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Thread: Dust Cap in or Out?

  1. #1
    Senior Member mbd7's Avatar
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    Dust Cap in or Out?

    As a companion thread to the foam one below I've also got the same situation with this same dipstick about the dust cap being an "innie" on my 2121's verses the "outie" he put on them because he said he had to remove the dust cap to get the "thousands of an inch" tolerance in the gap between the voice coil and the magnetic structure. Once again he states that there's no sonic difference in the cap being in or out. And my response is the same... if JBL wanted them to protrube outward then there's a damn good reason they did it inward plus I told him to repair the speakers and make them look the same as when the were brought in to you. Alright all you guys who have the answers post them now!

  2. #2
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    Much shorter answer for you on this one besides that's what JBL designed.

    The space under the dustcap forms a volume which acts as a resonant chamber. When the dustcap is inverted, the space is defined as a very small area. This characteristic defines some of the driver's frequency response.

    When the dustcap is dome shaped, the space is considerably larger which will alter the frequency response. This is a change to the driver and is not original.

    There are other considerations to the issues of where the cone is glued and how much surface area is in contact with the surface of the dustcap (cone vibration, resonance, and breakup modes) which can be clearly audible.

    I can appreciate his need to take the dustcap off to shim the gap evenly (yes it is in the thousand'th of inches). Pity he doesn't seem to believe that level of tolerance and accuracy is important to the rest of the precision parts and their placement in a JBL Transducer.

    'nuff said.

    Regards,

    Bart

  3. #3
    RIP 2011 Zilch's Avatar
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    From Vance Dickason Loudspeaker Design Cookbook, 7th Edition (current), P.10, Section 0.35 Dome Shapes:

    "Concave dome radiators usually have much greater efficiency in the high frequency range, but a narrower directivity pattern. The higher efficiency is due in part to the wide peak caused by cavity resonances (although this can be damped to some extent) and the fact that convex domes are usually made of hard materials. Convex domes have a wider directivity pattern in the high frequency range, and lack the efficiency of concave domes in that range."

  4. #4
    Senior Member mbd7's Avatar
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    Concave dome radiators

    Thanks so much for your precise answers to my questions

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    I never gave any of this much thought but I sure learned a bunch from reading the responses in both of your threads.

    I am a carpenter/cabinetmaker and I carry one motto to every job I do- Give the customer what they want or don't take the job!

    Sounds to me like you need a new speaker rebuilder because he does not follow a time tested safe business practice- Give the customer what he wants!
    I can't begin to tell you how many times my customers did not like exactly what they asked for and received. I have alway been able to re negotiate and re do the work to the way it should have been and it always falls back on the customer because I did exactly what they said they wanted.

    What part of being the same as they were does your rebuilder not comprehend?

    Gary

  6. #6
    Senior Member edgewound's Avatar
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    Quote Originally Posted by Zilch
    From Vance Dickason Loudspeaker Design Cookbook, 7th Edition (current), P.10, Section 0.35 Dome Shapes:

    "Concave dome radiators usually have much greater efficiency in the high frequency range, but a narrower directivity pattern. The higher efficiency is due in part to the wide peak caused by cavity resonances (although this can be damped to some extent) and the fact that convex domes are usually made of hard materials. Convex domes have a wider directivity pattern in the high frequency range, and lack the efficiency of concave domes in that range."
    That explanation is valid for tweeters and small diameter midranges where the frequencies reproduced are short enough be affected by driver diameter.

    The 2121 has a vented pole-piece so resonance and pressure under the dome's not really a problem. The 2122 has a convex dome and is used in later variations of the same application. The frequency band that the drivers use in these midbass applications are larger than the diameter of the dust dome, so they don't have much influence on midrange or high frequency propagation. They are more in the piston band.

    Now the dust dome on a guitar speaker like a D/K/E 120(F)/130(F)/140(F) or the others that have an aluminum dome and run full range....that's a different story....they act as high frequency radiators attatched to the top of the voice coil.

    In all, though....the 2121 dome should have been left alone. It's possible to do a fine refoam without removing the dome. That's found in numerous discussions on refoaming here. The factory doesn't recommend a refoam...only a recone, when OEM kits are available.
    Edgewound...JBL Pro Authorized...since 1988
    Upland Loudspeaker Service, Upland, CA

  7. #7
    Tom Loizeaux
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    Giskard, on several occasions, mentioned the "break up" of the 2121 due to its inverted dome. Still, all in all, I would expect (and demand) that a speaker service center keep with the factory design - unless discussed beforehand. JBL probably had a good reason to invert the dome on the 2121.

    Tom

  8. #8
    Senior Member edgewound's Avatar
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    Quote Originally Posted by Tom Loizeaux
    Giskard, on several occasions, mentioned the "break up" of the 2121 due to its inverted dome. ...

    .... JBL probably had a good reason to invert the dome on the 2121.

    Tom
    One reason is it looks cool. Just like the LE10A. It can also offer the cone neck a bit more strength and high frequency damping.
    Edgewound...JBL Pro Authorized...since 1988
    Upland Loudspeaker Service, Upland, CA

  9. #9
    Senior Member GordonW's Avatar
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    Actually, a "normal" dustcap orientation (ie, an "outie") will stiffen the cone more than an "innie". This is what Giskard was probably referring to, in terms of breakup... an "innie" dustcap will "move with the cone", since they're mostly in "parallel" (ie, the edge of the dustcap lays against the surface of the cone, in the same "plane"), where as an "outie", will mostly be perpendicular at the attachment... and therefore will "triangulate" the joint, making it stiffer.

    The gist here is: If the crossover and the rest of the speaker was designed around the "innie" dustcap driver (and its accompanying natural frequency response), the change to an "outie" will change the response, to a mild degree. It may still be listenable, but it ain't as it originally was... and in this case, that IS the point, with something as "collectable" as a JBL 4340 or 4343 monitor...

    Regards,
    Gordon.

  10. #10
    RIP 2011 Zilch's Avatar
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    Quote Originally Posted by edgewound
    That explanation is valid for tweeters and small diameter midranges where the frequencies reproduced are short enough be affected by driver diameter.
    Dickason substantiates his contentions with empirical results using 5-1/2" woofers, op.cit., Fig. 0.19.

    The action occurs between 2 and 10 kHz, in the particular case shown....

  11. #11
    Senior Member edgewound's Avatar
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    Quote Originally Posted by GordonW
    Actually, a "normal" dustcap orientation (ie, an "outie") will stiffen the cone more than an "innie".
    t... and therefore will "triangulate" the joint, making it stiffer.
    I said stronger. Effectively two layers thick of material that the voice coil is pushing/pulling on. Stiffness and strenth are not necessarily the same thing. On this 2121 driver...and LE10A... with a 3" voice coil...a 4" dust cap is inverted and overlaps the cone by approx 1/2" around the perimeter of the cone apex....along with the adhesive that holds it in place.

    The gist here is: If the crossover and the rest of the speaker was designed around the "innie" dustcap driver (and its accompanying natural frequency response), the change to an "outie" will change the response, to a mild degree. It may still be listenable, but it ain't as it originally was... and in this case, that IS the point, with something as "collectable" as a JBL 4340 or 4343 monitor...

    Regards,
    Gordon
    That I agree with
    Edgewound...JBL Pro Authorized...since 1988
    Upland Loudspeaker Service, Upland, CA

  12. #12
    Senior Member duaneage's Avatar
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    Quote Originally Posted by Zilch
    Dickason substantiates his contentions with empirical results using 5-1/2" woofers, op.cit., Fig. 0.19.

    The action occurs between 2 and 10 kHz, in the particular case shown....
    Dickason was very accurate in his testing, hard to argue with the results. If your crossing it over at 4500 hz you should be well out of th range of it making a difference anyway. The crossover network is going to be 24db down at that point. Most of the problems were around 10K in his testing.

  13. #13
    Tom Loizeaux
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    The "inverted dome" 2121s in question come in around 300Hz and leave around 1200Hz... so it sounds that it's OK. "Break up" issues...well I don't know.

    Tom

  14. #14
    ralphs99
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    Doug Button, JBL's director of transducer R&D at the time the LSR's were being developed commented on the inverted dome on the C500G midrange:

    "The only reason I can say is: because it works! The interesting thing about designing transducers is that you try everything you can find in the laboratory- you put little bits of glue here and you put coatings there, and you put things together in every possible way you can. A loudspeaker is always slightly misbehaving, and what you want to do is come up with a geometry and damping that minimses that misbehaviour. Putting the dome on that way worked very well. It also minimised the air cavity under the dustcap, which was an important feature because the air cavity can be very problematic."

    Cheers, Ralph

  15. #15
    Senior Member DogBox's Avatar
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    2121 Vs 2122

    Quote Originally Posted by ralphs99 View Post
    Doug Button, JBL's director of transducer R&D at the time the LSR's were being developed commented on the inverted dome on the C500G midrange:

    "The only reason I can say is: because it works! The interesting thing about designing transducers is that you try everything you can find in the laboratory- you put little bits of glue here and you put coatings there, and you put things together in every possible way you can. A loudspeaker is always slightly misbehaving, and what you want to do is come up with a geometry and damping that minimses that misbehaviour. Putting the dome on that way worked very well. It also minimised the air cavity under the dustcap, which was an important feature because the air cavity can be very problematic."

    Cheers, Ralph
    As was mentioned above and shown [Thanks for the pic, Heather!]
    Name:  2121 Foil and Vent.JPG
Views: 118
Size:  93.7 KB
    ..so, "under the dustcap" wasn't a problem.
    Yes, I saw Giskard mention a few times where "the 2121 suffers from break-up" that 'could be heard'. Maybe not everyone heard it which on occasion 4313B said to
    "leave it in." He could hear it and I am thinking - he could measure it - although I haven't yet seen that. He also said the 2122 was "a better driver". Never on a fix
    for the 2121 - only to 'use the 2122 instead'. 'Recone the 2121 with a 2122..'
    Maybe - the inverted cap 'was' the "best" position for it - with the rest of the design of the 2121. Take a look at the THEIL SMALL LOW FREQUENCY DRIVER PARAMETERS AND DEFINITIONS paper at the different specifications between the two...
    I had always thought that if the dustcap was the problem - then, turn it the other way!?? Well, maybe doing that introduced other problems that haven't been fully
    documented. Anyone got a proper Test Set-Up and all the parts to test this thoroughly? It would be interesting and I think clear up a lot of supposition.
    As recones are only now the personal stock of a very few, I doubt that any will be put to Mythbusters testing for the satisfaction of knowing.
    You have the choice now of finding a Proper recone or get it done as a "2123" - as that's all that is available at this time. Bit of a difference there..!

    Steve.
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