Originally Posted by hoffmand34
So are there any concerns with too much absorption on the front wall or is that what we're shooting for?
Burn the witch! Heaven forbid that it takes 5 posts before the notion even surfaces, when it should have been consideration 1 or 2 in the process.
Originally Posted by Brad Horstkotte
The front wall seems to be the one surface that there is consensus on completely deadening it.
(cue the dissenters)
And of course we have the acknowledgment of the all too common common assumption (nay, mantra...) that has prevailed for the past 20+ years in sphere of among the more enlightened in consumer acoustics who astutely realize the importance of room treatment - not at all like those nabob buggers who have not progressed to the point of recognizing the importance of room treatment...
So...maybe we should subtitle this per the plaintive recurring mantra on this forum: "Isn't doing something better than nothing?"
First, the problem stems from the assumptions implied in the question.
Let me explain.
First, what would be the purpose and practical impact of "completely deadening it"?
I am going to assume that this implies exactly what it literally says. You are going to damp ALL reflections from the surface, be they low frequencies or gamma rays.
Do all reflections have the same affect on the system? Are all 'evil'.
(What follows is NOT aimed at anyone in particular - it is a summary of the all too common attitudes commonly expressed that have dominated the 'consumer audio' community for the past 20 years ever since a few read the early articles - (primarily the article by Don Davis published in Audio in 1987) where only PART of the message was absorbed, and the body of the piece totally ignored!) and proceeded to overly simplify and misrepresent what was actually said; and which has resulted in a consumer market that is literally 20+ years behind the pro market.
Unfortunately, this is the common misnomer that has resulted from well-meaning folks mis-interpreting acronyms and simple generalizations without bothering to read the body of the work and becoming aware of the critical 'rest of the story'... After all, you need panels to eliminate first reflections, right? And you need to establish a Reflection Free Zone, and the front of an LEDE is a "dead end". And we all know that "you can't have too much absorption", right? (and I can literally source this last quote on forums stated by principles of some of the popular commercial sources!)
Every common conclusion so often tossed about here and elsewhere to that effect is ABSOLUTELY WRONG!!!!!
Welcome to the modern popular commercial and online history covering the last 20 years that constitute a perversion of what was learned in the previous 20 years upon which so many claim to base their conclusions - but of which these conclusions are little more than the result of simply reading article titles, generalizing, and failing to read the critical qualifying details.
By the way, are you aware that NOT ONE major acoustic response model ever posited that the front of a room must eliminate all reflections? Not One! Not even the Non-environment room that is the closest thing to a 'dead room" format that we have!
So get you panties in a wad with what I say, and then get over it...as the fact is that it’s the truth! And it gets old reading the same nonsense again and again and again. (Again, no one needs to become personally upset. But folks should realize the reality of the situation and recognize the substantial schism in awareness that exists. And its each individual’s choice as to what they choose to do about it…And if you do nothing, at least understand the nature and source of the schism…)
And for the purposes of this post, the desired goal is an accurate, optimal result. One that maximizes the positive without introducing destructive elements that merely serve to move the problems around and associating that with "better".
And this has also caused a false (and very tired) debate between the so called 'advocates of early reflections" and those they falsely accused of being for the elimination of "early reflections" (aka the Toole debate). A debate the is only real if you start with the same mis-characterization based upon an incomplete understanding of what was originally posited.
So it sure would be nice if a baseline awareness was established to put an end to this perpetual straw man debate!
There - it’s been said...
So...Do all reflections have the same affect on the system? Are all reflections 'evil'. And if not, WHY would anyone want to damp all reflections?
I know, I know...but all the online forums and folks selling absorption all say that you need to apply "absorption"!!!
OK, what is really known?
In small spaces we DO need to control the low modal frequencies in order to damp resonances.
And we know that "high" gain early reflections interfere with localization, imaging, and intelligibility. We also know that SOME, but not all, later arriving high gain reflections interfere with localization, while still later high gain reflections can be perceived as discrete echoes. But we also know that later arriving reflections can add a pleasing sense of spaciousness making a very small room seem larger than it actually is.
So, suddenly, not All reflections are bad! Oh my!
I'll pause for a moment to allow the shocked and dismayed to recover a bit from the notion that SOME reflections can be good, to get a drink...
We can do several things.
First, while the mirror trick is a wonderful concept hung onto and advocated fervently by a few, it originated to illustrate a concept that specular reflections functioned like rays - virtual pool balls. But there is a reason that it was NOT posited as a substitute for the tool it was used to illustrate 44 years ago! And that reason is exactly what we are trying to deal with here!
The mirror only illuminated Some of the POSSIBLE vector paths early arriving energy MIGHT take. So, if you blindly follow the mirror, you will damp every possible low order reflective path. And if you damp every possible path of early low order reflections, and you then apply the proper amount and type of absorption, what is the logical result? You end up eliminating ALL early low order reflections (with the exception of diffractive sources of which the mirror is blind!).
Congratulations! You are now contributing to a problem, and not to the solution. (Unless your goal is to build an anechoic chamber - as even the non-environment room maintained a highly reflective front wall! Go figure!)
And in response to the oft cited mantra of "yeah, well if that is all someone can do, isn't that better than nothing?" Actually, no. Its different all right. And you might even learn to live with it.
But is it optimal? Have we maximized the positive while ALSO minimizing the negative?
NO, we have not.
(And we are also ignoring that fact that the front wall is mistakenly being addressed out of context to the larger room - which is a mistake. But we will focus simply on reflections to keep things simple...)
So, let's assume that we do want to control the Low frequency Modal resonances (and mitigate any potential related SBIR issues).
To that end, in addition to the physical aspects of room dimensions, wall construction, etc. which are beyond the scope of this post, we need bass trapping. And regardless of the type of bass trapping employed, be it passive or active, velocity or pressure based, or electronic, we need to insure that it is frequency specific...that it mitigates the bass frequencies but does NOT also damp the mid and high frequencies (for reasons apparent later...)
As most are limited to porous velocity based absorption that means we need to use LOTS of it. Not 2 or 3" thick panels stuck to a boundary.
But Substantial amounts of porous absorption placed as close to the 1/4 wave spacing distance from the boundary as possible. And you can then 'remove' 1/2 of the spacing worth of material closest to the boundary as it does little to help with the cause. And what little it does do is effective only against much higher frequencies that are not our goal to stop.
So, assuming we have established a sufficient amount of bass trapping, how do we control the frequencies absorbed and make it into a low pass filter that returns the mid an high frequencies back into the room, where they can be appropriately controlled?
We apply a reflective front facing barrier.
The MINIMUM barrier that becomes sufficiently effective from about 600 Hz up is 6 mil plastic. But this can easily be replaced with more substantial or rigid facings such a 1/8" thick MDF.
The caveat? We simply make sure the angle of the facing does not reflect high gain early reflections back-into the listening position, so angling this appropriately is a simple tool to insure this. (Gee, finally a legitimate use for your mirror!)
OK, and for those lamenting the fact that they cannot use sufficient porous trapping, well, you can treat additional corners and surfaces with LF oriented porous material as well, and if that's not enough, employ complimentary active or passive systems. And if that's not enough, well, it’s not enough.
So, we have addressed the modal frequencies.
What about the early arriving specular frequencies?
First, let's dispel the nonsense that we need to eliminate ALL reflections. We not only do not Need to, we don't Want to!
What we want to control are early arriving indirect HIGH GAIN reflections of sufficient gain to trigger the ear-mind psycho-acoustic imaging, localization and intelligibility capabilities. the reason is that in this time range, our ear-mind is simply not up to the task as this is beyond the ear-brain's limit of resolution, and the result is a smearing and over simplification of sensory triggers as well as a loss of intelligibility. Now this is even more important with spoken word, as articulation is important to intelligibility - and with music we can generally get away with more indistinct 'smearing' of signals - and given the content of most stuff available now, maybe even ALLOT of smearing...And if you listen to rap, perhaps you can ignore this altogether, as the context is nothing that need be understood.
But how do we identify the HIGH gain early reflections from the low gain non-destructive reinforcing reflections? Can we use a mirror that is unable to distinguish the real from the possible, or the low from high gain reflections, and misses the very early Very destructive diffractive reflections? Hardly. But if you do, just realize the fact that you are compromising the system as much as you are improving it...
The way one easily identifies the real from the possible, and the high gain from the low gain indirect signals including diffraction, is to use the ETC response. With this one can rather easily resolve the vector path and point of incidence for the ACTUAL REAL issues. And then you STOP!
(Oh, and we are assuming that your room has sufficiently adequate resistive walls that essentially reflect all of the specular energy!)
You surgically apply ONLY enough broadband absorptive panels sufficient to mitigate the FULL range of the incident reflective energy. That's it! No more, no less.
And for those who have any number of reasons for using less, understand that absorption that is not sufficient to damp the full specular bandpass will result in coloration of the first signal. Its that simple.
And a some point in the precess, hopefully earlier rather than later, one should stop and think: just where are the reflections off the front wall originating, and what is their nature? Are they direct first order reflections from a speaker featuring terrible pattern control? (If so, get new speakers!) Or are they higher order low gain semi-diffuse reflections that have bounced around the room that have somehow managed to avoid the 'kill all the reflections' routine that have managed to persist long enough to be incident on the front wall? And do we really
want to get rid of semi-diffuse later arriving reflections????
So, do you still want to indiscriminately place absorption all over the front wall?
First, let’s examine the case for modal and ALL early arriving specular reflections, and then we will narrow the issue to the front wall…
And then, is it 'thick enough' to actually control modal frequencies?
If not, it is simply going to damp ALL of the early and late higher frequency (but not necessarily all of the indirect signal bandpass) specular reflections incident off the front wall. And do you really have that many early high gain reflections off the front wall that impede on the listening spot(s)? (If so, i would examine your speakers!!! as i would suggest that you could benefit from a speaker with more controlled dispersion...).
Is the front wall REALLY a prominent source of early arriving specular reflections? (Just out of curiosity, how many have tried using a mirror to see???? And how many using the ETC have identified front wall high gain early reflections??? Anyone? Is this a REAL problem, or an imagined problem?
And do you really want to damp all of the early low gain, as well as the later low gain reflections (that contribute to a positive sense of spaciousness to the room) off the front wall? What acoustical response model, aside from an anechoic chamber suggests that response?
And assuming the power response of most speakers, where the low frequencies tend to be more cardioid and the highs beam, what frequencies would comprise the majority of early incident energy. Here, let me help – it will be LOW frequencies. And how effective is 2-3 inch porous absorption applied directly to a zero velocity boundary against LOW_MID frequencies? But the 2-3” of absorption applied directly to the boundary is GREAT for eliminating the high frequency energy that is already in too short supply!
The result: we remove still more of the beneficial low gain high frequency energy and totally ignore the problematic low-mids.
So, is “doing something better than nothing” if we create more problems without resolving the imagined problem that we have let to even verify exists???? Namely, do we have a problem with early high gain reflections? (Just to clarify - we MIGHT, as who knows what degree of cabinet resonance or uncontrolled Q the speakers exhibit! So while a well controlled spatial dispersion speaker most likely will not, all bets are off for the majority of speakers out there!) But what does common sense, let alone measurements that will CONCLUSIVELY provide the answer – one that the mirror can’t even guess at?
You see, instead of beginning with the mistaken mindset that we must "eliminate reflections" and stopping to examine both the modal and specular regions, and also WHAT specular reflections are actually detrimental and which are reinforcing, the situation become both more complex, but actually simpler in many ways.
So what is the 'better way':1.)One, we focus on addressing modal resonance with frequency selective high pass bass traps.*
2.) Using the ETC, we specifically identity ONLY the early high gain reflections that require treating and resolve their specific points of boundary incidence.
3.) and then we Surgically treat ONLY those limited regions with a sufficiently broadband absorptive panel**
- which saves you time, money and effort buying, building and installing all sorts of additional insufficiently broadband absorption that only makes the existing problems LARGER.
And note that the finite set of broadband panels must be sufficient to absorb the full specular bandpass from about 300 Hz up or you will not eliminate the destructive effects of superposition that negatively affect imaging, localization and intelligibility, but you also ADD an additional problem called coloration to the mix. So if you are ready to scrimp on broadband absorptive panels (as opposed to employing other more advanced/complex treatment types of which little has been mentioned), you decide is 'something inadequate' is better than nothing...
So, you see... it’s really not that complicated. But the question and assumptions you begin with had better be well formed or the result, however well intentioned will not be optimal. And the result will be one of either a dead 'anechoic' claustrophobic environment where imaging cues are easily misinterpreted; or perhaps just as bad, a result where you have simply moved problems around in an environment that still suffers from modal resonance, as well as imaging, localization and intelligibility issues, along with the addition of coloration to the mix.
So you tell me, was "merely doing something (at a pretty fair expense in terms of material and time) better than doing nothing (that costs you nothing)?" That's for you to decide. Me, I suggest doing it properly.
And if you have compelling reasons why this cannot be done, and if you are going to spend the money, I would suggest calling a pro who is capable of possibly providing additional OBJECTIVELY verifiable alternative technologies that result in OBJECTIVELY verifiable measured results, be they passive, active, resonant, or whatever, to address the issues in the manner prescribed - being sure that you get a REAL return on your investment. (Or, failing that, you can convince yourself that fancy new expensive interconnects will make a real improvement and go out and buy and install them and sit back convinced that you just radically improved your system. And I dare say if you have already convinced yourself of their value, you will (most likely) at least feel
like it sounds better!)
And beware of anyone who claims they can provide a solution that magically overcomes the issues mentioned (thin porous bass traps, thin non-spaced porous specular broadband panels, etc.) using traditional techniques.
And in the end, baring some really bizarre spatial dispersion characteristics of the speakers, I would suggest that the 2-3" of porous absorption applied directly to a zero velocity boundary will neither mitigate low frequency modes nor is it sufficient to mitigate any broadband (or the more likely low-mid) high gain specular reflections that most likely do not even exist off this surface And if that were not enough, it will be ineffectual against any low mid specular energy that may result from the speaker's uneven power response that can still detrimentally impact localization, imaging and intelligibility in addition to coloring the direct sound. And to think you spent time, energy and money to augment that! Congrats!.
And the net result will be allot of money, time and effort spent that will damp low gain ambient mid-high frequency energy that, depending upon its arrival time, is beneficial to the sense of envelopment and spaciousness while missing any high gain low mids specular reflections that will still cause the problems you originally imagined you would solve. Ouch!
But you will have a sense of accomplishment and the all important sense of "having done something" that a few have posited as being so important.
So, in the end I guess we can, without exceptional objective verification of a real problem to solve, better label the "doing something rather than nothing" approach a therapeutic exercise rather than an actual acoustical exercise. Hey, but that can be worth something...
* ...as well as other appropriate complementary methods aside from porous absorption.
**We have intentionally left out diffusion, simply to make the analysis simpler. But if the actual reflections meet the best practice standards for the use of diffusion without adversely impacting the listening spot, diffusion can certainly be properly employed. And the appropriateness of this option can be assessed with the ETC response.