Hi all

I recently moved to a larger house and so am making some new panels for the first reflection points on the side walls and ceiling. The fabric I used 4 years ago I can now no longer get, so I've been hunting for something new that will match the old fabric. I've got samples of Camira Cara but none of the colours work for me. Not a fan of the hessians, etc. either.

I've got samples of these from BBrown:

BBrown

BBrown

both of which I like the aesthetics of. But they do not seem as breathable as the Cara - which I'm using as my yard stick seeing as it's so popular. I can blow/breathe through them, just not as easily as with the Cara. There's some resistance. And visually the weave is a lot tighter than Cara. Although the Trilogy does say it is suitable for acoustic paneling:

BBrown

I read with interest what SAC said about the "grazing angles" of fabrics. I've read some who say that even Guildford of Maine is not totally transparent......and others who say that as long as the material is soft and won't asphyxiate the missus if you tie it round her gob for a few minutes (and enjoy the silence), it will work fine.

I sometimes came across threads suggesting the use of foam on the front of the panels...... Arksun ran some interesting tests in one thread where he found foam to have excellent absorption at very high frequencies. So although I'm all spent out financially, I'm thinking of seeing if I can get funds together to buy some foam and go down this road.

Just wondered what people's thoughts are? And also, if I go for the foam what sort I should get (thickness, sculpted, etc. but also whether cheaper foam would be suitable)?

Cheers all

Max

PS - I have a few samples of the BBrown Trilogy I linked to above. If anyone is interested in volunteering their knowledge and expertise, I would happily send a sample for people to test and report back on RE suitability. The fabric looks great, is fire resistant, a good price, etc. So if anyone could confirm its suitability it could become another popular recommendation alongside Cara for those of us on the side of the pond where GOM cannot reach.

I will simply provide another source to corroborate the claim regarding the 'transparency ' of materials.

Exposing Acoustical Myths by Russ Berger

Check out #4.


While many may recognize the author's name, I suspect few understand why this reference is SO significant.

In 1978, when the TEF licenses were first made available with Don and Carolyn Davis appointed as license 'executors' for Dick Heyser and the Jet Propulsion Laboratory, Russ obtained a license (with a funny story behind the scenes here as well) and assembled the (literally!!!) $30,000-$40,000 worth of HP and GenRad equipment necessary at the time to assemble a TEF analyzer and then proceeded to measure anything and everything that did not run faster than he could!

Few have the depth of experience Russ does in this area. When he speaks, people would be wise to listen. Few have the first hand experience that he possesses!

And I am curious to know why one would think that foam would exhibit any less a tendency to reflect at high incident angles than would fabric?

Thanks for the reply.

I'll admit first of all that this is a total guess..... but I thought that if the foam was sculpted as opposed to the flatness of fabric, that the HFs reaching the panel at high incident angles would reflect differently.

More generally though, I thought foam (on top of the mineral wool slab - in my case Rocksilk RS60) would do a better job at absorbing HFs than fabric/RS60 alone would.

Pardon me as I temporarily engage in a bit of curmudgeonly digression...heh

The frequency of reflections and of absorption...
Not energy content, not gain, not arrival time...but frequency.

I am curious to know why folks keep arbitrarily breaking specular reflections down in terms of frequency. And variations of this theme abound in the majority of posts regarding reflections.

What is actually happening in far too many posts on the forum is a roundabout discussion of what in many cases actually amounts to equalization, in that we tend to focus on addressing spectral components of the reflections instead of the energy associated with the totality of the reflections. And the former is exactly what EQ accomplishes in the direct signal.

With all due respect, as high frequencies have much less energy content, and as they have such short wavelengths they are MUCH more easily damped, why are so many worried that the high frequencies will not be absorbed, but the high energy longer wavelength mid frequencies will!?

Energy reflects more easily at high incidence angles. And if we are using broadband absorption, there is no reason to manufacture some frequency dependent issue. We desire the mitigation of the full range of energy, not just portions of its spectrum. The only 'frequency' dependent 'issue' should be the low frequency extension of the broad band absorption - but few ever express concern about this as the focus continues to be some misguided imagined EQ of the rooms' response!! Generally speaking, you are not going to encounter broadband absorptive materials that absorb low frequency energy and reflect high frequency energy! If anything, it will be the opposite. But I cannot recall a single post where someone was concerned that the mids or lows will remain while the highs are absorbed with regards to broadband absorbers. Instead, our concern should be with the broadband absorption of reflected specular energy. Note, that aside from the implied reference to the low end cut-off Schroeder critical frequency (in other words, as we dealing with modes or specular reflections), that there is no mention of frequency.

And to take this further, it is common to propose using materials that absorb highs to the exclusion of mid band energy. Why does one think that we generally discourage 1-2" thick broad band panels as well as most foams? It is because they are not efficient broadband absorbers, as their mid and lower range of absorption is severely limited. The goal is relatively even or controlled absorption over a given bandpass.

Its time more people looked at the time domain ETC and focused on what it shows.Its characteristics are displayed in terms of gain and time. There is no frequency scale. There is a reason the time domain takes precedence here, as the time relationships are causal with regard to the derivative anomalies that appear in the frequency domain.

While there are potential cases where one has reason to focus on a bandwidth limited issue, we have yet to encounter one in the general discussions of absorption and diffusion on the forum. And thus far in this general area, as has been mentioned, the only significant frequency issue of note in the discussion of reflections is that of the Schroeder critical frequency that marks the transition 'point' from modal standing wave behavior to specular reflection - of which about the only real ramification is the choice of proper treatment method based upon an identification of the issue at hand.

And it serves little purpose to have a mess of posts now explain why some people imagine the incorrect issue being of import instead of folks beginning the form a better grasp of the proper behavior models. I understand that this is a change from how many have always thought of sound. But simply holding onto the frequency response like Linus does his blanket is not the answer. The first step is to let go of the dependency of frequency.heh

Hi

I must admit, I cannot fully understand the points you make (because of my lack of knowledge and understanding that is!). I still hugely appreciate you taking the time to respond in such detail.

But I'm still a little unclear as to whether a layer of foam on the face of my RS60 100mm reflection point panels would be more effective that simply facing them with fabric. And I dare not draw my own conclusions from what you've said for fear of getting the wrong end of the stick!

Would also be good to hear from anyone in the UK who might have used Trilogy from BBrown. Also, anyone anywhere who would be happy to receive a sample from me and give their opinion as to how effective it would be...

Cheers

Max

Without knowing anything about the foam but ASSUMING (yeah, right!) that it is 100% absorptive, the only real benefit in adding the foam would be in adding "thickness" to the absorber.

Foam in general tends to be more reactive than either cloth or porous fibrous material, and as such I would not recommend adding the material simply in order to reduce reflections from the surface.

The problem with this scenario is that we can speculate and speculate for days over this issue and still be no closer to a practical solution. heh


But here's a simple way to avoid all of the unfounded speculation about the speculation based on the speculation! As instead of making this process more complicated and filled with even more speculation... as the goal should be to reduce the uncertainty and to increase the effectiveness of the absorption.

Simply make on axis and off axis ETC responses (with RoomEQWizard or FuzzMeasur.)!

They will tell you exactly if and what reflections occur!

Reflections tend to increase as the incidence angle deviates from 90 degrees (perpendicular).

So the first thing to try if you have reflections off the panels would be to re-orient your panels to reduce the grazing angle (Angle one side by spacing one side of the panel so that the panel is a little further out then the other side).

Then repeat the measurement to verify the results.
That would be the simplest solution!

And if problems still persist, then by varying only one variable at a time, you can repeat the process and note the changes...hopefully refining the process and increasing the desired results.

Isn't that a whole lot simpler than all of the speculation and angst and additional time and money spent on additional materials that still results only in uncertainty?

SAC,

I think what's going on here, and I know you know it, is the OP is concerned about what a tight weave fabric can do to the effectiveness of a broadband trap in higher frequencies. Of course he's not ONLY concerned with high frequencies, or he could simply hang a tapestry and forgo the trap. The concern is what the fabric is doing, and I can't see why this isn't a legitimate concern, at least not one worth chastising over.

I've seen interactions between you and Max on several threads now, and he's been nothing but courteous and appreciative. It seems you are attempting to push towards breaking. Why?

The issue of "HF" reflections has been addressed, as well as the basic issue of reflections off surfaces at grazing angles.

As one might note, I was cited as the source of much of that information. What I was not responsible for was identifying the issue interns of frequency content. The issue is real if framed in terms of energy.

Whether foam is a solution is questionable at best. The reason (which I guess one should not be told) is due to its acoustical impedance.

A proposed simple solution to both determining the actuality of the problem as well as the effectiveness of a solution has been offered.


As to the digression - identified and labeled as such and directed to the larger issue beyond simply this thread - it is also valid. And it was directed at no specific individual!

(But it is interesting to note that some are quick to adopt the victim mentality. That is an issue for them to address elsewhere.)

The preponderance of questions on the forum regarding reflections continue to focus on the frequency content and not the broad band energy.

And while many still remain dependent upon the frequency response (and domain) and want to address specular reflections by frequency rather than as "energy" that must be managed, it is, for the vast set of instances, a mistake. Unfortunately, hearing this is apparently at odds with the ever developing educational and social norms where one avoids noting if a contribution is valid or invalid and instead rewards participation - as if it is all good. If only physics and acoustics followed those rules.

Thus a general admonition to cease conceptualizing and dealing with specular reflections in terms of frequency and to instead focus on the behavior of specular directed energy in the time domain.

People can either continue to ignore the issue and continue to chase snipe, and then become upset if they hear opinions to the contrary; or they can accept a different (time based) approach to the concept (and heaven forbid seek more information regarding the topic to the degree that it is confusing within the "difficult subject") and at the very least 'struggle' with a more productive line of reasoning. Especially as so many other visitors to the forum have proposed 'new alternatives' that deviate from the 'accepted' solutions while in the same breath admitting that they do not even understand the basis for the 'accepted' solutions.

%%%

I have used pyramid sculpted foams for HF control and been very happy with the result. I would have no hesitation in using Auralex, Prima, etc.
However, I would have thought it to be much more expensive than fabric and it does decay. I wouldn't sweat this too much. Why not divert the energy to more area and careful placement, including angling of the panels if necessary.This would trump small fabric issues IMHO.
I have seen it stated somewhere that Hessian is a perfect complement to 703.

DD

Thanks to one and all for the responses.

I think what often happens in forums such as these is you have some people with a huge wealth of knowledge and expertise mingling with others who have very little. So whilst I really appreciate your answers SAC they are unfortunately a bit over my head!

johndykstra has pretty much got it right. I want to know if my fabric choices are suitable. And whilst I looked into that the idea of foam as a cover dawned on me as well.

But the foam was only an idea because I had gathered the impression that it might more effective at dealing with high frequencies than the front face RS60 mineral wool covered with fabric. Perhaps I got that wrong..... But either way, I've now ruled that option out in any case.

In the meantime, I'm feeling quite confident that the BBrown Trilogy will work well on the panels, but it would still be nice to know for sure before I go ahead and get the money out. So if anyone has used it, or has an opinion, or would volunteer for me to send them a sample or two of fabrics to blow through (and perhaps a thank you token or two for you to spend on a pint!)....then please let me know.

All the best

Max

The thing is, acoustic treatment is not a simple matter, so you can't do a good job if you don't take the time to learn how it works. You can't treat a room with vague generalities.

SAC is giving you everything on a silver platter. You may have to study a lot more before it makes sense but you won't do a good job of treatment until you do.

I totally agree Paul..... but in this instance I simply want to know whether a particular fabric is suitable/acoustically transparent.

Seeing as fabric is commonly used with these sorts of panels, I'm hoping that some people - esp in the UK - may have used or tested the Trilogy fabric from BBrown and be in a position to comment on its suitability. Failing that, I was hoping that someone who knows what they were doing would volunteer their skills by agreeing to receive a sample in the post and take a look at it/try breathing through it, etc.

I would happily try SACs tests but I only have small fabric samples at the moment and would rather only spend the money if I know it will be suitable.

Cheers

Max

Folks, part of the problem here is that what is wanted a simple "answer" to a very complex issue. And not to disappoint, there is a rather simple answer - but one that has not thus far proved acceptable.

A discussion of all of the physical parameters and considerations surrounding the specular reflectance sampling technique is far beyond the scope of this post or even a thread.

Most wave theory has been historically modeled in optics, where we have the advantage of dealing with select wavelengths, as opposed to acoustics where we generally are dealing with a range of wavelengths and frequencies.

Reflections are also modeled in general terms regarding their incidence on smooth and rough surfaces, with roughness being defined in terms of the relationship of surface variations relative to the wavelength of the incident signal.

When sound strikes a flat surface, it is reflected in a coherent manner provided that the dimension of the reflective surface is large compared to the wavelength of the sound. As audible sound has a very wide frequency range (say from 20 to about 17000 Hz), it exhibits a very wide range of wavelengths (from about .79 inches to 56.3 feet). As a result, the overall nature of the reflection varies according to the texture and structure of the surface. For example, porous materials will absorb some energy, and rough materials (where rough is relative to the wavelength) tend to reflect in many directions—to scatter the energy, rather than to reflect it coherently.

And when you consider that the wavelength for a 17 kHz signal is .79 inches, many surfaces that one might conventionally consider rough are in terms of the model considered smooth.

Using optics as a simplified guide, in the attached diagrams, the incident wave represented by Itheta strikes a sample, n2, possessing some refractive index determined by its acoustical impedance, at an angle of incidence, θ1.

Some of the incident signal is reflected from the sample surface, represented by IR at the incident angle, θ1. Some of the incident signal is transmitted into the sample represented by IT at an angle of θ2.

As predicted by Fresnel equations, the percent of reflected vs. transmitted light increases with higher angles of incidence of the illuminating beam. Furthermore, the refractive index of the sample, surface roughness and sample absorption coefficient (as per the acoustical impedance of the material) at a given wavelength all contribute to the intensity of the reflected signal.

At high angles of incidence, between 60 and 85 degrees, reflected energy is typically increased relative to a near normal angle of incidence.

This is complicated by our having to deal with a range of wavelengths. Additionally, we do not have ready access to the acoustical impedances of the various substrates that define the incident boundary - complicated by the fact that this boundary is not simply a single layer (as in the first diagram), but rather it is a complex boundary consisting of multiple materials with various acoustical impedances (as in 2nd diagram).

Calculating just one such scenario is formidable at best, but now it is proposed that we accurately predict the variance based upon interchanging the thin film boundary material (the cloth) based upon the change in acoustical impedance that it offers. And to complicate matters still further, neither the users nor the manufacturers of the material nor the underlying substrate provide acoustical impedance values.

Like modal behavior, it often appears to be a simple calculation IF we severely restrict the variables to overly simplified models operating withing severely restricted degrees of freedom. In other words, if we simply ignore a whole lot of contributing real world factors, we can come up with a very general guesstimate.

With acoustic wave incidence with wavelengths considerably larger than the surface roughness and/or perforation, the easiest similar guesstimate is to state that with the increase in incidence, the amount of energy absorbed decreases and the among of reflected energy increases.

And like modal behavior, the precise calculation based upon an accurate model based upon a great deal of complex data that we quite frankly lack, is not reasonably possible.

But, as with modal behavior, there IS an rather simple method for determining the precise behavior! And that is to measure the reflectance relative to the source signal and to obtain the boundary transfer function in terms of gain. And that is to measure the ETC response. Not only do we obtain the gain of the direct signal, but we obtain the gain of the reflected signal. And allowing for absorption due to air, we can reasonably estimate the degree of absorption of the material.

Hopefully one will realize that minute changes in surface characteristics of a material relative to the wavelength of .79 inches at 17kHz is not going to mean a whole heck of allot. And I would venture to say that neither is it going to make a significant difference to an incident wavelength of 56.3 feet at 20 Hz.

In fact, one might even say that with such relative differences, that the surface is essentially a smooth solid barrier whose determinate characteristic with regards to absorption is literally its acoustical impedance. The significance of this being that for any given acoustical impedance, the greater the angle of incidence, the greater the amount of reflection!

Something we suggested long before we entered into this discussion. And thus the single most significant factor under our control is the orientation between source and barrier, where the single most determinate factor is to reduce such angle of incidence in order to reduce the reflection.



And if the issue is that significant, invest in a yard or two of the material and test it. And take into consideration just how many people here are actively equipped to perform such testing! And how many have an impedance tube to determine the acoustical impedance of small samples, let alone a walk-in device! Thus the question is not really a general question directed at many at all - unless one simply wants allot of guesses - educated or not.

And even if they are and have done the test, I can confidently predict that the angle of incidence is going to be a larger contributory factor than the material itself - as with the ratio of wavelength to surface dimension, ALL will be smooth rather than rough, and all will be reflective at high incidence angles.

And we can save a discussion of porosity and perforation transparency relative to the gas flow resistance at near normal angles for another time.

Here is the problem with applying a thin corrugated foam surface layer in order to ‘prevent’ reflection. (Note: This is not a thread simply comparing foam and fibrous material! Nor does it address acoustical impedance or permeability.)

First, we are concerned with controlling reflected energy – ALL reflected specular energy.

Energy content is not distributed evenly across the frequency spectrum. Long wavelength low frequencies have the largest energy ‘content’, while the energy content steadily declines with advancing frequency and the shorter wavelengths.

So when we refer to broadband absorbers, one might say we are really primarily concerned with low-midrange absorbers, as those are the more difficult frequencies to address. But, as addressing the low mids also effectively addresses the highs, all is well if our absorbers are indeed ‘broadband’.

And as such, we are not interested in effectively EQing the room’s energy content. We are content in large measure to simply control the dispersion of the direct energy in its original spectral content to the largest degree possible, as there are more appropriate places to EQ the direct signal.

Thus, with regards to broadband absorbers addressing specular energy, we do not want the treatment to selectively remove more of one part of the spectral content relative to another.

To those ends, it is important to recognize that the porous absorbers require ‘size’ in order to be seen by various frequencies. The lower the frequency of the energy, the longer the wavelength, and as such in order for a porous absorber to be seen and to prevent the wave from simply diffracting around it, the absorber must possess sufficient size.

A common depth for a broadband absorber tends to be 4” with a gap of 4” – making the panel effectively about 8” thick. (Using a simple model)That thickness alone will be effective to about 1687 Hz – and utilizing quarter wave mechanics, the effective thickness is thus (4 x 8)=32 inches and thus the lowest effective frequency becomes about 421 Hz. (Note, this is assuming normal incident angles for simplicity.)

Above, we already discussed how the amount of absorption decreases and the amount of reflection increases with increasing incident angles until the degree of reflection is maximized in the ‘grazing’ incident range of between about 60-85 degrees from normal (perpendicular).

Thus, the single most effective technique we can use to increase the degree of absorption in a panel is to orient it so that it is as near to perpendicular to the source as possible. But we often accept reasonable compromises here.

But as has been observed, reflections can indeed occur. If these are of sufficiently low energy content and gain, we tend to ignore them. But many times they are above the minimum levels we would like to see.

Which brings us to the topic of which fabric is most acoustically transparent in order to minimize this. This actually involves many factors, including acoustical impedance, porosity/permeability of the surface, the k factor, and other variables… subjects that are beyond the scope of this post (and which we simply do not know in the case above!). But let’s simplify things and assume that the permeability of the fabric is acceptable and that the largest determinant variable is the orientation of the panel with respect to the incident angle.

Thus, to use a metaphor, we are talking about whether we are dropping a rock directly from above (normal) to the surface of a body of water, or if we are throwing a rock so that it glances and skips off the surface at a high grazing angle like one skips rocks on the water. In order to reduce the refection, we ideally will reduce the angle of incidence of the incident signal.



So, if we do use a corrugated foam layer on the surface, what happens? (Oh, and for this we will posit that the corrugation is aligned perfectly perpendicularly with respect to incident energy for maximum effectiveness!)

The corrugations will present a less oblique angled surface – but ONLY for the small wavelengths that are smaller than the corrugations. The remaining energy simply diffracts around them as if they are not there.

And what wavelengths would those be?

Let’s see…So, what size corrugation do you want to use? ½”; 1”; 1.5”?

So, if any of these dimensions are chosen, the effective low frequency cutoff will be ~27,000Hz; 13,500 Hz; or 9000 Hz respectively. In each case the effective frequencies are excessively high, and they effectively remove only a small part of the spectrum of energy and effectively EQ the reflected signal, leaving the remaining energy to be reflected.

And quite frankly, we don’t even care about those high frequencies as they are so short as to be stopped by ANY impediment and because they contain Very little energy. So you see, while such an application can have an effect, it is not a comprehensive solution, and creates undesirable spectral imbalance. (In the process of partially solving a problem, it introduces still others...)

Another common possibility that is often seen is that people will not simply keep the basic panel in place and add a foam surface, but often they will instead use, say, a foam panel that has a total thickness of 4 inches and from the 4 inch thickness, mass is scooped out (removed) to make the corrugated surface – with the net result that we are now missing part of the ‘original’ material – the stuff that dissipates the energy as heat as the waves moves through it. So now we have even less ‘stuff’ to effectively resist the wave energy and we lower the overall effectiveness of the panel to absorb the lower frequency energy. And this too tends to ‘EQ’ the reflected signal still further from the source signal. In order for corrugation to work, it needs to be large enough to be seen by all of the incident wavelengths with enough material present to effectively dissipate the energy – which requires that such material would be thicker than the original panel to maintain sufficient mass and to also provide the angled surface. But that is not what has been proposed, and as such, it fails to satisfy the broadband requirements of our broadband panel.

And in either case, we tend to affect the overall tonal quality of the total sound by modifying the spectral content of the component energy – which was not our original intent, which was to control all of the reflected energy without modification except in terms of gain.

So this is why we certainly “can” use foam as a covering, but we “may” not without unequally affecting the spectral balance of the reflection. And why, rather than adding a material that will affect one part of the spectral content, we prefer to address the issue in a manner that will address all of the specular energy in total.

I can't believe the sheer amount of things I've learned from SAC on this forum.

It takes both the knowledge and the dedication to write posts like these ...

Thanks a million times for your contributions SAC !

Nick

Yes I agree Nick and appreciate it hugely. I've had a quick read through SAC's posts, but am at work so will look in more details and take time to digest it when I get home.

One thing I am interested to know SAC is whether you yourself have fabric covered panels at first reflection points and if so which fabric you chose to use.....

I think I will go ahead and - like you suggest - buy some of the fabric and test it. I must admit, I don't know how to do the test or what it is, but I'll get the program and read up. If anyone has any links to how to do the test that would be great. But I'm sure I'll find something if not.....

Cheers

Max

If we are discussing panels intended to absorb first order reflection; how often is there a problem with incident reflection angles of say 45 degrees or more?

Max, is it really worth all this detail? Any fabric you can blow through will work fine, including weed control netting and such. Cara is used by a well known maker, most likely tested. Hessian is available in colours and fireproofed. There is old data showing it to be a great match for 703 type panels. Foam is expensive, doesn't cover the whole panel, crumbles with time, and explodes in a fire. WTF.

If you really are into this topic, get REW or FM, study how to do ETC measurements. Search for stuff by LUPO or just ask him. It's quite simple.

The most important thing I note in all of the above could easily be lost with all the foaming.

Side Reflection and Cloud Panels may perform much better at the listening position by being angled. That's for real.

DD

Thanks DanDan. But "all this detail" you mention has been provided not requested!

As for the foam, as I said in post 12 - I have ruled out that option and all I now want to know is whether someone here has used the product I refer to from BBrown (Trilogy), and if not if someone would mind receiving a sample from me (along with a the price fo a beer or two for their trouble) to give their opinion.

I too have read in many places that any fabric you can blow through would be fine. I also recall Eric Desart saying it is far more difficult to choose a wrong fabric than a right one (he also says nearly all fabrics in a store would be suitable). However, some points raised here have led me to question this, which is why I feel the need to make sure the Trilogy from BBrown would be suitable.

I would happily use Cara if it came in a colour that I liked/was suitable.

Cheers

Max

But what cloth does he use? Does he have it? Is all the detail necessary? …..

The purpose was simply to illustrate considerations regarding what actually is going on and the ramifications of some commonly considered options that may at first seem to be quite reasonable – and to examine the options a bit more closely in terms of our desired goals.

The answer still lies, as has been the mantra, in process rather than in a particular whiz bang singular solution.

And still, with responses like – well, what do you have and use – instead of examining the particular topology of the space relationships and availing ourselves to the tools at our ready disposal - I fear we are still at risk of missing the larger point.

Folks are still looking for “the answer”. Well, I have good news and bad news. The answer is available. But it’s not simply in the form of “this brand” or “that brand”.

And even more importantly, what was presented above is just a small example of examining the behavior of Band-Aids before assuming that they actually solve the problems without creating additional problems.

The first thing that seems to have some really bothered is to learn that “absorbers” – those things we assume ‘absorb’ also have a reflective component. And it’s not just due to the cloth covering.

Is this a problem? Maybe, maybe not. But it is an aspect of the behavior of one's tools that one should be aware. Can this behavior be identified? Sure! This is just one more benefit of learning to use the ETC response. Used properly, it not only helps you determine the actual behavior in a space, but it provides you with accurate feedback to know what happens after treatment is applied. Thus the notion that consists of: ‘Well, we finished putting up all the treatment, so we’re done’, is not quite accurate. Verified proof of performance and iterative adjustment as necessary is prudent as well.

As should be the case in all error analysis (as distinct from MISTAKES), we examine a system for the attributes which present predictable deviation of behavior in a system and examine how they skew total system performance. And via critical evaluation, we better understand the how and why of the system behavior such that we can create an hierarchy of potential solutions, identifying those steps which will have the most positive impact with the least amount of work and expense (the largest positive ROI).

What one should be most concerned about is the incidence angles. How is your room set up. Where are your speakers positioned and how are they oriented, and what is their power response (polar dispersion). In choosing a room topology and imagining that panels will solve whatever problem we literally throw at them, do we take orientation into consideration? And if we do have reflections off the panels, do we want a topical covering that will only address very high frequencies and ignore the preponderance of the potentially reflected energy? I would suggest that we do not. (Hence why I took the time to explore the notion of corrugated foam coverings...)

But there are other considerations. What is being done with said energy and how does it affect the space? Does the eflected energy impede upon the ISD? Do we need it to be dissipated 'here and now'? Is it simply being redirected to another absorber capable of containing it? Are we seeking to return it in a latter arriving diffuse soundfield? Will it be incident upon a secondary seating area of significance? There are many questions determinate upon the specific space. Some may apply, some may not, depending upon the specific setting.

But in all, with regard to this particular situation, which is easier? Obsessing over which brand of material is imagined to magically solve a potential issue, or would paying a bit more attention to incident angles and thus potentially minimizing any negative impact throughout the process such that it would have been easier to have avoided creating the problem to begin with?

(And again, let me reiterate – the problem with reflection off panel absorbers is NOT simply a function of the material covering! We have simplified the issue to focus solely on the immediate interface while ignoring the larger acoustical impedance of the boundary!)

And the irony is that the common response is to now hold the material covering choice accountable for solving these issues! The real issue is far more fundamental (when the fact is, the absorbent substrate also exhibits similar characteristics!).

While one should attempt to insure that the treatment is not overly dense and reflective, the overall position and alignment of the various components relative to the source is a bit more determinant of this situation than is simply the fabric covering. And a failure to plan with the entire system in mind is not resolved by a last minute rush to find a perfectly non-reflective material. Even if you go to one of the more open weave speaker grill materials, you still won’t find a “smooth surface” material that will magically stop all grazing reflections.

Thus may I suggest folks expand their awareness of what actually happens with each component in the system and view the slightly more complex nature of relationships that result. The solution is typically not with some magic component that resolves this complex web of interaction, but in paying attention to the details (intended and unintended) within the total system.

Oh, and to revisit an old subject of contention, this is a primary reason I advocate the use of such tools as the ETC response rather than the mirror technique. Don’t get me wrong! The mirror technique was ironically first used to demonstrate the concept of the reflection behavior the ETC illuminated. And it works fine within a limited scope. But it is not enough. There are many behaviors (diffraction being a ‘big one, and near field behavior being another) that it does not illuminate. It also cannot provide information about reflective behavior off absorptive materials – as its use assumes that absorption absorbs!

But beyond the use of the mirror for gross general positioning, the ETC provides a wealth of information about heretofore unseen or unimagined behaviors. It has the ability to ‘see’ the distribution of acoustic energy through a space and provide us with both time/distance/path and gain information. This allows us to identify where energy is coming from and to where it is going, and at what gain levels – relationships of which would be beneficial for more to understand as well (as we too often simply assume interaction behaviors at or near unity gain levels!)

Sometimes it uncovers behaviors that we might have otherwise missed. Other times it verifies that we need not be further concerned with problems that do not exist – and BOTH are important.

But the larger aspect is that the tool effectively affords us the equivalent of an acoustical “X-Ray” vision the ads in the comic books always promised us! It seems simple to me, but I am amazed that more do not avail themselves of such a powerful tool. There are many good reasons why this tool is indeed the Swiss Army knife of acoustics, and why the ETC response easy supplants the frequency response as the one most useful patient chart if one is so limited.

Well, half of my lunch is now gone (the time, not the food!) and I still need to eat and then do more important things, so this 25 minute foray into the swamps of things acoustic must come to an end. And as always, for those who wasted your time reading something you find worthless, well, the jokes on you. For others, if you have questions, you are always welcome to PM me.

Thank you SAC.

Typical, I would call that a contradiction, leading to inevitable confusion.
Only testing can really answer your question properly.
Without considering in detail I wonder how one would do such a test and what conclusions could one expect?
e.g. Test Trilogy and you will get visible lines on an ETC graph.
The question will remain, how good or how bad is it? And compared to what?

I just looked up Trilogy on the BBrown site.
How about you email or call them and ask if they have reassuring evidence that this stuff is perfect for acoustic panels?
In my experience with other fabric suppliers, they did have answers to this question. (Camira)

DD

Hi DanDan

I tried that one and unfortunately they had nothing to say of any use with regard to using it for acoustic paneling.

Also, you mentioned searching for threads/posts by Lupo for info on ETC measurements, but I wondered if there are any other guides or references/"go to" sources of info about taking the measurements, "etc."!

SAC did one post which said all you need to know about ETC.
You may need to contact him directly to get the link.
However, note my caveat. An ETC measurement may show how much reflection there is, at different angles if you wish.
And what use would that be? How much is good or bad, and compared to what?
I do know that Camira is used by notable UK acousticians.

DD

I have used trilogy as specified architecturally, it is thicker and visibly less porous than Cara or even Lucia. BBrown insist its ok for acoustic panels but i'll reserve judgement until some testing is done - I guess impedance tube testing would give comparison to other fabrics, setting up a proper lab test would be pretty costly

There are not that many contract verticle surface fabrics available to choose from though, Camira do a huge range, but only 2 or 3 of the range get regular specification, Bradleys produce a fabric almost identical to Cara, while BBrown do a few different ones, trilogy is relatively new but other fabrics in their range are much more frequently used, such as Expofelt, which is very open. Kvadrat fabrics are now getting more use, they produce 'dedicated' acoustic panels...

These fabrics are all very good, but at least 2 stretch fabric companies insist of putting a heavy cotton backing cloth behind cara when doing wall linings, so add tightweave cotton 5mm behind cara fabric and 3mm infront of absorbant materials, I dont think that the combination will give the same results as the test results they publish that match everyone elses test results..

Welcome Amioa. I wonder if you too were a student of Bob's?
Thanks for the input which rings loudly of real world experience.
Would it be OK to name the actual fabrics you see in use? Also perhaps share your opinion on each specifically?

Impedance tubes are a tad hard to access, although Dange....
Can you measure at different angles of incidence in these tubes, by say simply angling the sample?

A while ago, I presented here an example of the destructive effect of a desktop reflection which resulted in visible combing in the FR graph.
Varying the angle of the reflective desk greatly diminished the problem.
Strangely the destructive effect seemed visually much more powerful seen as comb filtering in a Freq Resp graph, rather than as a reflection level in the more obvious ETC graph. Perhaps this would be a useful way to show up the small differences in amateur test scenarios. One would still need samples of the various fabrics to compare. Furthermore one would have to chose one as a reference. We have seen that the very commonly used GOM is quite reflective. One wonders if Cara and such have really been tested or are we all just Lemmings? heh

DD

Thanks for the reply Amioa.

I ordered a sample of Lucia at the same time as the Cara (because I'd seen Lucia mentioned). To my surprise you can hardly breathe through the Lucia at all! It is FAR easier to breathe through the Trilogy than the Lucia. But you're right, Trilogy is much thicker - and certainly NOT as visually transparent as Cara.

It is easier to breathe through the Cara than the Trilogy, although there isn't too much in it in my opinion. You can also feel your breath a bit more on the other side when you blow through the Cara, compared to the Trilogy. Again, not too much in it though.

A couple of questions if you don't mind -

1. Aside from Lucia and Cara, which other Camira fabrics "get regular specification"?

2. Do you have a link to Bradley - whom you mention? Can't seem to find them on the interweb.... And if possible the name of the fabric you refer to?

3. I will check out the expofelt and get a sample, although there seems to be mixed opinions on the use of felt. If you know of any other BBrown fabrics that might work please do let me know and I'll order samples of those at the same time....

4. The Kvadrat website is clear as mud and I'm not having much luck finding anything on there. Any pointers would be fantastic.

5. When I spoke to BBrown they seemed a bit non-committal and blase about the use of Trilogy for acoustic panels (even though I pointed out that the description states this). So I wondered if you wouldn't mind telling me your source RE: when you say they insist it is suitable.

Sorry to chuck so many questions your way but if you get a moment I'd really appreciate it.

Cheers

Max

1. Our posts crossed.

2. I feel like a lemming!