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Thread: JBL 4355 clone with different woofers

  1. #91
    Senior Member Odd's Avatar
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    Nice work.
    43XX (2235-2123-2450-2405-CC 3155)5235-4412-4406-4401-L250-18Ti-L40-S109 Aquarius lV-C38 (030) 305P MkII

  2. #92
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    hello, the work, albeit slowly continues. I fixed the supports for the 2445 and the reflex tubes. Before screwing the cones, I have to insert the sound-absorbing material; they recommend the 3 cm high ashlar polyurethane foam, what do you think?
    Giuseppe

  3. #93
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  4. #94
    Senior Member RMC's Avatar
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    Giuseppe,

    RE "sound-absorbing material; they recommend the 3 cm high ashlar polyurethane foam, what do you think?"

    I looked at a few web sites to see what your foam is, saw an italian and a german site too. Assuming i got the right foam, it seemed somewhat compressed materials, more like closed cell than open cell for example. Didn't appear like a very proper material, again if i got the right ones.

    Damping material has to be soft, fluffy, easily "spongy" = absorptive. I tried a foam in the past (see 1" foam pic) and when tacked in place it didn't meet these adjectives anymore due to some compression from tacking. Glueing it wasn't better having to cover and hold on braces for e.g. So the result wasn't good and i removed all of that.

    You're possibly better using 3 cm thick fiberglass wool, its easier to install inside. Then to prevent fibers from flying in the box or vent tubes, you simply spray a VERY LIGHT coat of spray paint on the fiberglass, this will contain the fibers. Some prefer using hair spray fixative, but in my view spray paint might last forever, maybe not hair spray? You can also cover the fiberglass with some cheese cloth for similar result.

    Another option is to use Polyester damping instead of fiberglass, since it has no flying fibers. I'm switching my LF cabs from fiberglass to Polyester, and exit the airborne fibers issue. Some don't like it as much, true it has little less density, but the one i purchase from Solen and others is 1.5" thick, i guess that compensates for fiberglass density but only 1" thick.

    At low frequencies it matters less with regards to preventing midrange sound reflections in the cab. In a bass only cabinet, or in an enclosure compartment where the woofer is crossed over pretty low (e.g. 290 hz), there aren't really mid frequencies involved, therefore no such reflections in that section. Moreover, here the low-mid driver's back wave is isolated in a sub-chamber and the mid freq driver is actually a horn (not a cone with a radiating back wave).

    Therefore, i think one has to examine the precise situation before condemning either fiberglass or polyester. In any case, i've attached a pic of JBL's own suggestions with regards to damping material in a speaker cabinet. Note that polyester isn't mentioned, probably because at the time of this older document it wasn't used as today...

    Richard

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  5. #95
    Administrator Mr. Widget's Avatar
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    Quote Originally Posted by RMC View Post
    Another option is to use Polyester damping instead of fiberglass, since it has no flying fibers. I'm switching my LF cabs from fiberglass to Polyester, and exit the airborne fibers issue. Some don't like it as much, true it has little less density, but the one i purchase from Solen and others is 1.5" thick, i guess that compensates for fiberglass density but only 1" thick.
    Years ago I also looked for alternatives to fiberglass. I compared egg-crate foam, polyester batting, and 1 inch “non-shedding” fiberglass. It was years before I had access to any precision measurement equipment so all of my experiments were performed by subjective evaluation.

    That said, I did notice audible differences between the different materials. I tried all three materials both as wall lining as well as completely internally stuffed cabinets. I preferred the fiberglass in every comparison. The egg crate foam was a very close second and the polyester batting was obviously inferior at reducing mid frequencies being reflected back through the woofer cone. Using more of it helped, but only up to a point.

    I have used fiberglass ever since.


    Widget

  6. #96
    Senior Member RMC's Avatar
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    Hi Widget,

    Thanks for the insight.

    Fiberglass i have bundles from replacing the house insulation in the attic (10" thick there, yup cold Canada).

    I also like the soft open cell egg-crate foam, when i can find some locally at reasonable price, mostly used in my small satellite boxes. I don't go for the more rigid type or closed cell. (Btw Solen, egg-crate acoustic foam, 1.5" X 40" X 80" (say 2 sq. y.), $73. + ship + tx = $100. i use a fair amount so $$$). My foam stock is almost depleted. Shipping companies (Amazon, etc.) use a lot so i don't see much around. But I'm not yet at the point of doing the garbage cans around town to find some, lol.

    RE "polyester batting was obviously inferior at reducing mid frequencies being reflected back through the woofer cone."

    There are no mid frequencies reflected in my bass only cabs, 24 db xo 250 hz or so, on to two-way satellites, bi-amp systems. LF box doesn't see the MF nor HF, and the satellite doesn't see the LF. MF reflections might occur in the satellite, however its absorbed by the soft egg-crate foam inside. The OP's situation is similar in my view, as previously explained. (I don't deny the merits of Fiberglass, its one of my suggestions to the OP).

    I remember reading that R.H. Small, in his monumental work, mentioned a minimal amount of damping material should be used. On the other hand, Eargle says low mass material, and Dickason says low density. Well its pretty much the same since they're related by Mass = Density X Volume. In that context polyester isn't bad. Regards,

    Richard

  7. #97
    Senior Member Sootshe's Avatar
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    Thumbs up

    Quote Originally Posted by Mr. Widget View Post
    Years ago I also looked for alternatives to fiberglass. I compared egg-crate foam, polyester batting, and 1 inch “non-shedding” fiberglass. It was years before I had access to any precision measurement equipment so all of my experiments were performed by subjective evaluation.

    That said, I did notice audible differences between the different materials. I tried all three materials both as wall lining as well as completely internally stuffed cabinets. I preferred the fiberglass in every comparison. The egg crate foam was a very close second and the polyester batting was obviously inferior at reducing mid frequencies being reflected back through the woofer cone. Using more of it helped, but only up to a point.

    I have used fiberglass ever since.


    Widget
    My experience as well.

    I changed my fibreglass lining to polyester & it changed the sound of the bass quite dramatically....not for the good.
    I ended up putting the fibreglass back in & covered it with black grill cloth stapled over the top of it to prevent the “flying fibres”.

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