can you overstuff a box with f/glass? - diyAudio
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Old 21st May 2009, 11:26 PM   #1
terry j is offline terry j  Australia
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Default can you overstuff a box with f/glass?

nearing the end of my long build, but along the way I managed to very cleverly not get the same volume in my new box as I had in the old...duhh.

The new box is prob about 10-15% smaller (?), and as I wanted to 'get the same sound' then that is one variable I am a bit concerned about. It may be more of a problem in my case than in others, as I use eq to get the woofer to go lower, and (well at least in my old box) 30-35 hz seems about as low as I can get the woofers to go. With a smaller box that may be harder to achieve.....?

Ok then, stuffing a box with f/glass makes the box 'bigger' to the woofer (of course my current box has f/glass in it too). So does that mean all I do is simply stuff/force/cram even more f/glass into the box to compensate??

I mean it's not some 'never ending' process here is it? more f/glass does not always means 'bigger' box does it? there would be some limit surely where more f/glass produces no more change - or even worsens things?

In any case, can we have too much f/glass? If we have too much, what are the results??

How can we know how much f/glass is optimum, is it just an empiric formula along the lines of kgs/cubic m (and are those formulas only guidelines, or can they be 'modified' as in my case for a certain purpose) , or are there ways to measure and from that determine when we have the right amount.

Indeed, I have seen what appear to be different types of f/glass, some kinda coarse and 'gluggy' or all bonded together, others seem kinda fluffy and free (audiophile descriptions there heh heh). I would presume the fluffy would be the 'best' yeah? ie sort of allows air movement.

just hope the smaller box does not come back to bite me...
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Old 21st May 2009, 11:41 PM   #2
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Yes, the effective volume starts to decrease beyond a certain point.
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Old 22nd May 2009, 01:37 AM   #3
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Default Re: can you [I]overstuff[/I] a box with f/glass?

Quote:
Originally posted by terry j
. . . The new box is prob about 10-15% smaller (?), . . .
How can we know how much f/glass is optimum,
just hope the smaller box does not come back to bite me...
If you are speaking of a purely sealed "acoustic suspension" design, 10-15% change will not change performance very much - try doing some box modeling in any of the free programs out there and you'll see.

The best way to "see" how the stuffing is changing things is to run impedance curves with a woofer tester. As stuffing is gradually added, both the system Fs and Qts will drop. At some point of stuffing (when density is too great) the trend will reverse, and box volume is robbed by the stuffing, and the dense stuffing begins to behave more like a solid than absorptive mass.

If you are doing some other design (ported, transmission line, etc) than forget all the above -- added damping will mess with box tuning and Q.

-- Mark
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Old 22nd May 2009, 02:15 AM   #4
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Hi Terry
I was going to send a link and now I can't find it in my bookmarks, damned old-timers disease; I think that the limit is reached when stuffing exceeds air density by more than a certain percentage, the figure I have in my mind is 30%, but graduated density seems to work better for me.
From memory ( as I said I can't find the link) acoustic fiberglass batts have much finer fibre size and is specially made to have a denser layer on the outside.

That said I always use normal fiberglass as i can walk down to Bunnings and buy a pack any day, but I always try and put that layer of BAM or Polyester between the glass and the woofer.


How's Cinderella going??

I'll go and get the "Cookbook" and get back
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Old 22nd May 2009, 02:25 AM   #5
56oval is offline 56oval  Australia
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Hi Terry
You could make a frame to go around the front baffle to increase the depth .Make a top plate & bottom plate to cover the ends .
Have a look at the Pass Labs Rushmore speaker .

Cheers
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Old 22nd May 2009, 02:29 AM   #6
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Just quickly reading ; maximum size increase seems to be at the 2lb per cubic foot mark when using standard 100% fill.
When using 50/50 fill there is a significant increase in apparent box size when using 2lb cubic foot against the walls and 1lb per cubic foot as the rest of the fill, Qtc shows big drop equal to 30%+ increase in box size.

So use normal stuff against the walls and fluff up the rest and hope it works, or try and get the proper acoustic stuff from ACI for the fill;
Owens-Corning 205 is the real stuff, I don't know if it is available in OZ

Regards
Ted
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Old 22nd May 2009, 02:42 AM   #7
terry j is offline terry j  Australia
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Default Re: Re: can you [I]overstuff[/I] a box with f/glass?

Quote:
Originally posted by Tubamark


If you are speaking of a purely sealed "acoustic suspension" design, 10-15% change will not change performance very much - try doing some box modeling in any of the free programs out there and you'll see.


How silly of me to leave that bit of data out. Yes it is sealed. Always 'knew' that 10-15% was no biggie, except as in my case I use tons of eq to get it low. Think in the box au naturale it starts rolling off at 70-75 hz or so, so to get it low in room requires a bit of eq. Dunno if that changes the situation of a minor percentage change or not.

Quote:
The best way to "see" how the stuffing is changing things is to run impedance curves with a woofer tester. As stuffing is gradually added, both the system Fs and Qts will drop. At some point of stuffing (when density is too great) the trend will reverse, and box volume is robbed by the stuffing, and the dense stuffing begins to behave more like a solid than absorptive mass.


-- Mark
Yeah, that articulates well what I was trying to say was my gut feel. At some point you have added so much or compressed it so much that it is no longer 'porous' and ends up just like adding bricks to the chamber, ie robbing volume.

Now I don't have woofer tester, but I vaguely recall seeing a post at the shack where he somehow used REW (maybe measuring across a resistor???) to get impedence plots. maybe not exact or calibrated, but enough to guage changes as we add f/glass....will try and find it, if I do I'll link to it here.

thanks for your help


Quote:
Originally posted by Moondog55
Hi Terry

From memory ( as I said I can't find the link) acoustic fiberglass batts have much finer fibre size and is specially made to have a denser layer on the outside.


I can get how thinner would help (more surface area), but on the flip side would not the denser cover inhibit?

Quote:
That said I always use normal fiberglass as i can walk down to Bunnings and buy a pack any day, but I always try and put that layer of BAM or Polyester between the glass and the woofer.
Is that to keep fibres away from the woofer? does it matter, or some sort of health thing?


Quote:
How's Cinderella going??

I'll go and get the "Cookbook" and get back
yeah that is what this is about, going well since my revitalisation recently. Prob next week can start the veneering of the baffle, the finishing will take a bit longer and then I guess that's it!

start planning for bathurst this year yeah?? you're running out of excuses you know.
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Old 22nd May 2009, 02:49 AM   #8
terry j is offline terry j  Australia
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thanks malcoHm

BUT, it took me years to finally get a nice looking box (so frankenstein can morph into cindarella dontcha know?) so at this stage no more bodgying yeah?

And BTW, you gotta start planning for bathurst too, you are coming aren't you???

cheers moondoggieboy, think I'll just use f/glass yeah? I can easily rob it from the house rather than buying special stuff.
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Old 22nd May 2009, 02:52 AM   #9
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http://www.nousaine.com/pdfs/Box%20Stuffing.pdf
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Old 22nd May 2009, 03:30 AM   #10
56oval is offline 56oval  Australia
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Quote:
Originally posted by terry j


And BTW, you gotta start planning for bathurst too, you are coming aren't you???
What are the dates .

Cheers

Mal
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