,

I'm in the middle of installing room treatments, both 2" thick acoustic panels from audimute and some DIY corner bass traps.

I just finished installing the acoustic panels... audimute helped me calculate how much of my room to treat based on the room size (12X17X8 high). I chose to add these panels at the first reflection point on the walls and ceiling. Basically, there is a row of 1'x2' acoustic panels on the ceiling covering the first reflection point for the width of the seating in the room (7 panels in all)... and then I added panels down the left and right walls, covering the first reflection points for the same seating.

It looks great. I just fired up a demo blu-ray and was shocked at the results thus far... not in a great way.:dontknow:

All of the highs are heavily muted and the audio sounds lifeless.

Without going into before and after REW comparisons, etc... I have some questions:

1) Is this result to be expected? When I re-calibrate my speakers using the MCACC on Pioneer Elite AVR, will some of the highs and mid range/high range that I loved so much return... will my AVR be able to balance the sound out? I'm a little concerned right now. :crying: The sound stage definitely narrowed A LOT... and sounds pretty lifeless.

2) If I've inflicted this much deadening with the acoustic panels... what do I need to do to make sure I don't deaden even more when I create the bass traps? I am going to construct 4' tall corner triangle bass traps. I've read some suggestions (some where) that adding a layer of plastic in front of the bass trap will help deflect highs. Suggestions?

Any basic insight would be great. Thanks,

27dnast


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## Prof. (Oct 20, 2006)

27dnast said:


> 1) Is this result to be expected? When I re-calibrate my speakers using the MCACC on Pioneer Elite AVR, will some of the highs and mid range/high range that I loved so much return... will my AVR be able to balance the sound out? I'm a little concerned right now. :crying: The sound stage definitely narrowed A LOT... and sounds pretty lifeless.


If you did an MCACC calibration prior to adding the acoustic treatments, then it will sound dead without any further calibration..
Also your Group Delay settings will be wrong for the new acoustic environment..

You will need to re-run MCACC and my suggestion would be to use "Symmetry" and the THX setting..I've found that these settings give me the most openness to the overall sound.. 
To open the sound field you will need to select 30-50ms. or 40-60ms.
Check that your speakers are set to "Small" and the cross over is set to 80hz.
After calibration, it is advisable to check levels with an SPL meter..to make sure that they are all set to 75dB..



> 2) If I've inflicted this much deadening with the acoustic panels... what do I need to do to make sure I don't deaden even more when I create the bass traps? I am going to construct 4' tall corner triangle bass traps. I've read some suggestions (some where) that adding a layer of plastic in front of the bass trap will help deflect highs. Suggestions?


Adding bass traps will only help to control and smooth out the lower frequencies..They won't affect the higher frequencies..


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## Todd Anderson (Jul 24, 2009)

Just out of curiosity (I'm only asking because I won't be able to test this out tonight...). Do you think I will hear a radical swing in the sound after I run the calibration (meaning, the same radical swing I just experienced pre and post acoustical treatments)?

I only ask this because, probably stupidly, I spent a lot of time mounting these acoustic panels in a rather semi-permanent way... and I'm a little afraid that I have done something that has cause a significant "negative" impact on the room rather than the positive impact that I was looking for. I had to make some alterations to the panels to hang them from the ceiling AND I dread having to repair/paint all the of the walls if I have to take some/all off...


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## SAC (Dec 3, 2009)

Is it to be expected?
Possibly, as we have no idea as to what the real problem was to begin with.

A common misconception is that ALL reflections are evil. And per some methods of determine where to treat (assuming one even uses any - such as the infamous mirror technique that was initially introduced as an explanation of concept, not as a definitive treatment protocol!), the tendency is to over-damp - to place panels everywhere an anomalous reflection MIGHT occur, not necessarily to apply them surgically as needed to resolve real issues -as should be done.

Also, the tendency is to apply panels that are not actually broadband, and which only absorbs the higher frequencies to the exclusion of the mids - resulting in an over-damped room with no highs - but where the high energy content mid range indirect energy is still wreaking havoc. (Also note, that most porous bass traps that folks install are not truly LF bass traps - they are full range broadband traps that will absorb ALLOT (read "too much") of the mid-high energy.)

Did you generate before and after ETC responses - one per speaker?

These will definitively show the actual specular energy content and allow us to accurately evaluate each and every significant reflection relative to the listening position.
They will also provide proof of performance data in order to compare before and after such that it is easy to determine if the expected results were accomplished (in terms of positioning and damping and the identification of any unforeseen residual reflection from applied absorption, etc.).

The process should be to generate the ETCs of the untreated space, and then per the desired acoustic room response model, determine which indirect signals to treat, using the appropriate treatment material, be it absorption, reflection/redirection or diffusion.

You then determinate the precise path of each anomalous indirect path and their points of incidence, and *surgically* treat ONLY those incidence points with a treatment adequate to control *all *of the specular energy (meaning the complete broadband spectral content - not all reflections) sufficient to create the total response desired. A followup sweep will provide feedback as to the success, or provide sufficient information to facilitate adjustment until such results are achieved. Thus you control the problem areas without removing the finite amount of energy we have in the room and thus over-damping (deadening) it.


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## Todd Anderson (Jul 24, 2009)

LOL... well, I'm probably the last guy you would choose the hangout with. 

I did not do an ETC test.

I did use the REW software and took some measurements... primarily looking at the waterfall generated because my bass is so boomy.

I guess I'm a little green here, but I had read that small enclosed rooms, such as mine would benefit from acoustic treatments and bass traps. I found the reflection points, via mirror - much to your chagrin, I'm sure... but at the advice of just about every panel manufacturer site I read - and talked to the folks at two different manufacturers. Both gave me a general amount of square footage that should be treated in the room (which is how I ended up with the amount of panels that I purchased). I treated the first reflection points.

Of course, as I write this, I have not taken the time to recalibrate my system. I am hoping **** very much so *** that I can breath some airiness and higher end life back into with a simple recalibration. If I can't, honestly, I am going to be heart broken and down-trodden as I have put holes etc. in to the walls and ceiling. Frankly, I'm feeling a tad panicked at the moment!!!!

My set-up... prior to the treatments seemed overly bright and is very - very - boomy.

I bought a parametric eq and used the REW software to help with the sub boominess... quite successfully I might add (at the primary listening position). But it made some of the other areas in the room even worse.

So, that led me to the path of bass traps... and I began reading how mid to high frequency treatments can really help make speakers sing and open the sound stage.

So... here I am. A little concerned that I just blew-it (along with a big chunk of $)!


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## SAC (Dec 3, 2009)

First, calibration is not necessary (except possibly for LF modal EQ setting) (this refers to REW, as far as MCACC, I would not use it for anything more than if it is capable of limited cut only LF PEQ - as you cannot EQ non-minmum phase specular signals, and that unit is NOT capable of identifying minimum phase (if any) regions in the mid-high spectrum at one point in space.))

You didn't blow it, but you are also at 'cross purposes' if you will.

The _first_ thing you need to do in order to evaluate the effectiveness of any treatment is to turn that MCACC ... OFF!.

The waterfalls are useful 'only' for low frequency (<~250 Hz) modal response.
They do NOT help with any specular behavior (the focused reflections that act as rays) that dominate above ~250 Hz.

As I mentioned, the mirror is a illustration of concept that was first presented to help folks visualize ray performance. It is capable of only indicating some of potential pathways - it provides no information as to which pathways are important (as in "detrimental"), and it provides no information as to indirect diffractive sources.

You may have installed a bit too many panels and as a result overly dampened the room.

Hopefully you have mounted the wall panels in a manner that allows them to be removed.

If so (or even if not so, that is a good plan - hint!) it would be relatively easy to simply remove them and to generate ETCs from each speaker at the listening point.

From there we can identify anomalous reflections and determine their specific pathways and thus to determine the optimal locations for each issue. 

Also, you did not mention the design of he panels. Hopefully they are sufficient to block the broadband spectrum of the specular reflections. Optimally this is >=4" thick with a(n equal) 4" gap. (Don't freak!) If they are too thin they will simply attenuate the high frequencies, leaving it dead while still allowing the destructive mid energy to wreak havoc with imaging, localization, tonality and intelligibility. 

If you would like help - or a bit more interactive zeroing in on the particular issues or procedures, as well as any explanations that are pertinent, PM me. It is so much easier to speak interactively by voice and to zero in on the actual points of concern and/or confusion that it is to try to write an acoustics text that covers everything from soup to nuts in several paragraphs in a manner that does not use the shorthand of a few technical terms about which all may not be intimate. In a conversation we can quickly zero in and explain what is confusing and focus on the real issues more easily.


Edit: BTW, I suspect the majority of your problem you hear is the MCACC....


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## Prof. (Oct 20, 2006)

27dnast said:


> Just out of curiosity (I'm only asking because I won't be able to test this out tonight...). Do you think I will hear a radical swing in the sound after I run the calibration (meaning, the same radical swing I just experienced pre and post acoustical treatments)?
> 
> I only ask this because, probably stupidly, I spent a lot of time mounting these acoustic panels in a rather semi-permanent way... and I'm a little afraid that I have done something that has cause a significant "negative" impact on the room rather than the positive impact that I was looking for. I had to make some alterations to the panels to hang them from the ceiling AND I dread having to repair/paint all the of the walls if I have to take some/all off...


Minor adjustments in MCACC can make noticeable differences in the quality of the reproduced sound..
I'll be very surprised if you don't notice a considerable difference after running the calibration again..

My own room is quite small (9'6" x 15') and I have floor to ceiling bass traps..front and back, with 4' x 2' x 2" first and second reflection point panels..plus a 5' x 5' x 4" panel on the back wall..and I still get an openness to the sound with good clarity and definition..

I presume you are using 703 or similar in your panels?..


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## SAC (Dec 3, 2009)

27dnast said:


> Hello all,
> 
> .....
> All of the highs are heavily muted and the audio sounds lifeless.
> ...



To again try to address the multiple interactive issues here...

1. Turn MCACC OFF! You cannot evaluate treatment with that imposing its processing on top of everything else! 

2. I guess I could ask just what makes you think this is a problem imposed by the application of treatment and not MCACC?

You can rather easily (aside from a bit of physical 'busy' work) verify exactly what the panels are doing and in the process you may as well verify that their application is optimal. 

Simply applying them based upon the use of a mirror can easily result in an overdamped room, as the mirror will indicate every POSSIBLE reflection path, but it will not tell you what paths are real. But by running a 'before' ETC we can determine exactly what the real issues (excessive gain indirect reflections) are and surgically treat ONLY this anomalous specular energy. We would then determine the precise paths and points of boundary incidence for ONLY the detrimental reflections, and then you will place panels appropriately at the verified points of boundary incidence with the effectiveness verified by an 'after' ETC. By addressing only the _real _problem reflections you can be certain that you do not overdamp via the application of broadband panels. 

As the Prof has mentioned, if this is done correctly, the effect is precisely the OPPOSITE of that which you have described. The preponderance of the early arriving direct sound will be direct energy, and by virtue of the Henry Precedence Effect & Haas corollary, the imaging, localization, tonality and intelligibility will be increased.

Also, please describe these panels - the material and thickness and mounting gap, if any.

The irony is that by doing this properly you will not need , nor want, MCACC to do anything in the specular frequencies above about 250 Hz!


And as far as bass traps - I suspect that this is where MCACC is going a bit crazy and trying to compensate.

You need to address the modal behavior. You can safely absorb porous corner bass traps and monitor the before and after with waterfall plots from 0-~250 Hz in order to determine the initial behavior and to verify behavior after installation. But again, you will want to also make before and after ETCs, to determine what effect their installation has on the specular energy above 250 Hz, as you both want to retain this energy, built at the same time you do not want to reflect it back to the listening position. Instead you want this energy to be directed towards the back of the room where it can become more diffused and contribute to a later arriving lower gain diffused ambient field.

As porous bass traps are actually full range traps, they absorb the mids and highs too. So, in order to avoid excessive absorption of these wavelengths, you can apply a reflective facing to the surface of the traps in order to reflect the mid and high frequencies and thus retain said energy into the room. 

And after this surface is applied, you will also want to run additional L&R ETCs to verify that said reflective surfaces are not reflecting the early arriving energy back into the listening spot. If this is the case (and also to avoid this likelihood, one can be proactive and make the corner superchunk style traps a bit asymmetrical, with the face being angled slightly outwards towards the back wall instead of at a 45 degree angle back to the center (e.g, the from wall interface at 2.5' and the side wall interface at 2', with a resultant ~38 1/2" face).

(BTW, actually doing what I am describing is a relatively simple process - much easier in fact than trying to describe the reasoning and method to the madness behind it. So if you would like any help actually doing this procedure, PM me and I can help you cur to the chase...)

Then, if you want to try using MCACC, you can...I guess.... Its need should be minimal. And then only with regards to remaining modal peaks. But at that point, what it 'does' should be easily isolated from that of the room treatment.


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## Todd Anderson (Jul 24, 2009)

Guys, I really appreciate your responses... SAC, you took a lot of time to give me some real insight into how I should have gone about things.

In answer to a few questions:

1) the panels are 2" thick... very similar to 703.
2) They are affixed to the walls and ceilings in a semi-permanent way that would be a total pain to move/repair walls

I re-ran the MCACC this morning... and the result was very positive. Of course, seeing as though the panels I placed are affixed rather permanently (which was an obvious error on my part), this is a huge relief.

I don't quite understand the the different timing selections for the symmetry mode (30-50 msec, for example). Is there a simple way to explain these and why they effect the sound stage? I noticed that one of the symmetry settings was as low as 10 msec. Anyhow, a simple explanation would be appreciated.

@SAC, at this point I am going to roll with what I have. I'm sure the proper thing to do would be to remove all treatments, run the tests and look for before and after responses... I can definitely hear an overall difference, actually, now that I re-calibrated my system. It is less bright, but not overly low-end loaded. The sound is much more focused. I actually like the results.

Thanks for you interest... I really do appreciate it!!!

My next task it to make bass traps, which is what I had originally intended to tackle before I got side tracked with acoustic panels. I do have room measurements for that, via REW. My room has some serious issues. I'm going to post a separate thread before I begin down that road... my room has limitations in terms of what (and where) the traps can go. I've read that bass trapping is fairly straight forward: trap the corners and place as many traps as possible. I hope that my impression is correct.


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## Prof. (Oct 20, 2006)

27dnast said:


> I don't quite understand the the different timing selections for the symmetry mode (30-50 msec, for example). Is there a simple way to explain these and why they effect the sound stage? I noticed that one of the symmetry settings was as low as 10 msec. Anyhow, a simple explanation would be appreciated.


The delay settings are there to take into account..room acoustics, size of the room, and distance between front speakers to surrounds..
When the room is correctly tuned for delay settings, there will be a smooth transition of sound from front speakers through to the back of the room..
For example, if a car was coming towards you in a movie and went right past you towards the back of the room, you would be able to track the sound all the way to the back of the room..

If the delay time is too short, most of the sound will stay towards the fronts, giving very little projection of the sound..If the delay is too long, the surround sound becomes over emphasised and distracting from the fronts..creating almost a hollow sound..

You can only try different delay settings to get the right balance..



> My next task it to make bass traps, which is what I had originally intended to tackle before I got side tracked with acoustic panels. I do have room measurements for that, via REW. My room has some serious issues. I'm going to post a separate thread before I begin down that road... my room has limitations in terms of what (and where) the traps can go. I've read that bass trapping is fairly straight forward: trap the corners and place as many traps as possible. I hope that my impression is correct.


\
Basically that's right..but you have two choices of the type of bass trap to use..
One type straddles the corners to the height required and needs to be about 6" thick..The other fills the corners with triangular sections stacked on top of each other to reach the required height..Both are effective and the choice is yours..


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## Todd Anderson (Jul 24, 2009)

Thanks for the input, Prof. I've fooled around with the symmetry settings... your explanation makes sense. I also dug out the manual from pioneer and they have an explanation that is similar (however yours is much easier on the brain)!

I've read quite a bit about bass traps, resonators, etc. I have budget restrictions and will be going at this the DIY route. My hope is to control as much of the bass as possible with the materials that I have... 3" thick 703 that I will double up into 6" panels for the corners...

Anything below that will be left to my parametric equalizer and I will move on my merry way.

I've read a ton of different solutions to reflective products that I can add in front of the insulation on the bass trap to help reflect some of the mids and highs... everything from foil wrap to plastic... .

There seems to be no right answer. Would I be wrong to go buy some thick mil plastic sheeting from lowes (something they sell in the paint section) and stretch that across the front (under the fabric)??


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## Prof. (Oct 20, 2006)

27dnast said:


> Thanks for the input, Prof. I've fooled around with the symmetry settings... your explanation makes sense. I also dug out the manual from pioneer and they have an explanation that is similar (however yours is much easier on the brain)!


Glad I was able to help you..




> I've read a ton of different solutions to reflective products that I can add in front of the insulation on the bass trap to help reflect some of the mids and highs... everything from foil wrap to plastic... .
> 
> There seems to be no right answer. Would I be wrong to go buy some thick mil plastic sheeting from lowes (something they sell in the paint section) and stretch that across the front (under the fabric)??


The bass traps should not have any reflective material in front of them..Just an open weave cloth covering..They are purely absorbers..
The only position where you should have a reflective facing is on the back wall area..so the highs are not totally absorbed..
For this you can use the commercial 703 with FSK facing or make your own..
Soft plastic is probably not a good choice..it needs to have some rigidity to it to be a suitable reflector..
Thick brown paper or thin cardboard will work well, or as in my case I used Mylar film for my large back wall panel


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## Todd Anderson (Jul 24, 2009)

Ah... Okay... So the trap is open on the front (facing out into the room) and the thin reflective material I choose should go on the back. Got it. I would have thought it was the other way around... To keep the high frequencies from even entering the trap... Assuming that once they enerer the trap... They are lost. 


One more question: am I better off with one 2'x4 trap hanging in the middle of a corner (say mid way between the ceiling and wall) or would I be better off dividing that trap in half... Making two 2x2 traps... Placing one in the floor in the corner and the other hung in the middle of the wall? (or, I guess a third option is a 2'x4 trap that sits on the floor in the corner?)?

I was thinking the 2x2 option, with one hung in the middle of the corner and the other put in the floor in the corner, would allow me to get more coverage in key areas...


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## Prof. (Oct 20, 2006)

27dnast said:


> Ah... Okay... So the trap is open on the front (facing out into the room) and the thin reflective material I choose should go on the back.
> 
> Got it. I would have thought it was the other way around... To keep the high frequencies from even entering the trap... Assuming that once they enerer the trap... They are lost.


If these are free standing bass traps that you're going to put across the corners, then you're going to need something to contain the insulation material within the frame..I would just use open weave cloth front and back..you don't need any reflective material..
The "back" I was referring to is the back of the room..Any panels on the back wall require a reflective material on the face of the panel..





> One more question: am I better off with one 2'x4 trap hanging in the middle of a corner (say mid way between the ceiling and wall) or would I be better off dividing that trap in half... Making two 2x2 traps... Placing one in the floor in the corner and the other hung in the middle of the wall? (or, I guess a third option is a 2'x4 trap that sits on the floor in the corner?)?


The third option is the best choice..or you could place it a little off the floor on legs..


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## Todd Anderson (Jul 24, 2009)

Ok... I just want to overly deaden the room in the high frequencies... 

Dow makes a rigid foam insulation board that has a shiny facing... I wonder if that would make a good reflective surface?? It would be easy to work with.

Is there a reason why 1 larger trap works better than two smaller separate traps?


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## Prof. (Oct 20, 2006)

27dnast said:


> Ok... I just want to overly deaden the room in the high frequencies...


To deaden the room of high frequencies you need good absorbers..not reflectors..
If you have a very live room now which is emphasising high frequencies, you need additional absorption to acoustically control the room..
The way to do this is to use 4" thick wall panels, spaced 4" off the wall (as SAC previously mentioned) and depending on the length of the room, have three or four panels along the walls..
You may even need panels in the wall/ceiling corners as well..
If you have bare timber floors ( don't remember whether you mentioned this or not) then at least a thick throw rug is needed.. 



> Is there a reason why 1 larger trap works better than two smaller separate traps?


You need the most absorption where the driver and or ports are located..Placing the traps mid way between the floor and the ceiling is not going to give much benefit..
You also need as much a large area as possible near the the corner and the floor..Separating the traps into two smaller ones reduces the effectiveness..


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## Todd Anderson (Jul 24, 2009)

LOL... What I meant was "I don't want to overly deaden the room". 

I have quite a bit of first reflection mid/high acoustic panels up... So I'm concerned that if I put trapping in all of the corners, that it will kill too much if the highs that are left.

Do bass traps at the front of the room need reflective material? Or should I only be concerned with the rear?


Thanks again... Sorry for the confusion. Sometimes typing on a smart phone makes you look not so smart!

27


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## Prof. (Oct 20, 2006)

27dnast said:


> LOL... What I meant was "I don't want to overly deaden the room".
> 
> I have quite a bit of first reflection mid/high acoustic panels up... So I'm concerned that if I put trapping in all of the corners, that it will kill too much if the highs that are left.
> 
> ...


LOL!..When I read your last post, I had to read it several times because it seemed to be the opposite of what You had said previously!..but I thought I must have misread what you had said in your earlier posts..:R

OK..so back on track again..Firstly bass traps can be applied as stated in my last post..
Using 2" thick wall panels at first and second reflection points is not going to over deaden the room..You shouldn't need any reflective components in those panels..
The only place where you need any reflective materials is on any panels on the back wall..and if you feel the room is too dead, then don't put any panels there..

If your highs are too subdued after this treatment, then they can be compensated with your EQ settings in the Receiver..


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## Todd Anderson (Jul 24, 2009)

Awesome. Time to get to work!!!


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## SAC (Dec 3, 2009)

27dnast said:


> Anything below that will be left to my parametric equalizer and I will move on my merry way.
> 
> I've read a ton of different solutions to reflective products that I can add in front of the insulation on the bass trap to help reflect some of the mids and highs... everything from foil wrap to plastic... .
> 
> ...



There are indeed correct answers!!! Physics is not some random science, despite the fact that some, who do not know physics, may feel about the subject of audio being _beyond_ science's ability to comprehend!:coocoo: :rofl:

There is VERY good reason to make the face of porous bass traps reflective to mid and high energies, while remaining vigilante about where that energy is reflected!

And just what is the basis for this notion of wanting "to over-deaden the room of the high frequencies"???

With all due respect, that doesn't agree with ANY proposed acoustic room response model! 
In fact, in studio and critical listening room design we don't even care about the frequencies above 10,000 Hz at all, simply because their energy content is SO low and the wavelengths so short that just about any incidence absorbs them. And NO model advocates effectively EQing the high frequencies to remove them!

Aside from the bass traps, where you DO want to make them band restricted LF traps, the specular region is measured, analyzed and treated in the TIME domain, not in the frequency domain!

And a 2'x4' velocity based bass trap wont even be seen by frequencies below ~280 Hz, as the energy simply diffracts around objects smaller than the wavelength is large! But it will make a wonderful mid-high broadband absorber!:doh:

And current gas flow resistivity based models indicate that lower gas flow resistivity material such as exhibited in the fluffed generic 'pink fluffy stuff' used in Large 'superchunk' format corner traps is not only less expensive, but outperforms the more dense semi-rigid material original dated assumptions that density was an indicator of LF frequency absorption. The fact is that the acoustical impedance and its ability to function as a frequency dependent resistance have much more to do with the material reactance than resistance.

And to jump around a bit, "liveliness" is *not* a matter of high frequencies! 
What determines whether a room is perceived as live or not is the ratio of the direct sound gain level to the first reflection gain level.

The time separation of the direct signal to the first reflection determines the sense of 'size' of a room. The longer the period, the larger the room seems.

Of course you can do anything you like. And I am certainly not interested in debating personal preferences. But whatever you desire, it should be based upon an accurate assessment of what is happening, and then actions to treat said space should utilize the proper techniques that actually and appropriately and effectively address real problems.

But I seriously challenge many of your notions as they are simply inconsistent with all of the various acoustic room response models as well as fundamental physics. And I would seriously suggest doing a bit more research before moving forward if you desire to base you actions on current acoustical physics and psycho-acoustics. 

And its pretty scary when I can rely simply on Toole's opinions to cite in a debate regarding so many of the points made! Ranging from the effect of thin absorptive panels, to small undersized porous bass traps, as well as dealing with specular issues in the frequency domain instead of the time domain....

A good place to begin learning about this behavior is D'Antonio & Cox's _Acoustic Absorbers and Diffusors_.


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## Todd Anderson (Jul 24, 2009)

If you would have read further, you would have seen that the statement that has you so excited was a mistake... Just left a word out while typing on a smart phone.

Relax. About everything.


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## basscleaner (Aug 22, 2011)

SAC:“And to jump around a bit, "liveliness" is not a matter of high frequencies! 
What determines whether a room is perceived as live or not is the ratio of the direct sound gain level to the first reflection gain level.
The time separation of the direct signal to the first reflection determines the sense of 'size' of a room. The longer the period, the larger the room seems.”

Dear SAC, let me carry a simple experiment: imagine yourself at open space with the critical stereo listening task. It’s easy to carry such an experiment like inside a fully absorbing room, for instance. If you will use your own hands like a reflecting surfaces close to your ears, you can easy hear big difference between use it or not. So, if you will move this “reflectors” far from your ears, but going to keep direction and ratio between distance and reflecting surface (making S bigger), you will sense nothing, and therefore can’t say anything about ‘size’ of a room!
Human ear can’t resolute time delay less, than 15 ms. That’s why first reflection and its delay, reffering to direct sound, is not the main parameter, which give us the sense of ‘size’ the room. I mean, the main is RT60.
And one more thing. I think, first reflection is integral parameter, but its behavior is different, depending on frequency. High frequency range has big influence on intelligibility, clarity, midrange of it - on brightness of sound. Because of it, think, it’s time to learn and control different parts of first reflections.


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## SAC (Dec 3, 2009)

basscleaner said:


> SAC:“And to jump around a bit, "liveliness" is not a matter of high frequencies!
> What determines whether a room is perceived as live or not is the ratio of the direct sound gain level to the first reflection gain level.
> The time separation of the direct signal to the first reflection determines the sense of 'size' of a room. The longer the period, the larger the room seems.”
> 
> ...


With all due respect, we don't have to cup your ears to do anything. I will also mention that in doing so you are not isolating the direct from the indirect signal arrival, instead what you are doing is only blocking the specular high frequency arrival (with wavelengths smaller than the size of the hand, which at about 4" wide would correspond to energy above ~3375 Hz) that arrives from behind the plane established by the hands. Specular energy below ~3375 exhibiting wavelengths larger than the hand will simply defract around the hand surface.

We have very sophisticated tools that not only precisely measure energy arrival behavior, but which can very precisely determine differences at each ear - commonly categorized as inter-aural cross correlation.

I am not offering some personal opinion, but researched fact. If anyone is interested in chasing down sources, PM me and I will provide them.

As far as controlling indirect energy arrival, one should be cognizant that below the Schroeder critical frequency(fc), low frequency energy acts modal and establishes standing waves. This distinctly differs from energy above fc which behaves specularly. 

Recognizing the different behavior in the spectrum between modal and specular, we employ tools specific to the two regions of behavior commonly grouped as 'bass trapping' (in ite broadest form) and treatments for specular energy commonly and broadly described by absorption, reflection/redirection, and diffusion.behaviors.

"Human ear can’t resolute time delay less, than 15 ms. That’s why first reflection and its delay, suffering to direct sound, is not the main parameter, which give us the sense of ‘size’ the room. I mean, the main is RT60."

Unfortunately that is not correct. 

You are partially referring to what is known as the Haas effect (only the interval is not limited to 15 ms) which is a subset of the Henry Precedence Effect, a period within where the initial signal provides the primary source of localization information, but where subsequent energy arrival is 'fused' by the ear-brain into a single smeared impulse - rather than experiencing them as discrete separate signals.. into what Richard Heyser referred to as "time smear distortion".

People commonly experience this in auditoriums and spaces such as airports and gymnasiums with PA systems where despite the systems exhibiting plenty of gain, you cannot understand a word that is said. That is due to the direct and uncontrolled indirect signals arriving at a sufficient gain level within the Haas interval and effectively being 'fused' into an unintelligible mess.

Also, there is no appreciable reverberant sound field in a small acoustical space. And at the high frequencies above our interest where it may be present, the levels are so low as to remain below the ambient noise level. We do NOT have a statistically random well mixed sound field where arriving energy is DEFINED to be equally probable from ALL directions! Instead specular energy can indeed be resolved with regards to gain and arrival direction - it is specular. Thus, instead of using RT calculations assuming behavior not in evidence, we use the ETC response which allows us to precisely view, identify and analyze specular energy wavelets and to determine their gain, arrival time, spectral content (if needed), direction, and spatial temporal quality (diffuseness).

And to repeat:

What determines whether a room is perceived as 'live' or not is the ratio of the direct sound gain level to the first reflection gain level.

The time separation of the direct signal to the first reflection determines the sense of 'size' of a room. The longer the period, the larger the room seems.

Reverberation manifests itself as an increase in the ambient noise level, reducing the signal to noise (S/N), which in severe cases (in large acoustical spaces) may become sufficiently loud to mask low level direct signals.


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## basscleaner (Aug 22, 2011)

Dear SAC. First of all, it’s so pleasant to take part in discussions by such a high style of it, thank you. Second, to my opinion, there is statistical base for research in such a range of room dimensions (excluding, of course, too small dimensions). Real absence of smooth diffusion field, like for big spaces, not exclude random calculating, (by Monte Carlo methods, for instance), for rooms. I don’t need in “well mixed sound field”. Look, if RT60 for room is about 500 ms and mean free path is about 15’-20’, thus there are approximately 30-40 counteractions with walls per ray, multiplied by 5 power 6, i.e. statistics>=500000. Therefore, the goal is to determine the model of probability density function, which depends on geometry, form, system position inside, point of listening, materials, furniture e.t.c.
Moreover, if I can calculate by Monte Carlo “nude” geometry, than it’s possible to continue calculations, involving next parameter (for instance, furniture elements), like a “perturbation”, with the help of perturbation theory.
That’s why not. I disagree with you, but it’s interesting to keep this discussion.


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## SAC (Dec 3, 2009)

I fear you miss the important point. This is not an issue based upon wishful thinking, feelings or belief. Rather it is based upon objective fact. Such behavior either exists in a meaningful manner, or it does not. And the pre-conditions for calculations are either sufficiently satisfied, or they are not.

This is a foundational issue that is important to understand in the measurement, analysis, understanding and treatment of small acoustical spaces.

No one doubts the potential to construct an artificial model that posits the existence of beau coup reflections that are of sufficient quantity and distribution that they may be posited to meet the one of the requirements of the RTxx definition.

Unfortunately, this definition includes a necessary pre-requisite requirement that “the RT60 is the measurement of the decay time of a well-mixed reverberant sound filed well beyond the critical point Dc” (where Dc is the critical point at which, closer to the source is dominated by the direct sound field, and further away is dominated by the reverberant sound field). 

The important distinction being that if we have a microphone located in a steady reverberant field, we could wander all over within the reverberant area and that we would not encounter any sudden change in the gain level. In a small room we do NOT have this luxury! Rather we have unique responses at each point within the room as defined by the non-uniform interaction of modes and specular energy.

The question is NOT, can we construct some method of analysis capable of artificially constructing a diffuse statistical model! The problem is that one would even consider using such a model tin an attempt to describe behavior which does not act in that manner! You are constructing a fantasy that is not applicable to the real space!

In a small acoustical space, the REALITY is that there is no appreciable well-mixed reverberant soundfield. Instead we have a space dominated by specular energy reflections above the Schroeder critical frequency. And below that critical frequency we have energy manifest as modal standing waves.

Folks like Ted Schultz, of Bolt Beranek and Newman, and Manfred Schroeder spent much time and effort to eloquently destroy the notion that significant reverberant soundfields exist in a small acoustical spaces.

Thus, the point is not to go to great lengths to creatively imagine models whereby we can erroneously construct mathematical models to pretend that such behavior exists.

What is enlightened is to instead acknowledge the reality that exists, and to analyze the REAL behavior. And to date, the problem remains that many remain unaware of the critical assumptions incorporated into the definition for the calculation of RT times, and of the actual behavior of the space – which is now able to be examined by virtue of such tools as the Envelope Time Curve. Thus, the problem is not one of fantasizing how a specular soundfield corresponds to a non-existent reverberant space, but rather of adopting our analysis to meet the real behavior of the space.

And RT times assume a behavior not in evidence, and calculate values that assume a uniform soundfield where one does not exist! In other words, they do not apply!

And this fact is foundational and fundamental to understanding the behavior of sound in a small acoustical space! And this has very real effects such as that the placement of treatment is NOT a random event, but the location of treatment is specific to the behavior. Thus you simply do not cover a third of the surface area and expect desired results as might be accomplished the far field beyond the Critical distance (Dc) in a large acoustical space featuring a well-mixed reverberant soundfield.

In other words, we become smarter than the available tools, and we chose the correct tool dependent upon its applicability to the actual situation and not simply because it exists.

If folks want to walk through the entire issue step by step, read the chapters in Large Room Acoustics and Small Room Acoustics in _Sound System Engineering_ by Davis &Davis (2nd ed) or Davis & Patronis (3rd ed). If you cannot find this in Google books or if you cannot obtain a copy of the text, you can PM me if you need a more complete explanation...Read the book first.

And you can also read comments by others such as Russ Berger (I HOPE that all are aware of who he is!!!) when they state this issue as Myth Number Eight in Exposing Acoustical Myths. But please be sure to read ALL of them!

This is an old issue.


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## basscleaner (Aug 22, 2011)

Dear SAC. You make me fall down from the discussion for a little bit time. OK, I'll do it, thanks, but I'll be back.


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