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Thread: Double k-145 with 2420

  1. #1
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    Double k-145 with 2420

    I’m planning to build two enclosures 9cu ft . There will be two K-145 per box with a 2420 with Mcaully 9 inches horn lens, with metal lens instead of the JBL’ plastic ones.

    Id prefer two woofers at the bottom then the horn at the top because of the listening height...

    il seems that 4,5 cu ft is ok For those speakers. Is it ok to double the cu ft wtith two woofers sharing the same box?

    What at if I put 4- 4 inches port at 5 inches deep each port.? Bue where to put them? Between the 15 inches? Or keep the two moofers close for better coupling? They will be concealed so no way to put them in the back!

    by the way they will be electronically crossed-over with a Behringer Ultradrive Pro DCX2496, a Phase Linear 700B (recapped, re-biased) for the woofers (parallel at 4 ohms) and a Crown D150A not recapped yet but they will...

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

    I have a few things for you but no time right now. I'll be back later... Regards,

    Richard

  3. #3
    Senior Member RMC's Avatar
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    Hi Maxserg,

    RE: I’m planning to build two enclosures 9cu ft

    That box volume is net I assume, therefore the boxes will probably need to be 11 cu.ft. or so gross to compensate for the volume taken by drivers, bracing, vent, etc. Plus being that large I hope you modeled them in speaker design software preferably using QL5 as box losses assumption.

    RE: il seems that 4,5 cu ft is ok For those speakers. Is it ok to double the cu ft wtith two woofers sharing the same box?

    I don't know where you took 4.5 cu.ft., certainly not bad in itself, however two JBL documents I have suggest 4 cu.ft. with box tuned to 40 hz, for the similar E145, though other options still possible. Yes its ok to double the number of cu.ft. with two identical woofers sharing the same box. The rule is double woofer, double box volume, double vent area, but same vent length as for a single woofer box (NOT double vent length).

    RE: What at if I put 4- 4 inches port at 5 inches deep each port.?

    I've done a few modelings in Winspeakerz of the K-145 in 4.5 and 4.0 cu.ft. both with Fb 40 hz. First, the minimum vent area recommended for the K-145 is 20 sq.in. (equal to a 5" vent). And your double 4" vents per speaker equal 25.1 sq.in. so you should be fine. For a 4.5 cu.ft. box the software gives me 5.31" long vents, a bit more than your 5". For a 4.0 cu.ft. box also tuned at 40 hz the two-vent length I get is 6.5".

    RE: Bue where to put them? Between the 15 inches?

    In most cases vent location is not critical. However, some manufacturers like Fane, E-V and a JBL case seen, they tend to have a preference for near the drivers when multiple used, if not in between them. This also depends on your specific box configuration.

    If you build your double woofer boxes on the basis of 4 cu.ft. for each driver, then you would get 8 cu.ft. net which would require 10 cu.ft. or so gross, therefore saving one cu.ft., since this is already pretty large.

    BTW I used a K-145 driver sensitivity number of 94 db from a 1973 spec sheet, instead of the 98 db seen in a K series brochure (500-2.5 khz) because I think these drivers are more often used for low end than mid... I also noted the spec sheet mentioned Fs 45 hz whereas the JBL T/S table says Fs 35 hz... This is a notable 10 hz difference. Regards,

    Richard

    P.S. I have more details on the two E145 JBL boxes having 4 cu.ft. I also have 1975 JBL plans for K-145: 3.75 cu.ft for one and 5.75 cu.ft. for two K145; plus 1979 JBL plans for two K145 in 8 cu.ft box

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    Thanks for your explanations RMC!

    i think that for woofer coupling I wouldn’t put between woofers , I would put them close to each woofer, each side the top lower one and two each side under the top woofer...

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    Senior Member RMC's Avatar
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    Hi maxserg,

    In your double woofer box you will have, on the front panel, two woofer holes, four vent holes and another hole for the 2420/horn lens. So that means seven holes on the baffle panel. This can be seen as extensive baffle hole cutting that reduces panel rigidity, in which case Fane Acoustics recommends an additional front to back panel brace.

    Personnaly I do a little more by putting an horizontal brace on edge about middle of box on the front panel's width. Then the front to back brace going to both front/rear panels also touches the horizontal front panel brace. F/B brace glued and screwed to F/B panels AND horizontal front panel brace also glued and screwed to F/B brace. So basically looking like a " T " shape thing. Add an horizontal brace between left and right panels. All this in addition to each panel being braced...

    In any case, those suggestions assume you keep a little space between the two woofers for the front to back and/or horizontal brace(s). Remember, two K145 in a box can rattle many things, including box panels, specially when powered by something like a Phase Linear 700B (I'm glad to see you still have one of those amps with their large VU meters that I could only admire ($) in the mid 70s, sure don't see a whole lot of those these days).

    Regarding your reference to the Enclosure Guide.

    If you follow the enclosure inside dimensions given in that guide you will not achieve an effective Vb of 4 (not 4.5 either) or 8 cu.ft. box. For the 4 cu.ft. enclosure the dimensions given lead exactly to 4.38 cu.ft. and for the 8 cu.ft. they give exactly 8.62 cu.ft. So that doc shows internal dimensions of a NET Vb, less useful.

    This is certainly not enough gross volume to get to the required net volume when all volume displacements are factored in: drivers, braces, vents, etc. I don't see where you get the required box overvolume if you follow the measurements given in the Enclosure Guide. Even more so when large boxes like 8 cu.ft. net should be modeled with QL 5 box losses assumption (therefore leading to increased box volume to compensate for those losses). Large cabs do tend to be more lossy (Bullock, Small, Leach).

    I clearly remember D.B. Keele using box overvolumes such as 20, 27 and 30% in designing some vented boxes, depending on cab size. Btw the 30% overvolume he used was for a 15"driver in a 7.5 cu.ft. box tuned at 40hz. Pretty close to your case.

    In another more detailed JBL vented box construction document (see note 1), seemingly written by Drew Daniels, things are presented and explained in a better way:

    " The enclosure dimensions given [here] are pre-adjusted for correct internal volume after bracing, drivers, ducts and fiberglass insulation are included." This way of presenting is preferable and less confusing for the speaker builder. The enclosure inside dimensions given in this case for the same E 145 as in the Enclosure Guide (similar to K145) are 25.2" H X 20.6" W X 17.8" D = 5.35 cu.ft. for a single driver box. So one more cu.ft. than the Enclosure Guide to account for driver, bracing, ducts, etc. This makes more sense to me. And that box is also tuned at 40 hz.

    To avoid ending up with a smaller effective Vb than the one really required, and therefore reducing the LF/VLF capability of your enclosures, I suggest you increase the 8 cu.ft. by 25+% = 10+ cu.ft. gross box volume, certainly no less than 10 cu.ft. to be on the safe side for your double woofer box.

    One can still look at the Enclosure Guide for many indications like Vb (net), Fb, vent(s) needed, or to keep the same type or so of box proportions. As for exact and required box gross inside dimensions to effectively get to the correct Vb to load the drivers, I wouldn't rely on the Enclosure Guide dimensions given. Regards,

    Richard

    Note 1: Vented Loudspeaker Enclosure Construction and Operation, Pages 3 and 4.

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    Well thanks a lot RMC. From now I do have a lot of reading from your generous post. Let me digest all of your informations! Ouff!😊

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    I think that I’ll make separate boxes? Two for each K145 and one kind of a smaller box for the horn.... it will be much stronger and no hole for the driver/horn/lens. And I will be able to tilt this if I want to have it properly adjusted.

    Ill have to properly anchor them. They will be on top of each other. Any suggestions on this aspect ara all welcomed

    Easier to move too.

    I will wire the woofers in parallel, the Phase Linear 700B can handle it. Anyway I won’t push them to the limit!

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    Senior Member RMC's Avatar
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    Hi maxserg,

    RE: "I think that I’ll make separate boxes? Two for each K145 .... They will be on top of each other"

    So If I understand correctly this will be ONE box for each K145 and the two boxes per side will be stacked. I see nothing wrong with this principle.

    However, since you mentioned in your first post "Or keep the two moofers close for better coupling?" I would build the four boxes total each with the same configuration.

    For example, that means putting each woofer in each box somewhat near the top of each box with the vents under. Made this way then you put the cabinet on top up side down on the other, therefore having the top and bottom woofers in each stack close to each other which is better for mutual coupling you initially wanted to achieve. Regards,

    Richard

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    Yes that was my intentions to make the boxes this way. Only two plywood of 3/4inches thick would separate the fifteen’s. Plus maybe another inch to leave space for some braces for corner assembly...

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    For internal acoustics, I would like not to use fibreglass insulation because of airborne particles. Or should I spray with some spraynet or spray with 3M contact glue(77)?

    I know that I won’t cover the back panels vis a vis the ducts but for the rest of the surface do I need to cover all?

  12. #12
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    Hi maxserg,

    You can use hair fixative spray (spraynet) or even spray paint, but in any case use VERY LIGHT coat to avoid this getting hard with time which would reduce fiberglass absorption of mid frequencies. You can also loosely wrap the fiberglass insulation in a layer of cotton cheese cloth (the type used when waxing the car for example) as this would help keep the fibers in place instead of airborne.

    Spraying the fiberglass with contact glue doesn't sound like a good idea to me as it may get harder with time...

    The minimum is covering 3 panels with insulation, however its best to cover more, except front baffle and near the vents approaching the back panel. As I recall JBL used/suggested 1" thick insulation material.

    When the port length gets close to the back panel its ok not to put fiberglass on that section of the panel. The logic being if a sound wave rebounds where there's no insulation then it will be absorbed on another panel covered with fiberglass, hence the importance of the suggestion of covering more than the minimum.

    Your ducted ports should have, for "breathing space" inside, at least the equivalent of vent diameter used from the back panel in order to work properly.

    There are other materials than fiberglass one can use, but I've previously posted in another thread two lists of those NOT recommended by JBL (e.g. felt, cotton, etc.). It should be low mass/density. Regards,

    Richard

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    Quote Originally Posted by maxserg View Post
    For internal acoustics, I would like not to use fibreglass insulation because of airborne particles. Or should I spray with some spraynet or spray with 3M contact glue(77)?

    I know that I won’t cover the back panels vis a vis the ducts but for the rest of the surface do I need to cover all?
    I used the white snow sheet material that's used at Christmas on displays (or pillow stuffing) for my latest project PA system. I worked in a insulation factory and worry about using any real glass type product for health issues.
    I didn't research how this stuff is made and maybe it's no better??
    It says Poly fill on the one bag.

  14. #14
    Administrator Mr. Widget's Avatar
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    I have performed objective and subjective tests on fiberglass and polyester “poly fill” and the polyester, while much easier to work with is significantly inferior to the fiberglass material from a performance standpoint.

    I do not use or recommend house wall insulation fiberglass, as it sheds terribly. I am not sure if non-shedding “Wrap-on” is still available, but this is what I use. I have also had very good results with acoustic foam like Sonex, and also very good results with the blue denim cotton home insulation.



    Widget

  15. #15
    Administrator Robh3606's Avatar
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    I use 3/4" fiberglass acoustic sealing tiles.

    Rob
    "I could be arguing in my spare time"

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