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Absorption Coefficients Question 703 v 701

Discussion in 'Room Acoustics / Studio Design' started by DonnyThompson, Jun 3, 2014.

  1. DonnyThompson

    DonnyThompson Distinguished Member

    Rod, Space, et al :

    I'm researching absorption coefficients on OC 703 and 701.

    According to Bob Gold's Chart, 703 2" (plain) mounted flush to wall has the following properties:

    125 250 500 1k 2k 4k NRC
    0.17 0.86 1.14 1.07 1.02 0.98 1.00

    source: http://www.bobgolds.com/AbsorptionCoefficients.htm

    The same values are listed on Owen Corning's SAC Chart for the same material, mounted the same way.

    Here are the specs for OC 701 2" (plain) mounted flush:

    125 250 500 1k 2k 4k NRC
    0.22 0.67 0.98 1.02 0.98 1.00 0.90

    My question is this:

    If I mounted 701 2" with an 16" air gap, I can increase the AC to 0.76 at 125 and to 1.02 at 250.

    But... 703 2" mounted with a 16" air gap has values of only 0.66 at 125, and 0.95 at 250, where the remaining upward frequencies have equal coefficients to the 701.

    Can you explain why 703 is better than 701 when mounted flush, (with the exception of 125hz) but when mounted with a 16" air gap, the 701 is better with low end than the 703 is?

    For broadband absorption panels (2x2), am I better off mixing the two different types in my room, or is the 701 the best bet for me all the way around when mounted with the air gap?

  2. MrEase

    MrEase Active Member

    I'm no expert but I suspect that the 701 will be more dense and or rigid than 703. When mounted directly on the wall the difference is not very significant for low frequencies but as soon as there is an air gap the density and rigidity will come in to play.
    DonnyAir likes this.
  3. Space

    Space Well-Known Member

    Typically the air gap matches the treatment thickness. A 2 inch thick panel spaced 2 inches will have better lf absorption than the same panel placed directly against the boundary.

    Sixteen inches is deeper than I have ever seen recommended for an air gap for any treatment in respect to back of treatment to hard boundary.
    DonnyAir likes this.
  4. DonnyThompson

    DonnyThompson Distinguished Member

    This is what my research had shown as well. I was thinking that this number - 16" - was high, but, not being an expert in the field, I thought perhaps I was missing a mounting technique I'd not previously heard of. With the exception of some clouds that I have seen, I'm positive I've never encountered wall-mounted treatment with an air gap more than 4".

    My original intention was to adhere two 2" 703 panels together, and then allow a 4" gap between this and the wall.

    I'm in the process of fabricating two 2x4 broadband absorbers; I have the 703... but after looking at the numbers, I was wondering if I should switch to 701, based on the specs I read.
  5. avare

    avare Active Member

    A late great friend of here has used up 3:1 air:gap ratios. I have facility designs with 2:1 ratios with 4" material and 8" airgap. As I recall it, the design was by ARUP.

  6. DonnyThompson

    DonnyThompson Distinguished Member

    Andre - did you or your late friend find a significant increase in LF absorption using this 4" material with an 8" air gap?

    I'm assuming that the material was of the OC 700 series, (or something comparative, like Roxul S&S)?

    I guess my question is, was there a substantial enough increase in absorption efficacy - proportionate to the 8" air gap - over a more traditional 4" gap when using 4" material - to warrant using those wider gaps?
  7. MrEase

    MrEase Active Member

    I have to assume that Avare is referring to the late, great, Eric Desart here. From my understanding (which is limited) the optimum air gap (if there is one) depends on the absorbant material itself. While I understand Space's comment about air gap being equal to absorber thickness, I don't think this is a "universal" rule of thumb. When creating a "classic" room within a room with decoupled flooring etc., then multilayered plasterboard is normally used with an airgap several times the wall thickness (as there has to be a supporting structure).

    This though, I think, is a different situation where the airgap can take a significant volume away from the final studio and a compromise must be found. This compromise will very much depend on available materials and the physical aspects and dimensions of the space to be treated.

    Sadly the Studiotips forum still seems to be down (although the website is still there) as it was a great source of relevant information - much of it from Eric Desart himself.

    I know that Avare, Bert Stoltenberg and J.F.Oros were regulars at the Studiotips forum and I have put out a feeler here as to what happened to it with no response. It would be a great shame if this resource was gone for good. I know that Rod Gervais was also thereabouts and if he is in contact with Bert Stoltenberg maybe could find out more.
    DonnyAir likes this.
  8. DonnyThompson

    DonnyThompson Distinguished Member

    Well, I've certainly been more than happy with the information I've seen on this thread thus far, but in regard to the site you mentioned, I agree that it would indeed be a shame for a site that held such valuable information and resources to go inactive.

    While I've been a recording and mix engineer for close to 35 years now, I'm just now beginning my journey into acoustic principles and theory. These past few months, I'm as excited about learning this new field of audio as I have been for quite some time with anything related to audio, maybe since way back ... as excited as when I set up my very first stereo coincidental mic array and heard the results.

    Perhaps it's more enticing to me now because for the first time, I've started working from home. Normally, I work at other studios- since I closed mine down in 2003 - and I've had the good fortune to mix in some very good sounding and accurate rooms along the way. I've also had the fortune to work in some very bad sounding and completely inaccurate rooms. And why would I feel fortunate about working in these places that sounded so bad? because it's given me the opportunity to realize the bad from the good, and to know what it is that I DON'T want for my own space.

    Back when I had my studio, I had a professionally designed control room, but I had virtually nothing to do with its design or fabrication, nor did I involve myself in its fine tuning with treatment afterwards -I hired a pro to handle those things for me. I was so busy with the business end of things, that I didn't have the time to get involved with the build. It's to my great regret, looking back now, that I didn't avail myself of the opportunity to learn this part of the craft when it was right in front of me... Hindsight being what is, I guess.

    But now that I'm doing more mixing projects here at home, and wanting to have the most optimal space I can get, I've found out just how much I don't know. But, it's a new journey, and an exciting one for me. ;)

    I think the thing that I like most about this project, is that when I've follow professional principles and standards, I can hear the difference. It's not placebic. It's not because of the power of suggestion, or because I'm assuming that it should sound better simply because I made these improvements.

    In short, it's not my imagination. I can absolutely hear tonal details that I wasn't able to hear before - with much improved clarity. I can also hear greater detail in the imaging of my mixes that I wasn't able to hear before. The accuracy of the translation of my mixes has astounded me. I can take mixes I've done in my room, play them back on car systems, home theater systems, cd-boomboxes, computers, and it sounds very accurate to what I'd mixed. I don't know of any greater pay-off than that. Principle to Application, Application = Positive Results. I've yet to see a downside. :)

    Thanks to all who responded with answers and suggestions, and feel free to post any further info you think would be important for me to know; and if not, that's okay too. ;)

  9. Space

    Space Well-Known Member

    Hey Andre. I agree that this can be achieved with the proper details of a room and the right person controlling the calculator. Eric could do it because he was a scientist and tested his work. But let us assume we are sitting around a coffee table talking about this aspect of a project.

    The isolation is unknown, the dimensions are here, exterior ambient noise levels and interior are unknown as well. Based on what I have seen from reading behind you guys...less is more and testing is a necessary evil to determine if what you are doing is productive or at the point of diminishing returns.

    Hopefully you understand what I mean.

    Mr. Ease good to see you, but as explained above it is about as good as you can get with so many variables unknown at this point.
  10. avare

    avare Active Member

    The acoustician is the great Eric Desart. He tested designs before implememting them. There was minimal compromise in his work. ARUP is one of the great acoustics companies in the world. Unfortunately they also a black hole when comes to spreading knowledge. I recall a third facility but do not recall the acoustic desgners of it. As I recall it, the 2 facilities not designed by Eric used 4/lb/ft³ mineral wool in their absorbers. I do not clearly recall if the great Eric specified what material he used. 3 independent sources using 3:1 ratios.

    Acoustics is very non-intuitive field. Sound travels more slowly in acoustically absorbent materials. 4" of physical depth may be 6 or 8 some other "acoustic depth." Not coincidentaly, that is depth of the material that has an acousitc impedance 4 times that of air. The 4 times acoustic impdedance ratio is the most common ratio used in absorber design.

  11. DonnyThompson

    DonnyThompson Distinguished Member

    "Acoustics is very non-intuitive field. Sound travels more slowly in acoustically absorbent materials. 4" of physical depth may be 6 or 8 some other "acoustic depth." Not coincidentaly, that is depth of the material that has an acousitc impedance 4 times that of air. The 4 times acoustic impdedance ratio is the most common ratio used in absorber design."

    I have grown to find this field of study absolutely fascinating.

    Well, it is a science, no? I would think that, as with any other form of science based on mathematics (is there any other kind of science?), that there is only right, and only wrong.

    You calculate room dimensions, take measurements, factor in other relevant quotients such as pre existing materials in use and their inherent reflectivity/RT60 at particular frequencies or frequency ranges, then you plug that data into a formula that tells you what type of material(s) to use, how much of it to use, and in some cases, where to use it. Doing so will result in success in gaining the improvements that you seek.

    I'm not saying that one couldn't improve their situation to a certain degree through guessing... even a blind squirrel finds a nut once in a while, but I would think that relying solely on intuition, or "guessing" off the top of one's head - unless your head just so happened to be someone like that of Eric Desart - would likely prove to be highly unsuccessful.

    I know for a fact that there's absolutely no doubt of that in my case.
  12. avare

    avare Active Member

    What does this mean?

  13. DonnyThompson

    DonnyThompson Distinguished Member

    I was merely saying that I've recently started to research this subject and have become serious about studying the science of acoustics, because it fascinates me.

    I've been a recording and mix engineer for about 35 years now, but I never immersed myself into the science of acoustics, and through recent study and research, I have come to realize that I know very, very little - and I would like to change that.

    My research has included reading as much as I can on the subject, and it also includes lurking here - as well as some other acoustics-based sites - and through reading the posts of people like Space, Rod, Yourself and others - I have grown more and more interested every day with the study of this field, to the point where I am actually considering going back to school - at the ripe young age of 55 - to study this science.

    I know very little about this field. I hope to be able to change that - through formal education if necessary -so that I can become less ignorant about it than I currently am. ;)

    I meant no sarcasm whatsoever.



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