Rise of the Eighteens - Page 10 - Home Theater Forum and Systems - HomeTheaterShack.com

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post #91 of 137 Old 04-10-09, 07:36 PM
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Re: Preventing cavities from resonating...

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bpape wrote: View Post
Ummm. IIRC, adding insulation actually makes the driver react as if the box was larger.

Bryan
I agree that is the case when it comes to box enclosures..but my understanding is that the reverse applies in the case of transmission line systems..
But then I'm no expert..

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post #92 of 137 Old 04-10-09, 07:54 PM
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Re: Preventing cavities from resonating...

I didn't see anywhere that this was a TL design. Sorry.

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post #93 of 137 Old 04-10-09, 09:31 PM
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Re: Preventing cavities from resonating...

I probably haven't used the correct term when I said "transmission line"..
Nevertheless, a path does exist for the sound waves to pass through..and on that basis, that flow would have to be restricted..to some degree..with the introduction of the chambers being filled with insulation material..Would it not?

With a tuned TL system..If too much or too denser insulation is added in the path of the back sound wave..the lower frequency db. levels are lessened from what the design intended, when the specific amount and or density of the infill has been changed..
Would this still not apply to some degree in Sonnie's system.?

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post #94 of 137 Old 04-11-09, 07:50 AM
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Re: Preventing cavities from resonating...

If it's a sealed system, adding insulation will make the box acoustically larger from the driver's point of view.

In any case, you're correct that you don't want to use a really, heavy, dense insulation.

If it is truly a tuned pipe, quarter wave TL, etc. - you want minimal insulation in the cavity.

I'm certainly not a professional speaker/sub designer. I've built a few and done a bit of research but it's hardly my expertise.

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post #95 of 137 Old 04-11-09, 08:56 AM Thread Starter
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Re: Preventing cavities from resonating...

What I can do is cutout a large section (green area) and also cut the dividers to the floor, allowing those areas to be opened up to the adjacent areas. Then I can add bracing and insulation, cover up the section that was cutout and add another layer of MDF.



Now... the question is how to add the insulation? Obviously I am not going to be able to "line the walls"... that is just not feasible since this is after the fact. My arms are not long enough. I could line probably line a good bit of it though, if that is what I need to do.

My main concern is making sure the enclosure does not create a large resonating cavity. I believe the bracing and add layer of MDF should take care of that. I am not sure what the insulation or polyfil will really do for me and whether it needs to be lined or stuffed.
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post #96 of 137 Old 04-11-09, 11:10 AM
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Re: Preventing cavities from resonating...

That's a start. A better solution would be to move the first green stripe forward, so the line of cutouts starts directly behind the areas colored in pink. Then add a second line of cutouts 1/2 the distance toward the back.

Typically ported boxes are lined with damping material, they're not stuffed with loose fill.

One option would be to use sheets of OC-703/705 or Roxul. Those can be cut to size and the pieces will slide into place through openings cut in the top.

That said there are those who use no damping in their ported boxes.

Damping is used to make the box look larger, that's not a consideration here. It also absorbs the higher frequencies not filtered out by the crossover. Finally when damping is used in conjunction with port length adjustments, the user is able to fine tune the "Q" and Fb of the system. Since this system seems to be all but done, the tuning procedure typically employed when building a sub isn't really an option.

HTH.

Aloha,
WB
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post #97 of 137 Old 04-11-09, 02:11 PM Thread Starter
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Re: Preventing cavities from resonating...

Thanks!

Sounds like I could just skip adding any insulation or polyfil and add braces and that other layer of MDF... along with making more cutouts in the current bracing for better airflow.

This is what we will end up with...



EDIT: I ended up going back, cutting out that section as noted above, added more bracing... and added another layer of 3/4" MDF for a 1-1/2" floor. The columns that hold the drivers have a 3/4" MDF back and sides with 3 layers of 3/4" MDF on the front. Bracing is every 9" with four routed square cutouts, except at the driver locations where there is three occasions of strip bracing on the sides.
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post #98 of 137 Old 06-13-09, 01:03 AM Thread Starter
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Re: Rise of the Eighteens

I cannot believe I either misplaced the images or just forgot to take them of the actual framing of the riser enclosures. I thought surely I took some pics with the drivers in there, but I cannot find them.

Maybe I can find them later, but it is not much to see that you really cannot tell about in the following finished pictures.









The end result is supposed to be approximately 80ft^3 tuned to 12Hz, looking at the port, maybe we tuned it a tad lower. I may need to remeasure.

The actual driver vs. port measurement are very strange it seems. This is only one nearfield driver measurement along with the port for comparison.




But what really counts is the overall response...



And let me tell you... these things are un-be-liev-a-ble!!! Awesome! Scott definitely has a winner here...
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post #99 of 137 Old 06-13-09, 08:18 AM
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Re: Rise of the Eighteens

Sweet! Never seen a design quite like that before.

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post #100 of 137 Old 06-13-09, 07:45 PM
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Re: Rise of the Eighteens

Do you actually feel/hear it under your feet, or is it more generally in the room sound?

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