[Red Dragon Audio]

arthurs,

Be sure to wrap the edges of the cut perforated metal with duct tape or a gasket to prevent it from cutting through the fabric (or your fingers by accident).  Have fun with your project and when they are complete take pictures to show off your glorious handy work.   

[arthurs]

Thanks Ryan, I'll be sure to cover the edges as well.  I'll take some pictures when they're done, but to level set expectations, "Glorious handy work" and me don't often end up in the same sentence, so I doubt my finished product will be on that level   But I am hoping with my wife taking charge of fabric selection, wrapping and finishing, they will come out to look nicely done.  aa

Next step is stuffing a few and measuring the room this weekend with some help from Brian (BRJ)  Should be interesting, currently have enough roughed out for 4 columns of 20" from floor to ceiling (corners), 4 columns of 16" from floor to ceiling (front and back walls), and 4 columns of 9" from floor to ceiling (side walls.)  Probably more than I need, but we'll see what we end up discovering this weekend....more to come, in fact I'll take some shots of the raw goods in the room once we tweak it this weekend.  Thanks again to everyone for their input and wise counsel, it's really helped, alot.

[brj]

To extend the questions a bit...

Over the holiday, I spent several hours playing with 4 3' long sections of 12x1" (12 inch inner diameter with 1 inch of 3-4 PCF fiberglass) Johns Manville Micro-Lok pipe insulation (PDF link) in someone else's room.  I had a calibrated mic and USB pre-amp hooked up to a laptop running Room EQ Wizard, and we tried dozens of placements.  Two of the tubes were stuffed rather heavily with paper backed fiberglass wall insulation.  We ran out of time before we had a chance to strip off half of the FSK on any of the tubes.  We didn't have multiple sizes to experiment with, so other sizes may have been more beneficial.  (I also wish the tubes with thicker insulation were not so expensive... the local insulation warehouse wanted almost $100 each for the tubes with 3" thick insulation!)

Some quick impressions on which I'd welcome commentary:

1) I'm not the only one with a really challenging room!    This particular room was a living room with a large opening to the kitchen to left front of the system, a set of french doors to the dinning room to the left rear, a large stone fireplace with glass doors to the right, a 36" CRT TV in front, a 75 gallon fish tank mounted in a huge wall-spanning shelving unit in back, and a cathedral ceiling over it all.

2) No matter where we placed the cylinders, we could effect a change in the measured FR, but it was never a universal improvement.  Some frequencies improved, while others were hurt.  Some placements definitely helped more than they hurt, but there was always at least some penalty.

3) Changes usually occurred within very tight frequency bands (usually 10s of Hz rather than 100s or 1000s), but could create as much as 10 dB of change within those bands.

4) In this room, placements in the rear of the room seemed more beneficial than placements in the front.  (Which is unfortunate, since none of the rear positions are feasible as permanent installation locations.)

5) With the FSK fully in place on the cylinders, FR effects could be seen all the way up to 20 KHz, although the effective dB change tended to be less significant than at lower frequencies.

6) Corner placements helped, but sidewall placements were also surprisingly useful.

7) Fiberglass stuffed tubes seemed more effective than unstuffed, but it seems to vary a bit with location.

I'm sure all of this varies with the density and thickness of fiberglass, degree of stuffing, as well as the degree of FSK removal.  Unfortunately, we couldn't play with those variables.  I'm also starting to believe that you need a *lot* of tubes to have a really significant broadband impact.  Of course, this appears not to be a problem for Art!

(As an aside, it really is amazing how many different things can affect the FR of a room/system.  Moving the mic, furniture, an artificial tree, or even a set of fireplace tools generated clearly visible shifts in the FR plots.)


Art, I can bring my measurement rig and camera over sometime this weekend if you'd like to experiment in your newly created "Sherwood Forest" (which also happens to be the nickname for the missile compartment on U.S. ballistic missile submarines! )  I have no doubt that others have better rigs than mine (especially on the software side), but we can at least get an initial grasp on what placements work best.

[zybar]

Great post Brian.

It reinforces a few concepts:

1.  Acoustical treatments work!!

2.  You need to take care where treatments get placed.

3.  You need to measure.

4.  You need to have a lot of coverage to be effective at taming low bass.


Have fun at Art's place and make sure you take pictures.

George

[Daygloworange]

Wow, given that those aren't large tubes, and their sizes, and the fact only 2 of them were stuffed, a 10 db reduction is remarkable.

I'm really interested in the results that Art is going to achieve with his traps. I'm anxious to read about your measurements and findings. Great work. Thanks for the effort, and info. Kudos brj .

Cheers

[brj]

Thanks for the kinds words, guys.  More thanks are owed to my brother, who actually plunked down the cash for the experiment based on some rather thin comments on my part - not to mention that he did all of the moving and thus bore the fiberglass induced itching!  We'll see if Art's more extensive experiment gives us a clearer path forward.

Wow, given that those aren't large tubes, and their sizes, and the fact only 2 of them were stuffed, a 10 db reduction is remarkable.

A couple of cautions...

1) I agree that in terms of bass wavelengths a 14" outer diameter doesn't sound large, but it turns out to be rather substantial when you see it in the room.  They were far more visually dominating that I expected, and would definitely need to be covered with a suitable fabric to have any chance of blending in to anything other than an industrial setting.  (The white FSK really stands out -  I swear they look like the world's largest cigarette!)

2) Be careful with that 10 dB number.  Again, we saw deltas that large only occasionally and only over very narrow frequency bands.  In addition, there was usually a penalty over a different frequency band, and often almost as large.


One other comment... I'm not sure how much pressure the self adhesive strip on FSK can take.  It might be worth bonding the FSK seam in a stronger manner if you plan to stuff the tube with a lot of fiberglass.  Liquid nails along the fiberglass edge of the seam may help, although it would be a bit awkward without someone else to help.

[TomS]

brj - this is great insight you're providing, so thanks!

Just curious to know why one would choose to implement tubular traps (other than maybe the ability to rotate the reflective half around) vs. "flat" traps like RealTraps, GIK Acoustics, etc.? 

- bpape or ethan feel free to school me but be gentle.

Tom

[brj]

The last time I checked, Ethan, Nathan and Bryan all have flat or corner fitted bass trap products out now, and based on their demonstrated expertise, I presume that they all work very well.

The tube trap idea is just appealing because it is a relatively easy way to experiment.  Less construction is required compared to home-built flat panels because tubes are inherently freestanding, and thus require no mounting hardware or external support.  Basically, buy, stuff, cover, cap if desired, and you're done.

In addition, there some situations where you might not be able to use flat or corner panels.  My room, for example, has no 90 vertical corners at all.  I intend to build a couple of tube-trap "plant stands" that I can strategically place in the back of my room!

[arthurs]

Thanks for the kinds words, guys.  More thanks are owed to kj, who actually plunked down the cash for the experiment based on some rather thin comments on my part - not to mention that he did all of the moving and thus bore the fiberglass induced itching!  We'll see if Art's more extensive experiment gives us a clearer path forward.

One other comment... I'm not sure how much pressure the self adhesive strip on FSK can take.  It might be worth bonding the FSK seam in a stronger manner if you plan to stuff the tube with a lot of fiberglass.  Liquid nails along the fiberglass edge of the seam may help, although it would be a bit awkward without someone else to help.

All the seams on the tubes here have been liquid nailed prior to the FSK being adhered to "close" the seams.  On the larger 22" tubes I went ahead and put a bead down the inside of the tube on the seam as well, I figured if I could crawl inside it, I should go ahead and liquid nail it some more...

Believe Brian when he tells you that the fiberglass is a pain, I've glued and prepped 32 tubes myself so far, and it really does get itchy (even with long sleeves and gloves) and they are a pain to move around.  My wife and I have also narrowed it down to one of three fabrics, and as soon as we finalize the number I'm going to use, we'll do a test run on a spare to finish them out so they look good....but hey, kudos to the guy who shows up with the microphone and software....

(for those who don't know me well, I like my sarcasm dry as dust...)

One other note, if you smoke (as I do) the giant cigarette look is oddly appealing....

[brj]

but hey, kudos to the guy who shows up with the microphone and software....

Hey, I hauled my rig 1000 miles to measure that room, so that should earn me a few points!    Besides, I actually did help out with the loading, unloading and moving a bit as well - I just managed to dodge the unpleasant stuffing part!

As for your traps, I'm happy to help out this weekend now that I'm back in town - just give me a shout.

One other note, if you smoke (as I do) the giant cigarette look is oddly appealing....

  No comment!

[BobM]

Just wondering what the effect of the different diameter tubes were for this test? I would assume that the larger diameter tubes absorbed better in the lower frequencies, and the smaller ones a bit higher in the band...

... but sometimes the counterintuitive prevails (think golf).

Thanks,
Bob

[arthurs]

I think Brian and his brother only had the 12" diameters Bob, so while I think most of us agree with what you suspect about diameter making a difference, we haven't tested for it yet.  We'll be able to measure/hear the effects of different sizes in the next steps of the experiment as I've got 3 different diameters (9" - 16" - 22") and we can also play with height variations which they were not able to do.  Keep in mind they were using four 3' sections and we'll be have the ability to mix and match 36 different sections in my room.

[brj]

That reminds me... I keep meaning to track down a reference that discusses tube size (for a given thickness and density of fiberglass) vs. trapped frequency.  Does anyone know of such a resource?

[bpape]

It kind of depends on whether it's filled or not.  If it's just a 1" or 2" tube hollow, then it will be some more of a filtering effect based on the diameter and the distance from the corner/wall to the front face.  Tie that in to the quarter wavelength of a given frequency and you can find where it will be most efficient. 

Just for giggles, let's take the 50Hz.  50Hz wavelength is approximately 10 feet so to have a trap centered and filtering at that would require one that is approximately 30" in diameter.  You'd get less absorbtion above and below that point but then once you get up to say 500Hz or so, it begins to come back closer to a 1.0

When said tube is filled, it will absorb maximally down to that point and then roll off slowly as frequency drops but perform just as well higher up in frequency - all the way into the kHz range.

If it's filled, then it should be maximally effective from the point described above and higher - assuming an unfaced tube.  If it's faced, the mid/HF absorbtion will roll off at approximately the same rate as a flat piece of the same density and thickness with facing on it.

www.bobgolds.com has a ton of absorbtion tables for faced and unfaced materials of various thickness, density, materials, and manufacturers.

Bryan

[brj]

Thanks for the explanation and exceptional link, Bryan!  That link has the single most comprehensive listing of acoustic specs by type and brand of insulation that I've ever seen.

Any comments on how the degree of stuffing, and thus the density, affects the performance of the tube trap?

When we were experimenting over the holiday, we didn't bother to pull the paper backing off the rolls of R15 fiberglass insulation we were using.  I suspect that the paper won't affect much acoustically since mid and high frequencies will already be reflected or absorbed by the scrim or tube, respectively.  Removing it, however, would certainly make the stuffing easier and more evenly distributed throughout the tube.

Personally, I'd still like to source some tubes with 3" of insulation thickness for my "plant stands" if I can find it for a reasonable price.  These will be both more effective, and better able to bear weight.  Of course, even the tubes with 1" thick insulation appeared surprisingly sturdy and will probably have few problems supporting the average plant or knickknack, but I'd rather err on the side of caution.


By the way, the plot shown in this sub-link of the page Bryan reference above shows the kind of effect we were seeing with our crude experiments.  Note both the various degrees of change in dB and the narrowness of the individual frequency bands over which the changes occurred.  (The scale of the plot is quite narrow.)

[Daygloworange]

I was surprised by the 705 versus 703 rigid fiberglass. At double the density, the 705 doesn't seem to do a heck of a lot more compared to the 703, in fact, it seems to do less at times in the low frequency region. Or am I reading something wrong?

Cheers

[bpape]

I know that room very well - I did the design for him. Would have liked to do more but there were spousal and physical restrictions.  Notice the sawtooth response at the bottom end.  That is from SBIR (Speaker Boundary Interference Response) where the spherical wavefront of the bass comes off the close wall surfaces and meets with the direct front radiated wave.  Some meets in phase and creates a peak.  Some meets out of phase and causes a dip.  

You can use this to your advantage sometimes to tweak other room response abberations.  Or, you can treat the surfaces direcctly behind and beside the speakers to minimize the intensity of the interaction by knocking down the reflected wave.

For the stuffing, there are a couple different things you can do.  For starters, it's not so much the density as it is the gas flow resistivity properties of the material.  Density is an easy thing to compare since normally a denser material will be more resistive - assuming it's the same material.  3lb mineral wool, 3lb cotton, and 3lb fiberglass all have the same density but different flow resistivity.

For the tubes, if you're going to use standard wall insulation, I'd compress it about 50% so it will end up about 2.4lb./cu ft.  Another option is to use progressively smaller tubes inside with layers of batting or insulation around them.  This provides not only solid absorbtion but also forces the wave to transfer through different mediums.  This creates progressive high pressure/low pressure areas.  As the wave moves out of a dense medium, it loses pressure then it hits the more dense material and creates higher pressure - and to a certain extent - a mini backwave to help cancel that coming in the lighter material.

Bryan

[bpape]

703 will actually outperform 705 down to about 50-60Hz.  I always tell people that 6" of 703 will outperform 4" of 705 for 75% of the money.  I almost never use 705 unless the customer insists because of something he's read that convinced him he needed it. 

Bryan

[Daygloworange]

Bryan,

Thanks for that. Any explanation as to why? Is there a difference in materials? Or does it have anything to do with the fact that there is less mechanical transfer from fiber to fiber due to the lesser density? Could you elaborate a little more on the reasons?

Cheers

[brj]

Another option is to use progressively smaller tubes inside with layers of batting or insulation around them.  This provides not only solid absorbtion but also forces the wave to transfer through different mediums.  This creates progressive high pressure/low pressure areas.  As the wave moves out of a dense medium, it loses pressure then it hits the more dense material and creates higher pressure - and to a certain extent - a mini backwave to help cancel that coming in the lighter material.

Actually, I had considered this exact concept.  Stuffing will be a bit more challenging, and you'd have to ultimately mount the tubes to a base to maintain the spacing, but it seemed like a good idea.  Depending on time and material restrictions, perhaps Art and I can try this approach this weekend.  9" inside a 16" inside a 22" anyone?

Would removing the FSK from the interior tubes provide any benefit or would it hurt by removing a potential membrane?

Thanks for all of your pointers, Bryan - very much appreciated!