By how much can fiberglass actually 'increase' enclosure volume?? - diyAudio
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Old 14th January 2004, 12:15 AM   #1
sardonx is offline sardonx  Canada
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Default By how much can fiberglass actually 'increase' enclosure volume??

I've read that by using fiberglass as stuffing in a speaker you can make the driver 'see' a bigger enclosure (something to do with its thermal properties). Does anyone know approximately how much 'virtual' volume you can gain by using this to its full advantage?

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Sardonx
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Old 14th January 2004, 12:44 AM   #2
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I've seen different values in different books, but the upper limit seems to be about 1.4 x nominal enclosure volume. How tightly you have to pack it is open to question; they never seem to get around to telling you that part. Lotsa places admit to more modest figures like 1.2.

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Old 14th January 2004, 12:46 AM   #3
MJK is offline MJK  United States
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Hi Sardonx,

The compliance of the air in a box is calculated using the following expression :

Cab = V / (rho x c^2)

where

V = volume
rho = density of air
c = speed of sound in air

For a given box, the volume V and the density of air are constant. The speed of sound is related to the type of expansion and compression of the air in the box.

If the air compresses adiabatically (the box is empty) then c^2 is proportional to the adiabatic ratio of the specific heats which is 1.4. If the air compresses isothermally (best possible fiber present to absorb the heat) then c^2 is proportional to the isothermal ratio fo specific heat 1.0. In this second case the box has a lower compliance and looks 40% bigger then the empty box.

The most a box can be made to look "bigger" by adding fiber is about 40%, this is the upper limit. This assumes the fiber volume is small compared to the box air volume. In real world applications, my guess is that the most you can gain in "virtual" volume is about 20%.

Hope that helps,
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Old 14th January 2004, 01:57 AM   #4
Bose(o) is offline Bose(o)  Canada
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MJK, would the equation that you posted change for a vented enclosure? Considering you now have three air masses instead of two. Rear wave inside enclosure, front wave outside enclosure, and the air-mass inside the port.
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Old 14th January 2004, 02:19 AM   #5
Mark Kravchenko --- www.kravchenko-audio.com
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Default Filled up and still more space

The max you can get in the real world is about 25% greater seen volume by the woofer suspension system. It's been described rather well. The air flowing in to the box through the port or air within a closed box will literally be slowed down when it is forced through the filler. Not the whole truth but it works. This makes the woofer think that it is inside a bigger box because with the filler it behaves like it is inside a bigger box.

Mark

Ps. BoseO where do you live so that I can check out your enclosures??????
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Old 14th January 2004, 03:56 AM   #6
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dont use fibreglas as a fill....go to a poly or wool fill...you doont want to be breathing in that
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Old 14th January 2004, 04:25 AM   #7
jleaman is offline jleaman  Belgium
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Old 14th January 2004, 04:51 AM   #8
Variac is online now Variac  United States
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I think the air being slowed is a different thing-an aperiodic port.

I've alway heard as was originally mentioned- that it is the mass of the fill absorbing heat, albeit a very small amount.
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Old 14th January 2004, 05:50 AM   #9
sardonx is offline sardonx  Canada
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Wow...

Thank you, Grey and MJK.

20% is not bad at all. I didn't know it was that much!
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Old 14th January 2004, 05:53 AM   #10
sardonx is offline sardonx  Canada
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and thanks everyone else! The server was down for a while.. i didn't realize there were more posts.

I would love to use wool... does it work as well as fiberglass for expanding volume? And where can i get wool?
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