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Discussion Starter #1 (Edited)
I couldn't find any threads were people tried to measure bass trap effectiveness (no bass trap then add bass trap etc) so I did a simple experiment.

I measured the reponse with an untreated room, then added one floor to ceiling superduper chunk column in the right rear, then added another superduper chunk column in the left rear and finally added a regular super chunk nine feet long along the front floor/wall boundary. After each addition I remeasured. The superduper chunks were humongous at 34"x24"x24" and the regular super chunk was the normal 24"x17"x17". So I didn't skimp on the material!

Here's what I found:

Baseline:
micatlistening_subonleftwall_untreated.jpg

Right rear column added:
micatlistening_subonleftwall_rightrearcornerchunked.jpg

Left rear column added:
micatlistening_subonleftwall_rightandleftrearcornerschunked.jpg

Front floor chunk added:
micatlistening_subonleftwall_rightandleftrearcornersplusfrontfloorchunked.jpg

I note the following:

1) Below 40hz there doesn't seem to be much effect.

2) Surprisingly the 40-50Hz region improved with each superduper chunk. Presumably reaching this low is due to using the bigger chunk size.

I was planning to cut these in half to regular size but now wonder which is better...larger chunks but only two columns, or regular size chunks which gives two extra column worth of material to use elsewhere. Any advice?

3) Adding the second rear column changed things as did adding the front floor column but did it get better? I'm not sure. Certainly there isn't a steady march to filling in the peaks/valleys...each one clearly better than the last...! In fact the first column seemed to smooth things the most. What is happening? Pehaps reflections from walls are starting to dominate that aren't being addressed by the corner chunks. Once again, comments from the cognoscenti appreciated!

4) Changes in the 80hz peak is interesting. It improved the most with only the right rear corner. Then regressed although in all cases it is better than untreated.

5) Some pretty deep nulls seemed to be introduced! Complete cancellation at some points where previously there was some energy.


Overall, to be honest, I don't see the improvements I had hoped. Especially given the huge size of these things in the corners. Since this was a test, I'm not going to necessarily keep it this way. But, based on these plots, it isn't so obvious what would be the best deployment of all this stuff either... :eek:
 

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The decay time down low has definitely been effective.

There is some smoothing of response but not huge. This is a very good example of the fact that while corners are a very efficient place to do broadband bass control, it's not the only place and isn't a magic bullet. Some of the nulls you're getting that really didn't change much can be caused by a variety of things such as SBIR, null off the rear wall, main to sub integration/phase issue, etc.

The nulls that got deeper are interesting. What's happening is that the treatments are capturing a reflection that was previously cancelling a problem so to speak.

Looking at the baseline, I'd say there's a bit of work to do with speaker, sub, and seating position before treatment is applied. I'm sure we can get a better starting point and then begin to address what's left, what's causing it, etc.

Bryan
 

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Discussion Starter #4 (Edited)
....now wonder which is better...larger chunks but only two columns, or regular size chunks which gives two extra column worth of material to use elsewhere...
Any comments regarding this trade-off?


...Looking at the baseline, I'd say there's a bit of work to do with speaker, sub, and seating position before treatment is applied. I'm sure we can get a better starting point and then begin to address what's left, what's causing it, etc....
It does appear to require an iterative process alright.

Do you always play with speaker/sitting position BEFORE any treatment? From this expirement it seems simple treatment can expose latent problems. So doing the basics (chunk corners at least since that is easy) might be a better starting spot?

In any case, I certainly agree. The baseline isn't the best starting point for this room!
 

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I always recommend doing as much as you can with locations of seating/speaker/subs prior to doing any treatment.

If it were me, I'd do twice as many of the smaller ones. They're still large enough to be effective down pretty low.

Bryan
 

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You never did mention the most important thing how it sounds. Did you notice a difference? I'm about to go through this same process and really enjoy reading other peoples journey's.

James
 

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Discussion Starter #8
Hehe...there isn't much to see...since it was just a test all I did was stack the raw fibreglass chunks. I didn't actually build anything. Anyway, if my daughter ever returns my camera I'll snap a picture.

Same for the sound, I was testing if one could measure the addition of the bass traps and how they would manifest themselves in the plots. I wasn't too worried about the sound since it wasn't representative of a real listening setup. Having said that, the room sounds quieter and less "slap happy" with these pieces added.
 

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Discussion Starter #10
Well...as mentioned it was primarily a measurement test not a listening test...but recognizing that, I'd say the audible difference was minor. Moving the sub or listening position makes a much bigger difference.

The test basically covered 25% of the dihedral corners and nothing else so perhaps one shouldn't expect much.
 

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I went from 70% corner coverage to 100% and the difference was dramatic. Also alot of difference going from 0 to 70, but less 'in your face'. I sure noticed when I removed the traps, though! :) So easy to get used to that sound and tight bass, that you really don't notice until you no longer have it.
 
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Just a comment that may diverge a bit from a few common assumptions. (in the hope that it may spur a bit of curiosity and further investigation.)

While porous (absorbent) bass traps can be effective to a small degree, they are not very efficient.
The reason is that 'particle velocity' of a sound wave (yes, I know it behaves as a compression wave at modal frequencies)near the walls and in corners is essentially zero for these long wavelengths. Additionally, their required depths to be effective at these wavelengths are prohibitive in a small acoustic space.

In fact, a porous/absorbent panel, assuming a 45 degree incidence would require a thickness of:

at 60 Hz 45 degree incidence: 3.33 feet 90 degree incidence: 4.71 feet

at 120 Hz 45 degree incidence: 1.60 feet 90 degree incidence: 2.26 feet

And that still assumes a high particle velocity!


A more efficient alternative absorber design, although a 'bit' more complex to calculate and design is a resonant membrane absorber (and its perforated variant). (For much deeper source detail into this topic, one might want to pursue the research of D.Y Maa)

The membrane converts pressure fluctuations into air motion. As the membrane sympathetically vibrates over a selective low frequency range, determined by its mass and the air spring compliance, it pushes air through an internal porous layer resulting in low frequency absorption. Basic relationships exist between the effective frequency and the membrane mass, stiffness and cavity depth.

Although beyond the scope of this entry, the design and construction of such an absorber is not overly complex assuming one has a bit of patience.

Overly simplified relationships are stated below, but their limitations should be noted, as for small areas the entire membrane may not vibrate freely due to edge mounting friction, and bending stiffness may tend to increase the resonant frequency.

f for normal (90 degree) incidence = 60/sqrt(md) for an empty cavity, and f= 50/sqrt(md) for a cavity with porous absorption.
f for oblique incidence = 60/{cos a sqrt(md)}

There is a bit more to account for real world variables before one run out and begins to model and build these absorbers! And in addition, one might find that porous membrane resonant panel absorbers are just a mite more predictable. But ala in all, the variations on the fundamental design are much more efficient at mitigating low frequency modal frequencies and they are also of benefit in that they do not absorb broadband specular reflections and thus avoid over-deadening the small acoustical space as is so common with the widespread use of absorption.

So...now you have an introduction to yet another more efficient variant of low frequency modal absorbers compared to the porous absorptive method that also exhibit the benefit of not being overly absorbent to the mid and high frequency energy that is comprised of focused specular reflections in the small acoustical space. And as such, they help avoid the all too common overly deadened room.
 

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Agreed. Porous membranes can be extremely effective.

Just to be clear - the calculations shown for thickness of standard porous absprbers are for 'optimal' absorption. This should not be construed as not being effective at those frequencies and lower if not that thick.

A 6" thick panel straddling a corner or a 17x17x24" chunk style absorber filling a corner can be pretty effective well below 60Hz. A 6" thick absorber spaced 2" off of a wall surface can be pretty effective at and below 60Hz. It's just not as effective as it could be until it reaches the 1/4 wavelength of the bottom frequency to be considered.

Bryan
 
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The behavior of porous absorption (eg, foam, rockwool, fiberglass,etc.) is very well understood.

When sound propagates in small spaces, such as the interconnected pores of a porous material, energy is lost. This is primarily due to viscous boundary layer effects. Air is a viscous fluid, and consequently sound energy is dissipated via friction with the pore walls. In addition to viscous effects, there will also be losses due to thermal conduction.

For the porous absorber to create significant absorption, it needs to be placed where the sound particle velocity is high. The particle velocity close to a boundary is usually small, so the portions of the absorber close to the boundary are not generating much absorption. It is the parts furthest from the backing surface which are most effective, which is why thicker layers of absorbent material are needed to absorb low frequencies.

For low frequencies, where the wavelength is large, one has to go a considerable distance from the wall to reach a point where the particle velocity is significant. (D'Antonio&Cox)

Hence this is why absorbent material can be placed away from the wall and still be effective, as it is being placed in the region of greater particle velocity. It is also why such companies as RPG have developed sinusoidally shaped panels (such as ProForm) which place material into this zone while providing attachment points (regions which are minimally effective).

A rough figure frequently quoted is that the absorbent material needs to be at least a tenth of the wavelength to cause significant absorption ( U. Ingard, Notes on Sound Absorbent Technology, 1994 ), and a quarter wavelength to absorb all of the incident sound energy, assuming the complete match in the acoustical acoustical impedance of the absorbent material.

Thus per the minimum quoted tenth of a wavelength (...and this assumes an ideal 100% absorptive acoustical impedance match):

at 60 Hz 90 degree normal incidence: 22.6 inches

at 120 Hz 90 degree normal incidence: 10.85 inches

...Still potentially prohibitive for many in an average small acoustic space.

The need for significant thickness compared to wavelength makes porous absorbers inefficient and not particularly useful at low frequencies. While they can be used if the associated trade offs are acceptable, it is why you see few of them in the product lines of the professional suppliers such as RPG.

Thus, this is part of the reason for not necessarily seeing huge changes in the CSD/waterfall plots by simply adding absorbent corner traps. They are relatively easy to build and to install, but one should be aware of their limitations.

And as such, much more effective and efficient absorbers are to be found in the resonant absorber design family, as they provide for much greater absorption for a given depth than porous absorbers.

This is not intended to scare anyone off from the use of absorbent bass traps.

But the behavior of such techniques are well understood.

The most significant variable involves the material itself, in the form of its acoustical impedance, which can vary widely. And few materials even come close to being 100% absorbent. In fact, the topic of acoustical impedance and its effects, just like in electrical impedance in the effective termination (absorption), reflection, transmission, diffraction, refraction, phase of reflection, etc. of acoustical waves,is fundamental and would be another worthwhile topic for a thread that should be considered prerequisite to the study of room treatment as it is completely predictive of the behavior of the material and technique. And I am already behind in another post!

Rather, having a bit more actual information on how they work and what determines their effectiveness allows one to make more informed choices. With a bit more information one can reasonably predict the effectiveness and determine the suitability of a particular technique in advance and hopefully choose the methodology that will most optimally compliment your needs, as well as your time, labor and pocketbook

...You pays your money and makes your choice...:bigsmile:
 

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Discussion Starter #16 (Edited)
...the topic of acoustical impedance and its effects, just like in electrical impedance in the effective termination (absorption), reflection, transmission, diffraction, refraction, phase of reflection, etc. of acoustical waves,is fundamental and would be another worthwhile topic for a thread that should be considered prerequisite to the study of room treatment as it is completely predictive of the behavior of the material and technique. And I am already behind in another post!...
Sounds interesting when you have time to start the thread!


mas said:
Rather, having a bit more actual information on how they work and what determines their effectiveness allows one to make more informed choices. With a bit more information one can reasonably predict the effectiveness and determine the suitability of a particular technique in advance and hopefully choose the methodology that will most optimally compliment your needs, as well as your time, labor and pocketbook...
Since I have no experience in this area I've been spending some time measuring and playing with my acoustic panels to develop a feel for all this.

So far my impressions are that broadband porous absorbers such as my rockwool (either panels or chunks) are quite ineffective in absorbing reflections below 100hz (there is some action but it isn't enough to solve a problem if it exists at those frequencies). I'm also finding it marginally effective between 100-200hz (it does seem to make a difference...but not large...and one needs a lot of the stuff). Above say 500Hz it seems quite effective (a 4" panel blocking first reflections seems to reduce reflections in the ETC curve 10-12 dB without any trouble.)

There may be benefits of the absorbers that I'm not able to measure or am overlooking but my initial conclusions are that porous absorbers (ie rockwool) works well for that initial time delay gap and not so well for anything else.

On the other hand they are easy to build, relatively inexpensive, well documented for the hobbiest and completely within the range of a DIY'er. One can also use quantity to compensate for (lack of) effectiveness to some extent.

Resonant membrane absorbers sound interesting but a search of the various forums for proven DIY construction techniques produced nothing. And even here this is little point unless it can materially address room modes in the 40Hz to 120Hz range. Can one even cover a couple octaves effectively? Without practical implementations for the DIY'er they are relegated to interesting theory only.

After spending an afternoon with various woofer and listening positions, I have little hope for seeing a smooth low frequency curve for one seating position, let alone two or three. There is just too much uncontrolled stuff going on at those frequencies and most of it is bad... :thud:
 

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Full, pure, non-porous membranes are much narrower in absorption range than the porous membranes or standard porous absorbers that were mentioned before. They can definitely have their place though.

As for the full porous absorbers, how effective they are is a function of impedance (gas flow resistivity), thickness, and distance from leading edge to boundary. The chunks you made are certainly thick enough to reach lower than 100Hz relatively effectively (not 100% though). You also have to remember that not all frequency related issues are going to be solved with corner placement.

If you're up for another experiment, try making 2 6" thick panels to hang centered on the rear wall behind the listening position - and maybe try them with and without a 2" gap behind them. To see which frequencies might be impacted, measure with them not installed at the current position, then again say 1' forward or backward so you can identify potential issues related mostly to the length dimension. Then install the panels and take a look.

Bryan
 
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Resonant membrane absorbers sound interesting but a search of the various forums for proven DIY construction techniques produced nothing. And even here this is little point unless it can materially address room modes in the 40Hz to 120Hz range. Can one even cover a couple octaves effectively? Without practical implementations for the DIY'er they are relegated to interesting theory only.
Here is where you encounter my rather charmingly jaded curmudgeon side.:bigsmile::devil:

First, I would be VERY careful of relying on DIY forums, unless you are content to rely on the limitations in understanding that may be present in the source.

For instance, as example of this problem occurred in well publicized errors (re. Desart) in resources precisely related to this subject!

For example, a site for HH calculations is
http://www.mhsoft.nl/Helmholtzabsorber.asp

This site has recently been upgraded to use the correct formula for "fo". Issues such formulas as well as trial calculations as this are critical to verify for accuracy, as problems such as this are unfortunately the norm rather than the exception in far too many places.

INCORRECT formula: fo= 2160*sqrt(r/((d*1.2*D)+(r+w)))
CORRECT formula: fo = 2160*sqrt(r/((d*1.2*D)*(r+w)))

Having others do your work for you is fine, provided you first verify their assumptions and VERIFY the correct implementations of principles!

Also, there are PLENTY of source materials that publish not only the ideal formulas but also a myriad collection of adjusted formula for various real world applications for exactly this issue.

I would suggest that the primary reason that you do not find more is that professionals do not run to the web to find calculators in DIY sites! They posses resources and texts that provide reliable source material and they have most probably spent some time studying such phenomena. And most consultants who do this for a living do not then post their work simply for others to copy.

Hence I hope that you will understand my (sardonic ;-) reluctance to empathize too greatly with your 'dilemma'. (And I hope that you can ascertain just a bit of my wry sardonic humor that may (without the benefit of vocal inflection, wink, and non-verbal cues) unfortunately seem 'mean', when it is meant as anything but... ;-)

There are a good many resonant technologies that are well understood, complete with many variations and hybridizations )ie dual layer). They are not necessarily hard to manufacture, but they do require a bit of verification and calculation of masses, percentages of perforation, densities, etc, of component materials, which, while not at all difficult to determine, do nevertheless require a bit of legwork, as you should not rely on published specs due to tolerances and variations which will effect tuning.

The information is out there. But you may have to do a bit of checking on your own. The ability to use Google is not a substitution for making the effort to learn and understand the principles involved. ;-) ESPECIALLY NOW, as much of the new paradigm in acoustics is still too new and is not published in sufficient detail in many of the books that many treat as references. (Hence why I would point you to Sound System Engineering by Davis & Patronis, as a good overview and introduction to the new paradigms that can further serve to point you in the right direction...)

I almost feel like I should be posting reams of PDFs and reference material here, but aside from time constraints and site posting restrictions, its not necessarily practical. (as evidenced by the posting time of this note!)

If you are serious in pursuing this and fail to locate the necessary (or sufficient) resources after checking sources (more reliable than DIY sites) such as the various journals, and you are willing to put up with my time constraints and limitations, PM me and I can try to help you with a few or at least point you to the sources. ...In other words, to paraphrase one of my former professors and mentors, I am not here waiting for someone simply tossing quarters up on the stage and expecting me to dance and do their (home)work for them (I have utterly unreasonable rates for such endeavors! ;-), but I am not averse to helping someone if they are willing to make a serious effort to acquire the prerequisite knowledge! :bigsmile: (BTW, his growl was pretty fierce, but if you actually bothered to seriously talk to him, you found his teeth weren't nearly as large as you might have expected! ;-)
 

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Utterly engrossing!

Spending the time to read your posts is time well invested in my opinion.

Looking forward to reading many more.

regards
Andrew:nerd:
 

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Discussion Starter #20
Thanks Bryan. I did the test as you suggested with a couple comments.

1) My panels are 4" thick so I used two each to double their thickness (ie 8" thick panels).

2) My rear wall (behind listening position) is actually a large patio sliding door to the outside. About 6' wide x 7' high in size so it is most of the height and approaching half the width of the rear wall.

Anyway, here are the results.
roommodaltest_paneleffeciencytest_initialcondition.jpg
These are the three mic positions varied along the room's longitudinal axis. The brown middle trace is the listening position, the purple top trace is 0.5' rearward, and the bottom is 1' forward. All with no panels.

I then chose the 1' forward position (this meant I didn't have to move the mic) and added the 8" panels against the rear wall (ie patio door) with this result:
roommodaltest_paneleffeciencytest_8inchpanels_vrs_nopanels.jpg

There isn't much difference. :hissyfit:

Are we having fun yet? The real world is so cruel... LOL
 
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