Damping

[acre]

Greetings All!
I just completed a set of boxes for the 416-8A, with the help of Dave Fitts and GM (Greg). The boxes are 10 cu.ft with internal dimensions that match the golden rule: 43.5" x 25.8" x 16". I put 1" Dacron damping in the upper 20" of the box on the sides and back. I have a very good friend who is a very competent designer that says once you get into the "larger" cabinet designs, you no longer need any damping because you start to lose LF output. Any comments? I remember testing these units before putting any Dacron in them and the low end seemed cleaner and louder. (The LF is mounted very close to the top of the box and the 811B sits on top. The alignment sounds ok with the 811B flange 3/8" back from the baffle front. I would like to attach a pic of the box. How does one do this??) Thanks...Lee B.

[GM]

>The boxes are 10 cu.ft with internal dimensions that match the golden rule: 43.5" x 25.8" x 16".

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Hmm, I see you didn't quite use my dims, but since neither these nor the ones I suggested are golden ratio, close though. As I mentioned, I juggled them a little to accommodate a specific height.

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>I put 1" Dacron damping in the upper 20" of the box on the sides and back. I have a very good friend who is a very competent designer that says once you get into the "larger" cabinet designs, you no longer need any damping because you start to lose LF output. Any comments?

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Yeah, as stated, it's at best misleading. For a given stuffing density, the amount of apparent increase in Vb/lowering of Fb is constant regardless of Vb. Also, as cab size increases, the eigenmodes fall in frequency/increase in amplitude, so damping requirements increase, not decrease if they fall into the driver's BW. Obviously, if they are above the intended BW, then in theory no damping material is required. Some lining is usually required with rear vented and/or steel frame drivers though.

The biggest factor is where the stuffing or lining is WRT the vent(s). Lining one wall, back, and the end opposite (or furthest away from) the vent works well in most acoustic/golden cabs. When the vent(s) are very near the driver, like many of Altec's consumer/studio designs, then all surfaces usually need to be covered since the vent location causes a rising response to Fb/steeper rolloff.

For high aspect ratio designs, the vent should be near/at the bottom in most cases, so the lining or stuffing should be from the top down to ~the midpoint, or a short distance below the driver if located this far down. If there's objectionable peaking at Fb, then extending it all the way down and covering the bottom will smooth the response out.

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> I remember testing these units before putting any Dacron in them and the low end seemed cleaner and louder.

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If you put the vents down near/at the bottom, then with no lining or stuffing you're getting an exaggerated LF response (Fb peaking) with the attendant poor impulse response, so I'm at a loss as to why you think they sound 'cleaner'. Try it with only the top, one side, and back covered down to the midpoint and let us know what you think.

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>I would like to attach a pic of the box. How does one do this??) Thanks...Lee B.

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AFAIK you'll need to put them on a web page you can link to.

GM

[acre]

Hi Greg,
Many thanks AGAIN for your quality input. As you mentioned, I built these cabs close to your original thoughts. I went with a 5"x15" vent located 5" off the bottom. (My calculations show an F3 of about 42 Hz-plenty low enough since I'm also running a Paradigm Servo 15" with this home theater.) I kept the LF as close to the top as possible. The 811B was "sledded" at first to check alignment. I did this process with an 800 Hz tone and then a white noise tone. I moved the horn back and forth until I couldn't hear any difference in phase. I think your comment about the boxes giving me a false sense of clarity is likely accurate due to the extended LF response acheived with no damping. I will try the damping scheme you mentioned-back, one side, and top, half-way down. I'll let you know. Thanks again....Lee

[acre]

Greg,

Correction, my F3 cutoff was closer to 48 Hz, not 42 Hz, as previously mentioned.....Lee

[acre]

I tried your damping idea Greg and all I can say is that you're a genius. I applied 1" of Dacron to the back, one side, and the top of the cabinets, about 3" below where the LF unit sits and what a difference!! I was clever enough to use the same cut off one CD for all tests; sort of a bench mark so that the same signal is tested. I used a Jennifer Warren CD with no fuzz guitar (there's something that needs to be un-invented right now!) Anyway, thanks for all the help Greg and Dave and anyone else I forgot. As a final note, I connected a LeSon tweeter in with the 806-8A driver just to extend the upper frequency to a full 20,000. It works fine. If anyone wants the plans to these cabinets just say the word......Lee

[dhook17322]

Anyone have a list of suppliers for the damping material? Or sources that can be found most places locally? Thanks.

[Fitts]

Hi GM,

I am wondering, has Lee put enough damping material in his cabinet to noticably affect the effective volume of the cabinet?

I am guessing that the total volume of the inch thick lining he described would have virtually no effect on the apparent box size and therefore very little if any effect on the Fb.

Notwithstanding, Lee has noticed an improvement in the qualtiy of the sound, is some of the improvement he is hearing due to the dacron damping resonances and standing waves within the cabinet?

Are the eigenmodes that you mentioned standing waves inside a cabinet or resonances of cabinet panels or can they be both?

Thank you for sharing your knowledge and expertise.

[GM]

>I tried your damping idea Greg and all I can say is that you're a genius.
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Not really, just an experienced tweaker. Anyway, glad you're pleased with them.
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>Anyone have a list of suppliers for the damping material? Or sources that can be found most places locally? Thanks.
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1" acoustic fiberglass is available at the local Home Depot, but if not in your area, check out heating/AC installers. Dacron and polfil sheeting are at the local cloth shop here. If not there, try an upholsterer shop.
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>I am wondering, has Lee put enough damping material in his cabinet to noticably affect the effective volume of the cabinet?
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Not even close. It takes from 1.0-2.0lbs/ft^3 depending on the material for it to be audible around/at Fb, though only 0.5-1.0lbs/ft^3 is required for it to 'muffle' the midbass/lower mids.
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>Notwithstanding, Lee has noticed an improvement in the qualtiy of the sound, is some of the improvement he is hearing due to the dacron damping resonances and standing waves within the cabinet?
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Correct.
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>Are the eigenmodes that you mentioned standing waves inside a cabinet or resonances of cabinet panels or can they be both?
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Both, though assuming decent construction using either stiff and/or extremely well damped materials, the materials resonances aren't an issue. Basically, what the lining does is attenuate reflections off the driver/adjacent boundaries and eigenmodes between parallel boundaries so they won't modulate the diaphram from the rear enough to be audible, but mostly it damps the vent's pipe harmonics, which can really make a mess of the FR above its 3rd harmonic.
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>Thank you for sharing your knowledge and expertise.
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You're welcome! Hope it helps.

GM

[dhook17322]

And my thanks as well for source tips on damping material.