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Cutting Rigid Fiberglass for Bass Trap: link to short video - Page 2

post #31 of 72
wouldnt it be more efficient if it was a resonator ?
like here :http://www.jocaviacousticpanels.com/...er/index.shtml
good response from 31hz to 125hz. i will experiment it soon (starting with one then 2)

fiberglass itself would need to be much bigger in volume to start being efficient at low FQs no ?
post #32 of 72
Thread Starter 
Quote:
Originally Posted by titch-- View Post

nice bass traps tweak

I got a question about them. I know you havent ran any tests yet, but how low (hz) do you think you traps will be affective to?

thx

Hi-

Thanks...
I got the idea for these traps from the StudioTips fourm. Here is a link to a comparison of some types of bass traps. They did some measurements of an Auralex MegeLERND, a corner trap, and a superchunk (with the larger, 34" face). I would imagine my bass traps are somewhere between the corner trap and the Superchunk performance, and closer to the Superchunk. Thickness, distance from the wall, and surface area (for total absorbtion) play into it.

Someone with more expertise would need to come in and forecast how low... I would guess as low as 40-50hz.

One thing people that measure these things will tell you is any measurement below 125hz is very tough. You cannot compare one test to another (though I believe you can compare a relative number made during a single test).
post #33 of 72
Thread Starter 
Quote:
Originally Posted by Digital2004 View Post

wouldnt it be more efficient if it was a resonator ?
like here :http://www.jocaviacousticpanels.com/...er/index.shtml
good response from 31hz to 125hz. i will experiment it soon (starting with one then 2)

fiberglass itself would need to be much bigger in volume to start being efficient at low FQs no ?

I am not sure... this one might be good to put on the master thread.

We'd really need to know what the material of those traps are made of. It might be the same as the Auralex MegaLERND. If so, the superchunk comes pretty close for a fraction of the price. I think my trap cost maybe $50, with most of the cost going to the rigid fiberglass.

As far as it being a resonator, I thought they worked across a narrow band. With broadband absorbers it comes down to material (and its density), surface area, and location, both distance from wall, and number of planes it touches (i.e. corner).

Again, I am just learning and think this would be better for the master thread.
post #34 of 72
hi
hi agree cost efficiency, you beat them

now, it seems this model has a wide spectrum of absorption and not one FQ in particular (for which often a resonator with a membrne is tuned on). this is why i find i interesting. the megalernds even are not efficient imho. foam is good up to 80hz maximum. but not below. only "membrane" based system work i think.

no other choice than to test it...

the most expensive but VERY efficient way of having great bass at all the seating area is a wall of subwoofers, either the full width or the two front sides vertically. (or both ).
but this an expensive solution. imagine for instance 6 or 8 $999 subs or even more expensive. virtually you cancell all room modes, the line of subs absorb also the bounced back wavelengths instead of the wall against which they sit. they (i think) play sort of resonators themselves, the launch bass but absorb the bounced back bass.
post #35 of 72
Sweet thx for the info. It would be nice with that much coverage down to the 40ish area, even lower if possible.
post #36 of 72
Quote:
Originally Posted by Tweakophyte View Post

Hi-

snip

I would guess as low as 40-50hz.

One thing people that measure these things will tell you is any measurement below 125hz is very tough. You cannot compare one test to another (though I believe you can compare a relative number made during a single test).

Hey Tweak:

That is a good guess I think... but a lot depends on what we mean by "effective".

If we are talking about the frequency range at which the device's performance drops out of the stratosphere of sabin numbers per unit we enjoy when we use corner mountings then I think you are dead on the money. At about 40 - 50 Hz we are going to see the a big droop.

But, you need to recognize that just because a device has dropped back to the more rational absorption performance range we might expect to see in the rest of the room - for example, the levels we see in a wall mount - that doesn't mean it isn't working anymore. To me that is just an indication that the "free ride" we get from corner mounting stops and we have to go back to normal performance expectations.

Look at the phenomena this way... a pair of 32" faced 703 SuperChunks running from floor to ceiling is going to add something on the order of half a dozen sabins per linear foot of corner treated at 100 Hz. That is a shocking number when you consider the device has 2.66 SF of face per linear foot and therefore 2.66 sabins would mean an "open window" of that size - no reflection of sound whatsoever. At first blush the idea you might get 6 SF or so of open window for every 2.66 SF of treatment seems to defy physics. Its an acoustical black hole, Wow!

But cool as that is, it aint magic. Instead its a byproduct of resonant behavior. Corner mounted porous absorbers typically exhibit a sharp peak in performance - typically somewhere in the 63-100 Hz bands. This little elf has been dubbed the "100 Hz peak" for lack of a better name. The 100HP moves around a bit, and gets broader or sharper, depending on the topology of the construct [wedge versus panel], the density of the foam or mineral fiber, whether you have upholstery and what kind of fabric is used, whether you space the devices or mount them edge-to-edge, whether you capture tri-corners, and the overall thickness and face dimension, etcetera.

The basic concept is however pretty straight forward, namely that porous absorbers in corners are a mass-spring resonate system. The air in the corner is the spring, held in place by the room's walls, and the panel or wedge of absorbent material is the mass. Mineral fiber or foam, naked or upholstered, panel or wedge, thick or thin, big face or small face... doesn't matter. They are all mass-spring systems with a resonance kick somewhere around 100 Hz, and, that kick has a front side and a backside on a measurement graph. You reckon [I think] that the backside of the free ride starts running uphill somewhere around 40 to 50 Hz - and from everything I have seen, that is a fair statement, but concluding that this means no work can be done using a porous absorber south of say 63 Hz does not follow.

Expressions of this peak can be quite large [on the order of a 50% increases over the median absorption outside the peak center band], and can appear very sharply expressed. Or, they can be so subtle as to not look "peaky" at all, but rather a gentle rise and fall. There are ways to soften the peak - and to broaden it - or you can even make it more "peaky" by adding a membrane [a generally nutty idea IMO - but it could fit the need for a particular application just so].

But, keep in mind that whatever you study as to this effect, to some degree the actual effect is masked by the 1/3rd octave band reporting technique. For example, a peak straddling a pair of bands could be quite "peaky" but not show up that way in the graphs - alternatively a peak of exactly the same magnitude but which falls completely in a single band will have a much more dramatic expression on a graph.

This is part of why it always best measure for low frequency performance in situ as the intrinsic properties of devices are hard to measure very low in the band, such measurements are hard to interpret [even if you know everything about how they were taken], and the results in any given room will vary enormously even using the same devices when you vary the placement. There are a number of "known good" devices, and known good treatment placement schemes, but nothing can beat verifying the installation with an on site measurement system [and a professional acoustician too if you can swing it].

PS: A couple of other comments:

The idea that foam doesn't work as well as mineral fiber is falsehood. Acoustic foam works great... non-acoustic foam does not. By the same token, there are thermal insulation materials that have excellent acoustic properties, and ones that do not.

Comparing absorption measurements at 100 Hz even or even lower, can be done both rationally and usefully provided you have all the information. It takes some intellectual honesty and a keen understanding of what you are looking at, but a lot can be learned from comparing measurements - even between measurements taken by different labs -if you have full details of the measurement procedures [sample size, sample placement, the % Uncertainty of the measurements, etcetera]. When measurements are proffered absent these details comparison becomes quite dicey stuff as you suggest, especially below about 80 Hz. This is not so much a limitation of the science - though physics does come into play - but rather, the key limitations are artificial constraints created by researchers and manufacturers who hide data.
post #37 of 72
Tweakophyte,

Awesome video, info and attitude. I loved it all, and it is very helpful.

Thanks for posting mate!

Joel DuBay
post #38 of 72
Thread Starter 
Scott-

Thank you for the comments. I am still digesting them!
--------
Joel-

No prob!
post #39 of 72
Thread Starter 
Hi-

Well... I built 3 of the 4 panels I had planned on. Two of the panels were for the front wall, and the other 2 were for the first reflection points on the side wall.

...then I asked for my wife's help...

She barfed all over all of the panels. In the end I "negotiated" keeping the 2 panels on the front wall. I post pics in the next few days.

The good thing is I got to do some listening. It was a short listening session, late at night (so lower volumes) and I could hear a definite tightening of the mid-bass. This run was the first 3 songs on U2's Joshua Tree. "Where the Streets Have No Name" has many strums of the same note in a row, so you can really focus on them. The notes were smoother and more distinct.

I also checked out a bit of Dave Brubeck "Time Out". On the "Blue Rondo..." track the Piano and Bass play the bass line in unison, and again, the same note in a row. Now for the fun part (as many tweaks create)... I heard something new in the music... a slight dissonance between the bass and the piano. I think, because the fundamental got out of the way so to speak, the tonal color of the bass became more clear. It was subtle but distinct. I had a similar experience with "Take Five".

Like I said, I'll try to post more pics this week.
post #40 of 72
Sounds to me like you got some treatments for sale ;-)

This is the same thing I'm struggling with in my family-room wannabe theater. Just how do I sneak treatments in? Or is the better question, when do I start building a dedicated theater in the basement?

Bobby
post #41 of 72
Thread Starter 
Here are some pics:








I used some chair glides and thich felt to give my panels some space away from the wall. I found that I liked having the space away from the front wall for deeper absorbtion, but the side walls looked better with thinner panels... until my wife saw them...

post #42 of 72
Nice stuff Tweak!

What's the WAF hurdle... why does she object... is it a color thing?
post #43 of 72
Thread Starter 
I actually made 2 sets of panels. For the first one I used 1x3 MDF. Even when I pre-drilled the holes, the MDF split. Also, over the 4' span it sagged enough to bother me when I hung it on the wall.

You can see one of the splits here (lower right corner):


For the second one I used B or better, pine 1x4s. I used the 6' lengths because I had trouble finding a straight board in the other lengths. I still had to spend some time in the lumber pile to find some good, straight 6' boards. Each panel took 3 boards. Two boards were cut 1/16" short of 4'. The remaining board was cut 1/16" shy of 25 1/2" (so the inside dimension is 2').

I cut them slightly short for two reasons. First, with a perfect cut at these lengths there would be a great, friction fit. Second, the guys at HD are careless with their cuts. I had to return one set because it was 1/4" too long (you could see his mark, and many of the cuts were out of square. (Don't go there before close on a Saturday night!)

The frames are held together with two 3 1/2" screws in each end. I tapped in some wire brads to hold the rigid fiberglass towards the front of the panel. I don't have pics, but I found it easier to wrap the frame and have the "seam" of the fabric be in the middle. I used some light, spray adhesive to hold the fabric together.

Hope this helps!
post #44 of 72
Thread Starter 
Quote:
Originally Posted by Scott R. Foster View Post

Nice stuff Tweak!

What's the WAF hurdle... why does she object... is it a color thing?

Thanks Scott.

It could be the color, but in the end I think she did not like the idea of "things" hanging on the wall. We agreed to make this room as "normal" looking as possible, and the panel take that away for her. I may try with her again.

I think I got my 80% performance with just the 2 panels. I am also going to do some small treatments in the shadow box area. The center sounds great at is it and I think the sub acts as a diffuser, but I have the rigid fiberglass, so I might as well play around.

In my next theater I will have room treatments as part of the plan, and make the walls with GOM or some other fancy fabric that SHE can pick out.
post #45 of 72
Quote:
Originally Posted by Tweakophyte View Post

snip...

or some other fancy fabric that SHE can pick out.


I think you are on to something there Tweak.
post #46 of 72
Quote:
Originally Posted by Tweakophyte View Post

I found it easier to wrap the frame and have the "seam" of the fabric be in the middle. I used some light, spray adhesive to hold the fabric together.

Tweak - thanks for sharing! I followed everything, right up until this quote. Can you explain more? Are you saying that you did not staple the fabric to the back (wall side) of the frame?

ND
post #47 of 72
Thread Starter 
Quote:


Tweak - thanks for sharing! I followed everything, right up until this quote. Can you explain more? Are you saying that you did not staple the fabric to the back (wall side) of the frame?

Hi-

No, I stapled. Let me try to explain (until I can post a pic). I can use this for an example for now:


See how the frame is in the center of the fabric? If I folded the sides over they would meet in the middle of the frame. I could still staple the perimeter of the fabric (which I basically did) but the fabric in the middle would still be a little loose (it was).

Since I was hanging them horizontally on the wall, the bottom piece of the fabric was really sagging. I lightly sprayed some adhesive (#45?) on the fiberglass and stuck it so it would not sage. The top piece of fabric still drooped a little.

None of this matters if you don't want to wrap the back of the frame. Since I have kids, I did.

Make sense?
---
Btw, the way I did the first frame is I shifted the frame over so one side of the fabric met the first edge with little overlap. The second side of the fabric reached all the way around and back to that same side (so the piece on the left stapled on the right). It worked, but it was a bit of a pain to do and made that side of the frame a little bulky.
post #48 of 72
That helps! Thanks, and good job!

ND
post #49 of 72
Quote:
Originally Posted by Tweakophyte View Post

The Johns Manville equivalent is JM 814 (make sure it's the 3psf density).
I got mine just off of 70 and Colorado

Can you tell me exatly where you got it? I'm getting tired of the run-around from the box stores and contractors trying to get the OC703. You'd think I was asking to date their wife, or daughter, or something...
post #50 of 72
I have been looking for but not finding

source for oc3 (or whatever today's recommended foam for panels and superblocks is - how about john mansvile

source for frk - actually, what is frk - t looks like spun glass

source for acoustic cloth.


I searched but, a reference to a forum entry would be fine.
post #51 of 72
Thread Starter 
Hi-

FRK is something like Foil Reinforced Kraft paper. It is used where you don't want to absorbe the high-frequencies.

I just used speaker cloth from JoAnns Fabrics. You can find a 40-50% of coupon if you sign up for their mailing list.

The Johns Manville equivalent is JM 814 (make sure it's the 3psf density).

click on spin glass

I got mine just off of 70 and Colorado (in Denver, but you can search here).
click on the HVAC when you search

A big nod to Bob Gold's site, and other's who have helped vet out which material to use.
post #52 of 72
I like this type of bass trap a lot, but I was a bit surprised when I found out how much it would cost to buy all the rigid figerglass insulation I would need. I can't find anywhere locally that carries anything equivalent to the JM or OC stuff and the places online seem to be about $70 for six 2'x4' panels. So, for two 7.5' tall corner traps using the 34"x24"x24" section, it would cost $280 + shipping just for the insulation!

Would stuffing an equal wieght (45 lbs) of normal fluffy fiberglass insulation into a triangular collumn of the same size would produce similar results?

I'm having trouble finding the density of fluffy fiberglass. From some info I got on Geisen's DIY bass trap page, I calculated 1.125 pcf. I called OC, and the person I talked to said 0.53 pcf. The 0.53 pcf value is almost identical to a value of 0.54 pcf that I calculated for blown insulation from some other info I got from the web. I'm waiting on a return call from Owens Corning, to see if we got mixed up on which insulation type the density was for.

If anybody happens to know this density info, could you let me know? Or if someone knows how much a roll weighs and what it's dimension are, I could calculate it that way.

Thanks,
-Stephen
post #53 of 72
Thread Starter 
I think I paid about $9 per sheet for the JM... I've heard that $8-12 is normal. Shipping will kill you. Did you look for HVAC insulation on the JM or OC sites?

I can't answer the fluffy question...
post #54 of 72
I tried calling all the places listed on OC's and JM's website around where I live and none of them have, or said they could get, the rigid fiberglass boards.

I just got a confirmation from the very helpful people at Owens Corning that the fluffy stuff has a typical density between 0.5 and 0.6 pcf. This means that, using the fluffy stuff, the cost for the insulation would be about $180 for two 7'5 foot tall 24"x24"34" triangular collumns. This doesn't include materials to build a light frame to stuff the fluffy stuff into.

So... it's not as much of a cost saver as I had hoped. I'm going to check some other stores around to see if I could get the fluffy insulation cheaper anywhere. I'm hoping that the loose insulation meant to be blown into place will be cheaper. I'll let you know what I find out.

-Stephen
post #55 of 72
very good work
post #56 of 72
Thread Starter 
Hi-

Thanks... too bad I couldn't find my 3m filter mask!
post #57 of 72
I have a quick question. I may need the traps aforementioned. Can I make the traps, and add a poster over them, or does that defeat the purpose?

Also, could I lay a clearish/see through fabric over the poster, over the treatment to make it more attractive to the lady?
post #58 of 72
Quote:
Originally Posted by ccarzoo View Post

I have a quick question. I may need the traps aforementioned. Can I make the traps, and add a poster over them, or does that defeat the purpose?

Also, could I lay a clearish/see through fabric over the poster, over the treatment to make it more attractive to the lady?


bump
post #59 of 72
Quote:
Originally Posted by ccarzoo View Post

bump


anyone?? Help???? :-)
post #60 of 72
Quote:
Originally Posted by ccarzoo View Post

I have a quick question. I may need the traps aforementioned. Can I make the traps, and add a poster over them, or does that defeat the purpose?

Also, could I lay a clearish/see through fabric over the poster, over the treatment to make it more attractive to the lady?



????
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