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first reflection panels density and thickness
Old 31st January 2011
  #31
SAC
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Gee, if we are going to become sticklers - considering how few folks' comments remain consistent to the too often amorphous titles of the threads and how threads morph into some amorphously related territory - we could simply, and more accurately, entitle almost every thread as "something abut something, maybe". heh

And while I am here, does anyone know anything about repairing the air-conditioning compressor on a 2008 GMC pickup with a 5.7L V8? And do you think that accidentally putting brake fluid into the coolant lines will adversely effect performance? heh

Maybe the forum would be better served if the categories were broken into more subgroups, such as sections for: general 'how to' approach room tuning; basic acoustics - or, I'm not sure what category this properly fits into; sound isolation & sound transmission issues; absorption; LF modal behavior and bass traps; specular reflections and broadband panel traps; diffusion and scattering; LF tuned resonate traps; acoustical room models; measurement and analysis issues; physical construction issues; studio layout, sizing, and equipment placement issues; etc.

And then we could still complain that the titles are insufficiently accurate and that the topic was still in the wrong category.

And we COULD benefit from more stickies keeping track of some of the basic concepts that are repeatedly asked ad nauseum, such as: what weights of absorbent material are optimal for each category of absorber; What are the generally accepted rules of thumb for the uses and application of facings/'FRK' in absorbers; What is the effect/benefit of providing a gap between boundary and absorbent panel?; etc...... In other wards - essentially a summary of the 'best practices' for each category to be perused prior to the same question being asked over and over and over again.

And then we can deal with posts like these, that have utterly deviated from the title of this thread as well!
Old 31st January 2011
  #32
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While jumping around, yes probably a summary of the best practices would certainly help to avoid the same questions being repeated again and again, as well as , if proper (still short) motivation is given, avoid people being lost on the ocean of marketing pitches.



Just an example: how can people believe low density fluffy is better than high density batts when all vendors don't sell any such product ! The only way to be convinced is a comparative graph. There is a thread which somehow provides that, but I found it by pure luck and after having performed my own computations (which few will do, cos' flow resistivity data isn't readily available).

Old 31st January 2011
  #33
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Quote:
Originally Posted by mhch View Post
Just an example: how can people believe low density fluffy is better than high density batts when all vendors don't sell any such product !
If I've understood things correctly, the fluffy is best used in really thick absorbers, like 12" (30cm) thick and more. Absorbers this size would be a bit awkward to ship and I don't think many (ordinary) people would want massive boxes in their corners anyway. Except for deep bass absorption you can handle the rest with thinner panels using denser insulation.

While I think a few stickies would be a good idea, I doubt it will cut down much on the noob questions. When you're new to acoustics you think your situation is unique and want to be told what to do.

It would be better to learn the general concepts, plan out a strategy, implement it then fine tune it. Not all the newcomers are prepared to do this. Some are, though, and they're the interesting ones. I'm sure they're also the ones that end up with good sounding rooms.
Old 31st January 2011
  #34
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Quote:
Originally Posted by mhch View Post
Please understand I'm not ranting ! I'm really glad to have finally found a reliable source of good information. Just trying to give some feedback.
same here
Old 31st January 2011
  #35
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Quote:
Originally Posted by PaulP View Post
If I've understood things correctly, the fluffy is best used in really thick absorbers, like 12" (30cm) thick and more. Absorbers this size would be a bit awkward to ship and I don't think many (ordinary) people would want massive boxes in their corners anyway. Except for deep bass absorption you can handle the rest with thinner panels using denser insulation.
This one first: some vendors ship DIY versions of their products. That would perfectly apply to fluffy based products as it would make it much less awkward to ship (specially if product is only the containing frame !!). But this is probably a marginal business.

And there exist massive corner trap products, expensive by the way.
Old 31st January 2011
  #36
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Quote:
Originally Posted by PaulP View Post
While I think a few stickies would be a good idea, I doubt it will cut down much on the noob questions. When you're new to acoustics you think your situation is unique and want to be told what to do.

It would be better to learn the general concepts, plan out a strategy, implement it then fine tune it. Not all the newcomers are prepared to do this. Some are, though, and they're the interesting ones. I'm sure they're also the ones that end up with good sounding rooms.
Now this one !

I definitely agree with the overall statement, and that's exactly what I've been doing and still do. But even though, it's far from easy to identify the right concepts and to define a strategy. Took me a couple of months.

For the noob questions, what about an FAQ ... I reckon most forum experts and moderators aren't ready to come up with this, as this requires some sustained effort to implement.
Old 31st January 2011
  #37
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Jens Eklund's Avatar
Naturally, the most important factor is the flow resistivity, but since no one seems to be bothered with this, here’s a graph illustrating absorption using only density of rockwool (5o mm, no airgap):

first reflection panels density and thickness-rockwool.gif
Old 31st January 2011
  #38
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Thanks for the graph. Jens.

Did you create the graph from resistivity data you had for these rockwool densities ?

I'm actually creating my own diagrams including the fluffy stuff (typically 10-20 Kg/m3) but not the 150 kg/m3 one, as well as trying to illustrate the effects of thickness and air gaps. Will post that later in the week.
Old 31st January 2011
  #39
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Jens Eklund's Avatar
Quote:
Originally Posted by mhch View Post
Thanks for the graph. Jens.

Did you create the graph from resistivity data you had for these rockwool densities ?

I'm actually creating my own diagrams including the fluffy stuff (typically 10-20 Kg/m3) but not the 150 kg/m3 one, as well as trying to illustrate the effects of thickness and air gaps. Will post that later in the week.
I used whatever the software (Soundflow) sets when using the density instead of flow resistivity (doesn’t say what that equals to). Regarding gaps:

https://www.gearslutz.com/board/5999249-post4.html
Old 31st January 2011
  #40
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DanDan's Avatar
Details

Quote:
i believe 3" is a bit too thin for flat absorbtion down to 250Hz, isn't it ? as i said, i bought some 3" thick (3.0 pcf) material, so i wonder what to do.
wouldn't 6" be best for flatter absorbtion ?
6" thick rockwool + 6" gap ?
The word 'flat' is bound to draw controversy. 3 inch PLUS a three inch airgap behind is a useful size. 6 inch with 6 inch gap will have considerable effect on the vertical and horizontal modes, both of which have deep nulls at approximately the mix position. PLUS it will do the HF/RFZ job. Win win. Yes, better, no doubt.
Overkill? Are you Pro or Amateur? Pros kill.
Look at any design for a Pro studio. The traps are vast, enormous.
Most of us can't or won't do this in our typically semi domestic studios.
So we compromise. Be assured though. A 2 inch RFZ treatment around the mix spot is well worth it. BTW, the Cloud is not really an additional option.
It is as or more important as the side panels. The floor also could do with some consideration.

The Q4 Avare thread was started to shine a light. It does to some extent, but unfortunately the same light illuminates some of the lesser human traits.
Controversy and Discussion differ.

There are simple effective guides provided by RealTraps GIK John Brandt, etc. etc.
For those who are really interested in what is going on behind the scenes, books are the only game in town.

Master Handbook of Acoustics by Everest
Recording Studio Design by Newell

Later, Cox, Davis and so on. For now how about this free one
Table of Contents


DD
Old 31st January 2011
  #41
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Jens Eklund's Avatar
3” wool + 3” gap (8,8 kPa*s/m², 30 kg/m³) vs. 6” wool + 6” gap:

first reflection panels density and thickness-3in-vs-6in.gif

The 0,7 mark is approx 100 Hz for 6” and about 200 Hz for 3”.
Old 31st January 2011
  #42
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avare's Avatar
 

Quote:
Originally Posted by Jens Eklund View Post
The 0,7 mark is approx 100 Hz for 6” and about 200 Hz for 3”.
Thank you for posting the graphs.

Andre
Old 31st January 2011
  #43
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DanDan's Avatar
Extra Octave

Yes thanks Jens. Worth noting that absorptive traps behave better in practice than in prediction or theory. e.g. Distributed they can have double the tested effect.
Andre, there was a link provided, I am assuming by you, which showed that absorbent traps, in practice, achieved an octave below the predictions. This would tie in nicely with the BBC results showing good absorption (essentially flat if I may bravely use that loose term) down to 50Hz for 12 inch traps.

DD
Old 31st January 2011
  #44
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avare's Avatar
 

Quote:
Originally Posted by DanDan View Post
Andre, there was a link provided, I am assuming by you, which showed that absorbent traps, in practice, achieved an octave below the predictions. This would tie in nicely with the BBC results showing good absorption (essentially flat if I may bravely use that loose term) down to 50Hz for 12 inch traps.
Could RD 1992-11 be the one you are referring to?

Andre
Old 31st January 2011
  #45
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Jens Eklund's Avatar
Quote:
Originally Posted by DanDan View Post
Yes thanks Jens. Worth noting that absorptive traps behave better in practice than in prediction or theory. e.g. Distributed they can have double the tested effect.
Yes, one can increase the number of Sabins achieved (maybe not double but a lot) if utilizing the edge diffraction effect but considering the topic of the thread, this is not applicable in this case (and usually not applicable in studio design in general since you normally don’t want broadband absorption in other places that reflection points (unless bigger control rooms).
Old 31st January 2011
  #46
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Hannes_F's Avatar
 

Quote:
Originally Posted by mhch View Post
Just an example: how can people believe low density fluffy is better than high density batts when all vendors don't sell any such product !
HAHAHA ...

Quote:
The only way to be convinced is a comparative graph. There is a thread which somehow provides that, but I found it by pure luck
You probably mean this one
Could someone help out interpreting material's gas flow properties

Quote:
and after having performed my own computations (which few will do, cos' flow resistivity data isn't readily available).

Originally I decided not to insist on this any longer since I seem to have a totally different opinion in that regard than many of the experts here. But you ask, so I try not to leave you alone in the rain. So I'll try to explain in easy words what I think I understood (there is always more).

Bass frequencies in a small room float like waves, not like rays. As a consequence this happens: If the loudspeaker fires at any untreated portion of hard, heavy wall ... and I mean any ... in front, behind, sideways ... any ... then this area will reflect bass waves towards the listening position and cause comb filtering. For higher frequencies you can get away by tilting or angling this part of the wall away but for bass frequencies this sort of reflection will happen, no matter what the orientation of that spot is.

Soo ... if you are using absorption (no matter where) then better make sure it is bass absorbent. Or else you will mess up your frequency response, and no bass trap anywhere else can cure this.

Now ... what can we do to make porous absorbers bass absorbent? The Whealey porous absorber calculator tells you all: Number one, make them thick, number two alternatively use an air gap, number three, don't make them too dense (see my posts in the link above for a rough understanding of that).

(Side remark: If you are up to using other devices like membranes in front of your absorber, angled walls or perforated panels then all the better. In this case you can control parts of the spectrum by reflection ... but only those that behave like rays, see above).

Now it has been argued for a long time whether or not the Whealey calculator is correct for low frequencies since it is not so easy to do measurements in that range ... and some people are inclined not to accept anything except it is delivered to them on the silver tablet of a measurement. And sometimes not even then.

OK, here comes my measurement. You see a configuration of my room where the front part is entirely covered with 30 cm* of 5000 Rayls/m fibreglass. Most of the back part is also covered but parts of them thinner, while the back wall has 50 cm of the same material. All fibreglass is wrapped into fairly thick plastic since I am only interested in the bass absorbing feature of these devices. In front of the back part absorbers there are diffusors (but really only in the back part).

*) To be exact, the heavy part of the front wall is covered either with 50 cm of this material or with 30 cm absorber in front of 20 cm air gap. There is a gypsum board part of the front wall and that is covered with 30 cm material flush on the wall, as are the sides.

The first of the following pictures shows the untreated room response, the last has all the absorbers in place. You see a dramatic improvement, the bass response is within +/- 4 dB with a RT30 of 0.3 sec.

In the middle you see what happens if I remove one absorber (measuring 120 cm x 60 cm). Note that this absorber was not in a reflection point but the bass response immediately gets a new dip of - 10 dB @ 67 Hz plus some overshoot below and above. This is what I tried to explain above in this post, any untreated portion of wall will reflect bass waves towards the listener and cause comb filtering. Not only in the mirror points.

I have seen so many threads looking for help along the lines of "I have a room full of absorption but there is still this nasty 120 Hz dip" ... that I hope this helps to understand why. Even only one area of 120 cm x 60 cm (2 ft x 4 ft) either untreated or treated too thin will already make a huge veil into the frequency response (we are talking about small rooms here).


Now for all those that still don't believe that thick fluffy fibre glass is wonderfully bass absorbent ... look at the spectrogram. This is a sort of waterfall chart but looked at from above. From left to right are the frequencies (20 Hz - 300 Hz) and from bottom to top is the time (- 300 ms to 1000 ms). In the upper picture(the untreated room) you see these flames going upwards - these are the room resonances that are ringing. Below you see the treated result, fairly even down to 30 Hz. And even means here even in the start frequence response and in the decay!

In the left part of the chart (between 20 and 30 Hz) you see one room mode left where the absorption becomes too small to eat it up. But you can clearly see that the absorption is effective down to 30 Hz!



Also please compare to the Whealey chart for 5000 rayls/m fibreglass that predicts short of 50 % absorption @ 30 Hz, opposed to higher flow resistivity material. If 50 % of the wave is absorbed then the remaining 50 % can make a - 6 dB dip. For 67 Hz the value should be 75 % absorption and which equals to 25 % interference that can cause a dip of -2.5 dB. Both is in good consistency with my measurement (I am leaving out geometrical factors grossly negligent here).




These are my consequences:
- If you use absorption, use it right (which means bass effective)
- Any thin absorber in a small room will cause more trouble than help in regards to the bass response. No matter how nice it works in a direct reflection point for higher frequencies.
- More is not always better - especially valid for absorber density (the correct term would be air flow resistance)
- Any untreated portion of heavy wall will cause trouble in a small room. That is why you will probably never get a really good bass response with only hanging a few panels plus using some corner traps.
- Try to find a good combination of absorber thickness and density (and gap) with help of the Porous absorber calculator. Keyword is "combination" here.
- Yes, absorption works down to 30 Hz (!)


Please note that I am not saying "stuff your room with absorbers and nothing else". There are plenty of other methods, and the most intelligent way probably is to control the bass by absorption/membranes/perforated and the treble by reflection/diffusion. But when it comes to absorption I hope this post helps somebody out there.

Cheers
Hannes

Last edited by Hannes_F; 31st January 2011 at 10:52 PM.. Reason: added calculated absorption charts
Old 31st January 2011
  #47
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DanDan's Avatar
Real Point

Thank you Andre. It should be noted that the full octave difference is a comparison of two extremes, normal vs random incidence.

My real point here is that we have side and vertical modes in action at the mix position. In a typical 8 foot high room the listener is at the nulls of both side and height modes. Then the combined peaks at X2 and so on.
I could make a case for asymmetrical listener position here.....LOL

It seems to me that side reflection and cloud panels should at least attempt to have a go at those modes. The extra depth of a thicker panel, an additional airpgap, FRK on the back, causes little extra expense or hassle as we are already treating HF in any case.

Ultimately the more space efficient designs seem worth consideration.
Helmholtz perf, Limp Membrane, Panel, VPR (floating steel panel), with a HF absorbent front seems to cover all bases.

DD
Old 31st January 2011
  #48
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avare's Avatar
 

Thank you Hannes! Fantastic data on low end absorption.

Andre
Old 31st January 2011
  #49
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avare's Avatar
 

Quote:
Originally Posted by DanDan View Post
Thank you Andre.
You are welcome.

Quote:
It seems to me that side reflection and cloud panels should at least attempt to have a go at those modes. The extra depth of a thicker panel, an additional airpgap, FRK on the back, causes little extra expense or hassle as we are already treating HF in any case.
I had been espousing that for years, but gave up as my remarks fell on deaf ears. It is good to read your stating it independently of my writings.

Andre
Old 31st January 2011
  #50
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DanDan's Avatar
Ditto

We were obviously writing simultaneously.
Thank you for that great post Hannes. Bookmarked.
thumbsup

DD
Old 31st January 2011
  #51
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Very good summary and data, Hannes. Moreover I believe it despite having read it on the Internet, thanks to the tiny background I already got from reference books


Looks like pointers to several great threads and posts might be worth gathered into a sticky .....
Old 1st February 2011
  #52
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Thread Starter
many many thanks for sharing all this knowledge. head is about to explode.
hjertelig takk for the graphs Jens. thanks Dan, Andre and thanks Hannes for blessing this thread with your data !

Old 1st February 2011
  #53
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Yes thank you Hannes, very insightful. I've been stressing face size for some time now, as I've seen a common thread that binds rooms treated with entire surfaces effected.

I helped out a gentleman in India design a room... in which every surface sans floor was covered in at least 1' thick fluffy insulation, fabric'ed and slated. For whatever reason, Amit is shy about publicly sharing the measurement results, but having seen them, it's comparable to your findings.
Old 1st February 2011
  #54
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Thread Starter
Quote:
Originally Posted by DanDan View Post
It seems to me that side reflection and cloud panels should at least attempt to have a go at those modes. The extra depth of a thicker panel, an additional airpgap, FRK on the back, causes little extra expense or hassle as we are already treating HF in any case.

DD
the rockwool i bought came with kraft paper on one side. i was planning to remove it for building these early reflections panels but reading what you all (Hannes, DanDan, Avare) say, i believe that leaving this on the airgap side of the panel would be beneficial for the overall bass absorbtion in the room, right ?
would this kraft paper act as a membrane ? or is it too thin and should i glue an extra layer (of thin foil maybe ?) to it ?
i'll definitely make 6" (2x3) thick panels (+ 6" gap from the wall), removing kraft from the outer rockwool panel, and maybe leaving this kraft paper on the inner one (the one closest to the wall), depending on what you recommend.
Old 1st February 2011
  #55
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avare's Avatar
 

Quote:
Originally Posted by neuro View Post
the rockwool i bought came with kraft paper on one side. i was planning to remove it for building these early reflections panels but reading what you all (Hannes, DanDan, Avare) say, i believe that leaving this on the airgap side of the panel would be beneficial for the overall bass absorbtion in the room, right ?
would this kraft paper act as a membrane ? or is it too thin and should i glue an extra layer (of thin foil maybe ?) to it ?
i'll definitely make 6" (2x3) thick panels (+ 6" gap from the wall), removing kraft from the outer rockwool panel, and maybe leaving this kraft paper on the inner one (the one closest to the wall), depending on what you recommend.
What is the density of the mineral wool? With 12" total depth, if the mineral wool is ~4 lb/ft³ you will have excellent low end absorption. We do not have enough test data to give a professional recommendation to keep the FRK. Having written that, without test data, I do not see it causing any significant degradation in the worst case that I can hypothesize.

Note all the qualifiers in the last sentence.

Andre
Old 1st February 2011
  #56
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Thread Starter
the density of the material is 3.0 pcf (50Kg/m3).
if i correctly understand what FRK is, i don't have that on my panels, it's just "kraft" (softwood pulp. "brown paper" ?).
isn't FRK kraft + foil ? that's why i thought about adding some thin foil.
hmmm, i'm well ready for making the panels. i think i'll just leave the paper and spare the foil.

Good first reflections panels building is 99% design and 1% construction.
Old 1st February 2011
  #57
SAC
Registered User
 

FRK and FSK are used generically for the purposed here. It can be any effectively reflective coating - paper, cardboard, hardboard, plastic, etc.....
Old 1st February 2011
  #58
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Quote:
Originally Posted by neuro View Post
would this kraft paper act as a membrane ? or is it too thin and should i glue an extra layer (of thin foil maybe ?) to it ?
i'll definitely make 6" (2x3) thick panels (+ 6" gap from the wall), removing kraft from the outer rockwool panel, and maybe leaving this kraft paper on the inner one (the one closest to the wall), depending on what you recommend.
If you angle your side panels and have a membrane in front of it you could even reflect some of the energy towards the back wall which would be clever. However I found that such light material is only effective for fairly high frequencies, not for the low mids or high bass. For that you would need heavier membranes (think wood, metal or gypsum board).

Be aware that every coating (thin plastic, thick plastic, paper, fabric) reflects a portion of the spectrum to an amount. An ETC measurement tells you much about that. At the places where I want the surface to be really dead I use ... structured foam (in front of the absorbers, not instead of them)
Old 1st February 2011
  #59
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Thread Starter
i thought the initial purpose of these side panels was to absorb mids and highs to avoid the comb filtering caused by the delay between the source signal and its reflections on the side walls. i understand what you said about making all absorbers effective in the low range.. but putting a membrane on the front of the panels ? i'm confused.
i may be wrong but i kind of hope these panels will attenuate the annoying feeling i have when i move my head left or right from the sweet spot (?). if the front of the panels reflects highs, i doubt that will happen.
Old 1st February 2011
  #60
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Jens Eklund's Avatar
Quote:
Originally Posted by Hannes_F View Post
If you angle your side panels and have a membrane in front of it you could even reflect some of the energy towards the back wall which would be clever. However I found that such light material is only effective for fairly high frequencies, not for the low mids or high bass. For that you would need heavier membranes (think wood, metal or gypsum board).

Be aware that every coating (thin plastic, thick plastic, paper, fabric) reflects a portion of the spectrum to an amount. An ETC measurement tells you much about that.
+1

/Jens
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