Johns Manville Fiberglass Duct Board type 800

Hi guys, I have some leftover rigid fiberglass duct board in 1.5" thickness. I want to use it as a broadband corner absorber. The question is, should I double-up in order to achieve a thicker (3" or more) piece? Also, the board is faced on one side. Should the facing be towards the outside, or facing the corner? Thanks, Jules

kidego32 wrote:

Hi guys, I have some leftover rigid fiberglass duct board in 1.5" thickness. I want to use it as a broadband corner absorber. The question is, should I double-up in order to achieve a thicker (3" or more) piece?

Yes. The thicker you make it, the flatter the absorption.

Also, the board is faced on one side. Should the facing be towards the outside, or facing the corner?

All the facing does is reflect high frequencies. Ideally remove it. Second, put it on the inside facing the corner. Andre

Doubling will give you more absorption, but won't change the range of frequencies much; ideally, you should remove backing on at least two layers and stack them for a broadband corner absorber... Steve

knightfly wrote:

Doubling will give you more absorption, but won't change the range of frequencies much; ideally, you should remove backing on at least two layers and stack them for a broadband corner absorber... Steve

Sorry Steve, I thought what I didn't write. :oops: At high frequencies, the thickness will not make much difference. As one goes to the lower frequencies, the more the material absorbs at the 1/4 wavelengths that the material is spaced away from the surface(s). The thicker the material, the wider the band where the material is absorbing, hence a flatter absorption curve at the low frequencies. Andre

Thanks for the replies guys. I recall reading on an older post that the backing actually helped with the lower frequencies. Is is this the actually the case?

IF the backing is facing OUTWARD, it will lessen the absorption of higher frequencies; this, and a slight tendency of the backing to act similar to a panel-type bass trap, could be construed as "trapping more bass" - However, I would think this effect would be VERY slight; normal panel traps operate by the front panel moving in response to sound pressure, then internally this pressure change is converted into air movement; finally, the air movement is converted to heat when it passes through (and back through) the absorbent. The absorbent (important point here) is NOT to be in contact with the front panel (unlike being actually attached to it) or else the panel becomes too damped, and little if any conversion takes place. Losses (trapping) in a panel trap occur first with the power required to flex the front panel, then by traveling both directions through the absorbent inside, and (if there's any sound energy left) by bouncing off the front panel and back into the absorbent again. Energy in the absorbent itself is converted to heat (by the resistance of the closely spaced fibers in the absorbent to the passage of air). Bottom line - Not so much... :?

Thanks Steve for that very clear explanation. I'll be removing the backing tomorrow ... :)

Steve, > I would think this effect would be VERY slight < Not as slight as you might think. I assumed you already saw my Density test report that also compares the FRK paper facing. If not, here it is: www.ethanwiner.com/density/density.html Look at the comparison between 3-inch 705 and 3-inch 705-FRK. Especially at the ~100 Hz mode and above, the FRK helps quite a bit. --Ethan

Ethan Winer wrote:

Steve, > I would think this effect would be VERY slight < Not as slight as you might think. I assumed you already saw my Density test report that also compares the FRK paper facing. If not, here it is: www.ethanwiner.com/density/density.html Look at the comparison between 3-inch 705 and 3-inch 705-FRK. Especially at the ~100 Hz mode and above, the FRK helps quite a bit. --Ethan

The tests are usefull as you now I have thanked you for them right on your site, but you also know the limits of the results due to the software limitations. PLEASE DO NOT present short general comments that DO NOT indicate the limitations of the interpretation of results. Straight to the point, in direct terms how much change did the material make? Quantified answer please, no generalities. Respectfully; Andre

Andre, > you also know the limits of the results due to the software limitations. < What limitations? All those tests were done at the same time in the same room, and all that varied was the absorbing materials. > PLEASE DO NOT present short general comments that DO NOT indicate the limitations of the interpretation of results. < Again, PLEASE TRY HARD not to be a jerk. If you have something to contribute, then please do so. Insults get you nowhere, and only show everybody reading that you have an attitude problem. > Straight to the point, in direct terms how much change did the material make? < Look at the two graphs I mentioned, for the changes I described, and you'll clearly see the difference in absorption. --Ethan

Ethan Winer wrote:

Andre, > you also know the limits of the results due to the software limitations. < What limitations? All those tests were done at the same time in the same room, and all that varied was the absorbing materials.

If you want. I can post the URLS to teh SOS threads where the weaknesses are detailed in depth. The sofware automatically scales the graphs to the top of the highest peak in the curve. You do not know from the graphs if A or B has greater overall absorption and how that would transalte tot the overall room.

Andre, > I can post the URLS to teh SOS threads where the weaknesses are detailed in depth. < Be my guest. All the negative comments there were more about attacking me personally than examining test methods. If this is the way you prefer to "discuss" acoustics, far be it from me to suggest otherwise. I consider the personal attacks by you and your friends as a badge of honor. In some twisted way all the name calling appears to be simple professional jealousy. > The sofware automatically scales the graphs to the top of the highest peak in the curve. You do not know from the graphs if A or B has greater overall absorption and how that would transalte tot the overall room. < Not at all. The magnitude of the peaks is irelevant. As my Density report page explains clearly, what you should look for is a lowering of each mode's Q and a reduction in its ringing time. To refresh your memory, this is from my report:

Note that the tests described here do not give any numbers! All analysis is done visually, looking at how the modal ringing decay time is reduced (the "mountains" come forward over time) at each mode frequency, and also by seeing how the Q of each mode is lowered thereby making the peaks broader.

You seem to be a bright enough fellow, but if any part of the above sentence is still not clear to you please let me know and I'll try to find another way to explain it. --Ethan

Ethan - "Again, PLEASE TRY HARD not to be a jerk. If you have something to contribute, then please do so. Insults get you nowhere, and only show everybody reading that you have an attitude problem." Andre may be a bit brusque at times, but I have yet to see him resort to name calling, unlike yourself. You will stop doing that yourself IMMEDIATELY, or you will be banned. I have no problem with "spirited discussion", and even less problem with accurate information. I would like to see the thread you refer to on SOS, can someone please post it? I am curious now as to how much "personal attack" is going on, and how much may be valid criticism of method. However, I prefer to come to my OWN conclusions instead of listening to a bunch more "he said/she said" stuff, so a link would be appreciated. Ethan, I noticed in one of your graphs on the link above, that NONE of the peaks in the waterfall reaches 100; I'm not sure why, I've not had quite enough play time with the SW to know if that's because of a higher frequency peak affecting it, or?? What I DO know, is that there is a "calibrate" function in ETF, which I've used - it's very easy and results in more "real" numbers. Not seeing that used, I also would wonder if you ran the CAL FILE for the RS SLM you show in the pix - I searched that page for "cal" and came up empty. Again, I'm in need of a lot more "cockpit time" on ETF, (I bought the entire package with plugins and calibrated mic/preamp) but as a cursory observation it seems to me that you might avoid at least SOME of the criticism by using the Cal function - it's not difficult, and might give people less ammo (and the tests a bit more credibility) Regardless, if I see one more personal attack in the form of name-calling, it will be the last. Telling someone they are wrong is not name-calling, linking to other threads is not name-calling; bringing it back HERE will get you deleted and banned. This is MY home, and that's about the ONLY rule here. Either live with it or leave. VERY sincerely... Steve

Steve, > I am curious now as to how much "personal attack" is going on < A lot, which is why I may have overreacted a bit. You apparently have no idea how much abuse I have received from these guys, and I am sick of it. Though in fairness to Andre, he has been less rude to me than his friends. But this is not the first time he's used an accusatory tone rather than simply stating points he disagrees with. If you want to police this forum for rude behavior, and I encourage you to do so, you need to do it evenly. If you want to see the kind of abuse and personal attacks I'm referring to, start here: www.soundonsound.com/forum/showflat.php?Number=77684 Then look here: www.soundonsound.com/forum/showflat.php?Number=65497 And don't miss this one: www.soundonsound.com/forum/showflat.php?Number=63346 Or this one: www.soundonsound.com/forum/showflat.php?Number=58040 You can tell when the trouble-makers arrived because there's so much name calling the forum mods have to lock the thread. And these are just at SOS. I'm sure you know there are half a dozen more at Recording.org, and many more in other forums. --Ethan

Steve, > I noticed in one of your graphs on the link above, that NONE of the peaks in the waterfall reaches 100 < That's not quite true. Unless you click Calibrate, which I purposely did not do for those tests, the highest point is set at 100. In many of my ETF graphs the highest point is the mode just above 200 Hz, which is exactly at the right edge of the graph. > I also would wonder if you ran the CAL FILE for the RS SLM < That aspect of calibration doesn't work, at least not using the files you find around the 'net. Every RS meter is different because, I assume, they buy whatever mike element is cheapest whenever they're ready to order. Further, it's not really necessary to calibrate when measuring low frequencies. I did a side by side comparison with my RS meter and my expensive AKG mike, and they were within a fraction of a dB below 500 Hz. Even more to the point, the absolute response is irrelevant because all that matters in my tests is the relative change from one run to the next. > it seems to me that you might avoid at least SOME of the criticism by using the Cal function < Calibration is not necessary for the tests I did. And as you'll see if you follow the links I posted above, it's not likely that anything I do will avoid all the name calling and personal attacks. > and [give] the tests a bit more credibility < If there's any aspect of my tests you're unclear on, please let me know and I'll explain it. Again, not calibating was intentional to keep as much information as possible on the screen. --Ethan

Steve, Further to your question, "I am curious now as to how much 'personal attack' is going on" here's the latest totally unprovoked BS from your pal Eric Desart: www.recording.org/ftopict-26170.html You can follow his link in that post to another bunch of recent nonsense. I know you're friends with Eric, but he's their ringleader and he's the main reason the unwarranted personal attacks against me and my company continue. --Ethan

Ethan, thanks for the links; I'm too deep in the proverbial doo-doo ATM to check them, but I will. The RS meter cal file I referred to is included with ETF, (or download from their site, IIRC) - but most of the buzz I see about that meter indicates that it's not that bad, especially compared to the humongous topography of most rooms' response - I'd consider myself fortunate to get the room under control enough to even CARE about the meter's response for LF stuff. I understand your reasoning for keeping the curves at the peak for better readability, but have (un-tested as yet) reservations about the results; (without testing it myself yet) I would think that it's possible a person could make a change in a room that would affect frequencies NOT being considered, and mistakenly think they had done one thing to one band when in fact they had changed everything BUT that band - I know this isn't a strong likelihood, but I don't like to make statements I can't back up with fact unless I at least label them "gut feel", "SWAG", etc - that's my reservation about NOT using the CAL function. I've played with this a small amount in my own (acoustically crappy) living room, and was kind of surprised by the results (didn't keep the files, that was some of the first part of my "learning curve", I know better now :=) As to evenhandedness, I'm pretty sure I've covered this before; your comment made no sense to me, since for "evidence" you posted several links to places I never go. I won't tolerate name calling on this board, period. I've told both you and Eric this. Eric's response was to go away, at least for a while. When he did return, he was civil. Andre and I have communicated about this, and he has agreed with me - no more. I saw the RO crap start up again, Rod has since edited and commented. My take is very similar - I've heard so much of this feud that I simply don't care anymore - if EITHER you or Eric considers me a friend AT ALL, you'll keep this crap the hell out of MY HOUSE, PERIOD. I've already stated the alternative, and it applies to ANYONE who joins in. ENOUGH, ALREADY !!! I don't know, short of dynamite, how to make this any clearer to you. The purpose of this forum is to help people accomplish what they could otherwise not afford, and to learn; it is not to air personal vendettas, no matter WHO is being wronged or who thinks they are. If either you or Eric had a CLUE just how disruptive this fight between you has been, you would take it offline and KEEP it offline - aside from a few shit disturbers, NOBODY WANTS TO HEAR THIS ANY MORE. Especially ME. So take it to court, or take it onto the deck of a burning ship and duke it out with chainsaws, whatever - but take it away from here. Any further "yeah but" crap will be taken as a direct refusal to abide by my request for an end to the feud, and you will NOT like my response. Please do NOT make me go there, it isn't at all productive. Any CONSTRUCTIVE discussion that is RELEVANT to the goals of this board, bring it on. If you're STILL somehow not clear on this, then be ready - it ain't gonna be pretty... Steve

Steve, > The RS meter cal file ... most of the buzz I see about that meter indicates that it's not that bad < It's not that good either, for the reasons I stated. Someone in another forum - AVS I think - said he tested a number of RS meters and found they all varied quite a lot from each other. I can believe that. But again, the variation is above the bass range. And again, absolute response doesn't matter at all for my density tests. > I would think that it's possible a person could make a change in a room that would affect frequencies NOT being considered, and mistakenly think they had done one thing to one band when in fact they had changed everything BUT that band < Look at how the span from each peak to its adjacent null is reduced. That proves the fiberglass affected those frequencies. > I won't tolerate name calling on this board, period < I agree completely. --Ethan