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Thread: JBL Speakers Internal Bracing

  1. #1
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    JBL Speakers Internal Bracing

    JBL Speakers boxes are just made of MDF with very few internal braces compared to many other hifi brands. How, in these conditions do they not ring like bells?

    I wonder whether JBL does "glue" the internal bracing with a damping material, in order to suppress de resonnances of the panels (as does KEF on the LS50 for example).

    Does someone know that?



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  2. #2
    Senior Member RMC's Avatar
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    Hi gbadaut,

    I think there are some misconceptions here and the comparison doesn't seem appropriate.

    The KEF is a mini monitor whereas the JBL is a larger one. That has some implications too.

    MDF/HDF, also known as particle board, is quite rigid material suitable for speaker enclosures (even if I personnaly don't like it nor use it).

    There may appear to be "very few internal braces" as you say, however the "shelf" type bracing used in the JBL seems appropriate, dividing the panels in three parts, each brace covering 3-4 panels and glue blocks are also used. No specific reason I see they should "ring like bells". Doesn't look to me like a faulty box design. One can always add more everywhere but at some point diminishing returns kick in, as well as cab weight.

    Each speaker Design Engineer has his own approach in view of speaker target market and makes a number of decisions, choices and compromises. Inevitable. That within a predetermined budget re selling price.

    As for the comparison with the small KEF, well it doesn't appear to be fair. First, the graphs shown don't compare the JBL cab vibrations with the KEF. Second, the material used in Figure 11 is "Budget Chipboard" not particle board or MDF/HDF as used in the JBL.

    Chipboard as we know it in North America is wood chips with glue then compressed. Density? This is low quality/low cost material, not used often for speakers, but often used in housing constructions re cost.

    Seems to me that for Fig. 11 and comparison the worst case was chosen, making the KEF look way better... But is it really? Lets assume a maybe, but the KEF is a very small box, and other things being equal, smaller panels have higher rigidity and less tendency to flex than larger ones...

    BTW the KEF flared vent seen on LS 50 left picture (inside cab, air intake/out) touching the center brace isn't glorious work in my view, too jam packed small enclosure. Much prefer JBL's less constrained vent air flow from all directions, but its a larger box...

    I'm attaching here three Chipboard pictures from the Net (also called OSB for Oriented Strand Board). First pic is about the best it gets, second pic say middle of the road, and last pic about as bad as chipboard can be. Which one KEF used for comparison?? Their "budget" description might point downwards... Regards,

    Richard

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  3. #3
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    The drivers themselves also add rigidity. If you had a flimsy 15” woofer bolted to the speaker versus a more rigid one at the interface, then the rigid one is adding more structural support. Laser interferometry is pretty easy to obtain so all but the smallest speaker builders are using it.

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    I think the salient point here is the extremes KEF have gone to in the construction of what is already a small rigid(small panels) box.
    The larger panels in the JBL are broken up with asymetric horizontal bracing. You can go too far bracing a large enclosure, shifting the panel resonance higher in freq and into low mid territory.
    Plenty of ways to skin this cat, Matrix bracing from B&W, thin panel BBC style, Differential materials construction from Tannoy which uses thick panels with a floating brace system using lossy adhesives and similar lossy coupling of rear of driver with the brace. The JBL boxes don't ring.

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    Hello,

    Thank you for your responses. I want to be clear: I'm not saying that JBL speakers do ring. They use an interferometer to design their boxes, so if they were ringing, they would detect and correct it.

    My question is really genuine: do they use "dampened braces", or a special glue that adds damping to the construction? The KEF paper shows that it is very effective. MDF do ring, so a good box design seem mandatory, and JBL knows how to do that.

    The reason why I'm asking this is because I carefully constructed a box following the JBL M2 plans precisely. The result that I get is that the panels do exibit some unwanted resonances that create a 'discomfort' while listening to music. If I put one ear directly on the panels I can clearly hear the resonnances (especialy on the top and on the rear panels).

    So I wonder what does JBL that I don't. After having read the KEF paper, I think it could be the glue. I used wood glue, maybe they don't...

  6. #6
    Senior Member RMC's Avatar
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    Hi Alan,

    RE "The drivers themselves also add rigidity. ... adding more structural support."

    Not sure at this point its that evident.

    Would seem to be somehow logical, and then imply the more drivers on a panel the more rigidity. However it doesn't appear to work this way according to British loudspeaker manufacturer Fane Acoustics (now called Fane-International; Fane Acoustics now being their Musical Instrument speakers division). Fane has been in the speaker business for a very long time, I assume they may have tested this issue.

    Baffle hole cutting appears to weaken panel structural rigidity instead of increasing it, even with driver(s) covering the hole(s): (Bracing section of their speaker enclosure design and construction manual, P.6)

    "In 4 X 12"enclosure designs, ... bracing is additionally recommended from center front to center rear. This method of bracing (RMC: prior described + center brace) should be used in any enclosure the baffle of which carries heavy drive units or which has been significantly weakened by extensive baffle hole cutting."

    Looks to me if drivers were adding baffle structural rigidity further bracing wouldn't be needed. On the other hand, not really difficult to get into extensive baffle hole cutting, e.g. 4-way system with 2 vents and you already have 6 cutouts in it, assuming only one woofer. Regards,

    Richard

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  7. #7
    Senior Member RMC's Avatar
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    Hi gbadaut,

    RE "dampened braces", or a special glue that adds damping ... I think it could be the glue. I used wood glue, maybe they don't...

    I'm not aware if JBL uses such braces or special glue. That would surprise me a little.

    Could it be that something in the cabinet (e.g. brace(s) is not properly fastened in place, hence the resonance? (since two panels resonate more than others). What is different construction wise about these two VS the others could be a place to start inquiring.

    Personnaly I don't use standard wood glue (like the white one) for my cabinets. I prefer using Carpenter's glue which has a stronger bond and higher initial tacking. Regards,

    Richard

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    Quote Originally Posted by RMC View Post
    Hi Alan,

    RE "The drivers themselves also add rigidity. ... adding more structural support."

    Not sure at this point its that evident.

    Would seem to be somehow logical, and then imply the more drivers on a panel the more rigidity. However it doesn't...
    What I meant is that once you HAVE a hole in the front baffle, the drivers matter. For a given diameter woofer, if you have a really flimsy driver there is more flex through the whole structure whereas an fancier driver, which may primarily be intended for power handling, can be stiffer.

  9. #9
    On Holidays Ian Mackenzie's Avatar
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    Quote Originally Posted by gbadaut View Post
    Hello,

    Thank you for your responses. I want to be clear: I'm not saying that JBL speakers do ring. They use an interferometer to design their boxes, so if they were ringing, they would detect and correct it.

    My question is really genuine: do they use "dampened braces", or a special glue that adds damping to the construction? The KEF paper shows that it is very effective. MDF do ring, so a good box design seem mandatory, and JBL knows how to do that.

    The reason why I'm asking this is because I carefully constructed a box following the JBL M2 plans precisely. The result that I get is that the panels do exibit some unwanted resonances that create a 'discomfort' while listening to music. If I put one ear directly on the panels I can clearly hear the resonnances (especialy on the top and on the rear panels).

    So I wonder what does JBL that I don't. After having read the KEF paper, I think it could be the glue. I used wood glue, maybe they don't...
    Are you saying you can hear the panels resonate while listening?

    The thing is that you are hearing this over the direct sound from the drivers so it must be bad!

    If that is the case you should be able to detect the source of the resonance with a sweep sine wave.
    Try tapping the enclosure walls with a hammer. It might be a brace that is not glued or pressure fitted.

    I personally glue and screw bracing in a diy project using 2 x 4 pine.

    Those shelf braces need to be cut with precision to fit and work as they should. In a production scenario with cnc that is all de-bugged with prototyping.

    BTW did you know that Black Holes (in space) create gravitational forces that can be detected on planet Earth with very sensitive scientific equipment? Its particularly bad when a Black hole is feeding on other planets and farts. If a Black Hole does not have a regular supply of planets it starves and it dies! The forces are such that if our Sun was sucked into a BlackHole it would be crushed to the size of a one inch solid ball of almost infinite density and mass. This was described on a documentary. (True).

    It could be that your Vulcan ears can detect the feeding frenzy....LOL.

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    Hello,

    I did the construction minutely. The panels and the braces were firmly attached. I've made a lot of other boxes before this one so maybe I've done a mistake but it would surprise me.

    I've already tryed to find the resonance with a sweep sine wave but I failed detecting it. I think that I should try again, but this time with the ear against the panel! I guess it should work.

    This is an opened original JBL M2. Does it looks like glue on the braces? I guess it is...
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    My opinion is that my box doesn't lack stiffness but damping. So few braces on such a big box cannot raise the frequency above the working frequency of the woofer (about 900Hz). So this box needs damping (unlike subwoofer that need stiffness) but there is not much inside.

  11. #11
    Administrator Robh3606's Avatar
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    Hello

    I use sine sweeps to test my boxes. I brace mine at odd 1/3 intervals to break up the panels and also add two coats of dampening compound the stiffens the panels and seal any air leaks. Could be a high Q resonance did you run slow?? Last "box" issue I had wasn't the box it was the way the woofer was seated on the box. Loosened it up re-torqued and all was well.

    Rob
    "I could be arguing in my spare time"

  12. #12
    On Holidays Ian Mackenzie's Avatar
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    Did you use mdf?

    Are you 100% certain the perceived noise is coming from the loudspeaker?

    Is your bracing exactly the same as in the M2?

    I am not being critical. It’s just that a perceived annoyance in the sound can have one of several origins. The sound can be a reflection, a window or from the driver cone. The mind is a thinking machine and will often over think a problem.

    Did you use fibre glass insulation on the panels? Fibreglass actual has a high coefficient of sound absorption and will markedly effect the amount of sound passing out via the driver cone.

    Next time you remove the woofer check the coverage on the sides and behind the woofer and on s as my bracing.

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    Hello,

    I used MDF with the same thickness but 20% lighter that the original's. I didn't put a gasket between the waveguide and the box (I don't know what it looks like). I used MDM-4 instead of woodglass. And the panel under the woofer is not as thick as their. The rest is exactly the same.

    My boxes are make exactly like this : https://www.youtube.com/watch?v=eoSUc_U-tWg
    Excluding te addtional panel under the woofer and I dont use the patented events.

    I think that JBL deliberately studied this construction to use as few materials as possible, to maximise the internal volume, and to limit weight and price. While keeping the resonnaces low. But if you don't do exactly what they did (exact same materials, exact same glue, exact same assembly...), you start getting colorations. Which are hard to suppress without professionnal tools.

  14. #14
    On Holidays Ian Mackenzie's Avatar
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    Nice YouTube.

    https://www.muziker.es/monacor-mdm-4



    Have you attempted to measure the system to see if you can identify the issue?

    On the gasket there might be a benefit to minimise vibration of the horn structure. Thin foam door seal might be worth trying.

    I would talk to Robert (Europe) who has done a lot with the M2 builds.

  15. #15
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    Hello,

    I did't tried to tackle the problem with measurements because I find the measures too difficult to interpret. I find it easier to knock on the panels and hear the sound it produces. Or put the ear on the box while listening to music. It allows me to ear the problems and find where it happens. Unfortunately, it doesn't tell me exactly 'why' it happens and how to fix it. So I guess my best solution is trials and errors...

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