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Thread: Foam In or Out?

  1. #1
    Senior Member mbd7's Avatar
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    Foam In or Out?

    I'm starting a new thread on this because I need an answer pronto and I've shifted thru the past threads without finding what I'm looking for. The dumba$$ that put the foam on the outside of the cones on my 2121's says that there's no sonic difference and he showed me several other speakers by other manufactures that came out of the factory with the foam on the outside of the cone. My response to him was that JBL puts the foam on the back of the cone for a reason plus I told him from the beginning that I wanted these speakers to look identical to the ones I brought in. So what exactly is the difference and what is my reason for suing this guy for putting the foam on the "wrong" side of the cone and now having to spend at least $700 for reconing with JBL cone kits verses $150+/- for new foam.

  2. #2
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    For Post #300, here goes,

    A lot of reconing/repair shops will tell you that the placement of the foam on either the front or back doesn't make any sonic difference (including some well known shops that are JBL authorized). However, there are some technical and aesthetic reasons.

    JBL is considered to be one of the best R&D, Engineering, and manufacturing companies in the speaker business. They design and manufacture a lot of stuff in house and what they outsource, they specify closely when it makes sense. They have a history of accuracy and precision in all their process which date back to Jim Lansing himself as an engineer and machinist.

    In the design of woofers, JBL has had a lot of engineering evolutions over the last 50 years. They have researched and created a lot of new and innovative technologies to get their woofers to do the things they want them to do. Every part of every woofer is designed a specific way with a specific place to be and a specific tolerance in size, shape, and measurement.

    The primary issue for the surround is to help it center and place the outer edge of the cone. When the surround is designed and placed on the back of the cone by JBL, it aids, with the spider, the voice coil in being in the EXACT static position the JBL Engineers want it to be. If we assume that that position is in the EXACT middle of the magnetic GAP of the pole piece and top plate, there is a reason for this. The reason is for symmetrical and uniform motion in and out of the voice coil and consistent, uniform interaction with the magnetic field gaps in, under and over that gap.

    JBL introduced the SFG (Symmetrical Field Geometry) magnetic design along with an Aluminum Flux stabilizing ring around the pole piece. Why did they bother reengineering a "simple" metal piece that conducts magnetism?

    From the 2235H brochure,
    "The 2235H has a low-loss magnetic structure that weighs 8.5 kg (18 5/8 lb) and incorporates JBL's unique Symmetrical Field Geometry (SFG). This design, in combination with the aluminum Flux Stabilizing Ring around the pole piece, reduces second harmonic distortion to inconsequential levels. The 100 mm (4in) diameter voice coil benefits from new adhesive technology and optimized coil-former construction of aluminum, plastic and high temperature paper for increased power capacity. The 19 mm (3/4 in) length of the voice coil allows increased linear excursion capability and the extension of usable frequency response to 20 Hz in equalized alignments and subwoofer applications. Careful choice of suspension elements, taking into account their interaction with the voice coil motion, results in a driver with tight, controlled transient response and complete freedom from dynamic instabilities."

    Symmetrical motion will be changed if the foam is placed in the wrong position, altering the static center position of the voice coil. Distortion increases as the motion becomes altered at higher drive levels. Also, the spider is placed at a slight deformation compared with factory stock neutral so also affects the way the cone moves.

    A lot of people think "what's a few millimeters up or down, they still work don't they?" Well, with the voice coil length from 1/2 inch to 7/8 inch (about 12 mm to 23 mm) and the gap heights around 3/16 inch (about 4-6mm) that precision of 2-3 mm can be critical. Some people can hear it, some can't. JBL can and has certainly tested it, measured it, and thought they could improve their product by redesigning it to reflect these changes.

    Here's a couple of analogies that people can get their head around.

    Automotive - Decenter your steering wheel by about 3 mm so they are pulling left or right without correction. (or do your tires the same way toe-in or out). That car will then be "crabbing" down the road, constantly needing your correction to stay straight and performance will suffer - car turns better one way but not the other, tires will wear incorrectly and badly, and other suspension components may wear due to aysmmetrical stress. Remember that alignment specs on high performance suspensions are specified not only in degrees but also minutes (very fine measurements). And these (tires and cars) are macro structures.

    Audio - Push-pull tube and transistor amplifier output stages - Balance has always been the key because the signal must drive both sides symmetrically, otherwise distortion is developed. But if offset voltage drive (deviation from a nominal zero midpoint) is different from positive or negative parts of the amplifier, significant imbalance occurs and distortion, measurably and sometimes audibly, will result.

    Perhaps the best reason to state is: JBL designed it that way and it should be put back to exactly that standard. Otherwise, its different. As you stated, correcting just the appearance wil be costly now unless you can put up with the residue on the front of the drivers. Just because other manufactures have their surrounds on the front doesn't mean they didn't put it there on a lark. Why doesn't he put the surrounds on the back of those if it doesn't matter? It was done simply because its easier, not because it was correct.

    Hope this explanation is usefull. Sorry for the verbosity and length. I'm just wired that way. (positive voltage to the black terminal results in outward cone motion!)

    Best regards,

    Bart

  3. #3
    Senior Member mbd7's Avatar
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    Thank You

    No problem with the length of your explanation, it contains a wealth of information. I'll get some pictures posted ASAP so that you can see as well as read about this atrocity.

  4. #4
    Senior Member BMWCCA's Avatar
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    I went through this discussion on a smaller scale a while back and got an answer from both the JBL-authorized repair station and Harman. Just follow this Link to read the back-filling BS replies from both. BTW, I did not buy the systems in question.

  5. #5
    Senior Member mbd7's Avatar
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    Great link to a Very Good Discussion on this issue

    Thank you for bringing this to my attention, this type of help is what makes this board such a great resource!

  6. #6
    jbl
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    Like you say. JBL did it that way for a reason.

    Ron

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