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Thread: Need help designing a subwoofer

  1. #1
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    Need help designing a subwoofer

    My current project is to build some subwoofers that will be identical in outward appearance to my Oracles, and contain 2 each JBL K140s. I know that this driver isn't ideal for use as a sub, but I have 4 (although 1 might be a D140, can't remember) of them gathering dust, so it's what I'm going to use. I have tinkered online with several calculators, and still don't have a firm grasp on the particulars.

    My basic idea is to take a box of this design,



    And orient them port-to-port within the confines of the Oracle cabinet. Can anyone offer some more precise values regarding tuning of the box, port, etc? TBH, I'm not even sure on how to select the basics, i.e. box frequency and so on. Part of the difficulties I ran into way trying to calculate the changes due to the trapezoidal box shape. The picture I linked was as close as I could get with that calculator.

    Any help would be appreciated.

    I too would be interested in finding someone who could build them, as I have seen several posters turn up goose eggs trying to find custom cabinet builders. I did have my WTB in the marketplace, but that seems like a long shot.

  2. #2
    Administrator Mr. Widget's Avatar
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    The D140 and K140 both require the same tuning, so a mismatch won't matter. The difference is mainly in voice coil power handling.

    You are correct, these speakers will not make a terribly great subwoofer, but with four of them you will probably be able to get the bass out of them that you are looking for as long as you filter out the extra midbass.

    In answer to your question, you need to build a 4 cu ft box for each woofer. Those boxes should be tuned to 40Hz. To get that tuning you will need two 4" diameter ducts that are each 6.5" long. Alternately you can build a rectangular port that is 12.6 sq inches by the same length.

    If you prefer two massive boxes, you can build a pair of 8 cu ft boxes with two of your woofers in each. These will each need a single 50.3 sq inch port that is 5" long or four 4" diameter ducts that are each 5" long.

    The precise shape of your trapezoid is unimportant, but the interior volume must follow those guidelines.


    Widget

  3. #3
    Senior Member grumpy's Avatar
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    So some uncharted territory then (aka bench racing) vs 4648 type dual 15 boxes (-10dB @30Hz):

    Oracle/Disco cabs are what... 6+ ft3?
    IF fixed on 2drivers/box without expanding the volume (if not the front profile, by going deeper or removing the wedge shape),
    you might want to shoot for closer to driver resonance for port tuning.

    Driver proximity and room gain may help with the resulting slower/sooner roll off. I assume you can set the sub level and crossover independently of your other speakers,
    and some EQ (e.g. peaking HPF 3+dB @45Hz will restore some of the difference from the larger box suggestion and provide some protection for the drivers).

    I'd also take my comments above with a grain of salt until you have some test cabs built, check the tuning, and can hear them in your room with the intended placement.
    Wouldn't be hard to end up with an impressive-looking bunch of boomy nonsense with no real low end.

    The Oracle system already goes pretty low... so curious what the goal is beyond using some extra speakers? Kick, boom, pant leg fluttering, maxed out on main amp power, reducing bass distortion by distributing it across more voice coils? All talk, on my part. Just curious.
    Folks have assembled systems with old D130's in large boxes to great effect with what I'd call naturally recorded acoustic instruments/voices, so sometimes unexpected scenarios can work.

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    Smile

    Quote Originally Posted by Mr. Widget View Post
    You are correct, these speakers will not make a terribly great subwoofer, but with four of them you will probably be able to get the bass out of them that you are looking for as long as you filter out the extra midbass.
    My old stereo system when I was a teen had a single 140 in a 6 cu ft box that was tuned at 40Hz, and that thing was a beast, but it had an active crossover that was low pass @ 100Hz on it. It's response was almost flat down to about 40Hz. The slight drop off after that was easily compensated since I was powering it with a BGW750. Of course, I don't have that room available here.

    Quote Originally Posted by Mr. Widget View Post
    In answer to your question, you need to build a 4 cu ft box for each woofer. Those boxes should be tuned to 40Hz. To get that tuning you will need two 4" diameter ducts that are each 6.5" long. Alternately you can build a rectangular port that is 12.6 sq inches by the same length.

    If you prefer two massive boxes, you can build a pair of 8 cu ft boxes with two of your woofers in each. These will each need a single 50.3 sq inch port that is 5" long or four 4" diameter ducts that are each 5" long.

    The precise shape of your trapezoid is unimportant, but the interior volume must follow those guidelines.
    So, the interior space available I have calculated at approx 2.6 cu ft available for each driver, that's what I have to work with. Is this a lost cause? I guess what I'm interested in at this point would be to model the behavior in this admittedly too-small enclosure, and see what the results are. I'd like to see what the outcome would be and see if it would be acceptable. Of course I could put one driver in each cabinet (much better match) but as I said, I've got plenty of drivers available.

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    Quote Originally Posted by grumpy View Post
    So some uncharted territory then (aka bench racing) vs 4648 type dual 15 boxes (-10dB @30Hz):

    Oracle/Disco cabs are what... 6+ ft3?
    IF fixed on 2drivers/box without expanding the volume (if not the front profile, by going deeper or removing the wedge shape),
    you might want to shoot for closer to driver resonance for port tuning.
    Under 6, actually. This is the sort of thing I was referring to, would that be an adjustment of the box frequency, or just the port? All of the online tools I have looked at so far are either too simplistic to cover these scenarios, or so complicated that I am unable to utilize them.

    Quote Originally Posted by grumpy View Post
    The Oracle system already goes pretty low... so curious what the goal is beyond using some extra speakers? Kick, boom, pant leg fluttering, maxed out on main amp power, reducing bass distortion by distributing it across more voice coils?.
    Overkill, really. Never can have too much of that.

  6. #6
    Administrator Robh3606's Avatar
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    Hello

    I would download WinIsd link attached. Unfortunately they don't have the T/S driver parameters for the D-140 so I used the E which should be similar. Here is a screen shot in 2.6 vs 5.2 tuned to 30Hz. As you can see 5.2 for a single driver would be your best bet. But as others have said not a good choice for subwoofer. The only advantage would be to but them within 1/4 wavelength to get +6 dB from mutual coupling and as Mr. Widget said roll them off so you don't get to much mid bass. You are -10dB @ 30 HZ plus 6 from the coupling so -4 dB from the reference level which is the drivers sensitivity so figure about 96 dB 1 meter.

    If you look at the K which should be close to the D it models a little better. Try both so you can see. The program is very easy to use and has a drop down driver menu with all the drivers pre-loaded. All you have to do is select the driver and it will give you an alignment. Then you just change box size and tuning to see what the response changes are.

    Rob

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    I will download that and play around with it. I forgot to ask before, how much of an impact would stuffing the box with insulation have? Doesn't that act to increase the effective size? I realize that wouldn't be much, of course.

  8. #8
    Administrator Mr. Widget's Avatar
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    I didn’t realize you meant to copy the external dimensions of your L220s exactly. The volumes and tunings I suggested are from the JBL enclosure guide and represent a maximally flat tuning. You can certainly deviate from that and get a variety of LF curves… since your application is to only use the extreme LF part of its output, you may find an alternate tuning better suits your goal.

    You can model a variety of tunings to get an idea of a direction to follow, but really you should experiment. You can do this with replaceable ducts and try one or two woofers in each cabinet. You can always cover the second woofer cutout if you find you prefer one woofer per cabinet, and you can do the same with the ducts if you decide you want fewer ducts… or if you want to really do it right, you can design your enclosures with replaceable baffles.


    Widget

  9. #9
    Administrator Robh3606's Avatar
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    Quote Originally Posted by enelson14 View Post
    I will download that and play around with it. I forgot to ask before, how much of an impact would stuffing the box with insulation have? Doesn't that act to increase the effective size? I realize that wouldn't be much, of course.
    Hello

    It helps but I would wonder if it would be worth it to over stuff. No matter what you can't have the stuffing interfere with the vent/vents. Even if you model a larger box @ 30Hz you are still going to be down and at 20-25 barely any useful output minus possible room gain. Try the 5.2 box with 25Hz you can see a shift with more extension at the price of a drop @ 30Hz.

    Rob
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  10. #10
    Senior Member RMC's Avatar
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    enelson,

    I'm with Widget post #2 in terms of proper box sizing and tuning, its about the best you can do with K140 VLF wise, i.e. with output and an acceptable response. Note the smallest JBL recommended K140/E140 box volume is 3 cu.ft. I know this game since i own the JBL 2205H drivers, which happen to have the exact same basic parameters to model a LF cab in software than the K140: Fs 30 hz, Qts 0.21 and Vas 297.3 L

    Larger Vb than those come with a penalty with regards to LF roll-off and output, unless you use a second or third boundary placement. Most speaker softwares already assume 2Pi box placement (one boundary, most of the time on the floor), second boundary would add cabinet to the back wall and third boundary means in the corner of the room. Under these conditions you get room low-frequency reinforcement.

    Your luckly having the K instead of the E series, since the former has little higher Qts (helps a bit on box size) and somewhat more excursion capability than the E version which is useful for bass reproduction. E140: Fs 32 hz, Qts 0.17 and Vas 297.3 L. in simple terms Vas determines the proper box size, however Qts acts as a cab volume "trouble maker": low/very low then LF response will drop sooner, higher/pretty high the driver will be able to maintain flatter response in lower frequencies or in a larger box.

    You seem to have a predetermined cab which doesn't help your cause. Unfortunately some things don't appear to add up. In post #1 i see net internal volume 1.9 and total internal volume 2.61 . First, 1.9 is lower than JBL's minimum recommended of 3 cu.ft. for the woofer.

    RE "So, the interior space available I have calculated at approx 2.6 cu ft available for each driver, that's what I have to work with." What you normally use for driver performance calculations or box modeling is the NET volume (i.e. 1.9). The space taken in the cab by driver, bracing, vent, etc. must be subtracted, hence the need for cab overvolume to compensate for those items.

    Also the computer modeling done using 2.6/5.2 would seem incorrect in view of the above, since it uses the gross internal volume. The reason being the net is what the driver actually "sees or plays with".

    Finally, you need to know or remember that in WIN ISD software the default box losses assumption is QL 10, which is not standard and is overly optimistic in most cases, it unduly reduces box size. The industry standard is QL 7, as was done in the modeling shown. This QL number can be changed by the user in Win ISD, as i recall go to box tab, at the bottom click on advanced, you'll see QL, QA and QP, click on QL and change 10 to 7, never mind the default QA (absorption) and QP (port losses), they're usually too small to bother with, or for special case only. If you save your WIN ISD project with QL 7 it should stay, but if you start a new project in ISD it may have default to QL 10 again, so you need to keep an eye on that software's QL number.

    I hope the above will help you.

    Richard
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  11. #11
    Senior Member RMC's Avatar
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    enelson,

    i modeled quickly in Winspeakerz the E140 (T/S for K140 are not in the software database, don't have time to add it now) and here's the response curves i got with 4 cu.ft box, lower curve tuned to 40hz and upper curve tuned to 50hz. The K140 would be slightly better and equivalent to the 2205H curve.

    Still not glorious low bass. This is a side effect of the very low E140 Qts of 0.17

    The black horizontal line is driver sensitivity at 100 db. Red lines at the bottom are related to driver excursion.

    Richard

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    I played around with the free version of WinSpeakerz and with WinISD all last weekend, and I guess it's just not practical to put 2 drivers in this size cabinet. I guess that was the reason for the PR in the Oracle to begin with? I even experimented with that, using some Dayton PR specs, but all of the theoretical designs I came up with seemed to perform worse than a ported box, I'm not even sure if I was doing the configuration with WinIsd correctly.

    The single 15", ported in this cabinet seems reasonable, if not ideal. I'm going to be working up some detailed measurements and thoughts later, maybe someone would be willing to double check my work later?

    My current thought was that I found an acquaintance who is a hobbyist for fine woodworking, so I think duplicating the cabinets visually shouldn't pose an issue. I envisioned the front panel as having two 15" cutouts, equally spaced, with a blank-off cover made for one hole. That would enable me to experiment with the position of the driver (not that I think that would change much.) Regarding the porting, a rectangular, front panel insert that could be removed, so that different ports sizes and lengths could be experimented with easily by just swapping out the insert.

    Any suggestions for improvement?

  13. #13
    Administrator Mr. Widget's Avatar
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    Quote Originally Posted by enelson14 View Post
    Any suggestions for improvement?
    Not to be flippant, but you should consider a different woofer. While the 2235H wouldn't be my first choice for maximum LF output, a single 2235H in each of those cabinets tuned to 28Hz would be lightyears better than the K140s. Try modeling it and compare.


    Widget

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    Quote Originally Posted by Mr. Widget View Post
    Not to be flippant, but you should consider a different woofer. While the 2235H wouldn't be my first choice for maximum LF output, a single 2235H in each of those cabinets tuned to 28Hz would be lightyears better than the K140s. Try modeling it and compare.


    Widget
    I'm not opposed to the idea, It's just that 4 K140s is what I have on hand. If i were to source 2 2235s, what would be looking at for getting rid of the 140s? Is there any demand for them? A quick glance through ebay seems to be that the 2235s are going for more than twice the price, and that's assuming I could get a decent amount for all 4 of them. Any one willing to trade?

  15. #15
    Senior Member RMC's Avatar
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    RE in this size cabinet.

    RE The single 15", ported in this cabinet seems reasonable, if not ideal.

    My understanding is the OP is still talking about his 1.9 cu. ft. box net.

    I don't see how he would make a valid 2235H cab in such small volume tuned to 28hz. The typical 2235 box used is 5 cu.ft. tuned to 30hz...

    Something seems to be missing
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