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  1. #1

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    Default Changing Size of Ports for dual MOMO 12" Bandpass design

    Well I posted this topic in the car audio corner, but no responses, so i'll try here.

    I can't find the 5" ports the box requires, but was wondering if the critical factor of the ports is the volume of the tube itself. Doing some calculations if I use two 4" ports which I have from the single momo bandpass design, and an additional 2" port, I would only be losing .59 cubic inches of space in the ports themselves. I'm assuming that with the 2 inch at 13" it will be in tune with the other larger ports?? This assumption may be wrong as I think about it, but I would really like to have some bass this summer. If I can't get this box to work I guess i'm going to have to cut it in half and just use the sealed chambers :(.

    Please help me solve my dilemmma.
    Derek S.
    1994 MTX SHO
    Polk MOMO 12"

  2. #2
    Polk Woodpecker
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    Where did you get your plans for a dual MOMO box? We only offer plans for a single MM100 or MM120, and they both use 4" ports. Let us know.

  3. #3

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    Hey Josh, Kim gave them to me eons ago when I built my single MOMO bandpass design. At the time I asked, he gave me both a single and a dual bandpass design.

    I'll upload what I got and you can have a look.

    Thinking about my calculations last night I know they are wrong, but I'd like to get this resolved soon :)
    Attached Files
    Derek S.
    1994 MTX SHO
    Polk MOMO 12"

  4. #4

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    You can step down the port size and it'll actually get shorter for the same tuning frequency, but the big problem is that the smaller the port diameter the more likely you'll be to hear "port noise."

  5. #5
    Polk Customer Service
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    Derek,
    We can do better!!!!!!! I have attached plans for a dual MM120
    iso push pull 2 chamber band pass. If you have any questions
    give us a call at 800-377-7655.


    Thanks,
    Kim
    Attached Files

  6. #6

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    Default could i adapt this kim?

    hey kim...

    is there any way i could adapt this to 4 DX 12's?

    obviously i'd have to buid 2 of these boxes, but just wondering if it'll work for the woofer.

    i'm bored and sick of how my car sounds and if the new amp doesnt' help im entertaining new box designs (port --- i dread the thought)
    "With your own attitude it is hard to survive here... But who gives a damn, we are here to change the world, and we dont need a password for that."
    - Anurag

  7. #7
    Polk Customer Service
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    Here are a set of plans for a 2 chamber quad DX12 iso band pass enclosure. Have fun!!!!! Let me know how it sounds.

    Thanks,
    Kim
    Attached Files

  8. #8

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    for some reason i am having trouble using the link that kim posted for the iso push pull dual gnx12 box....i was wondering if i could get the document emailed to me maybe at steevo@breakbeat.com and i was also wondering if there was maybe a way to adapt the design and add a third gnx12 (maybe in it's own sealed chamber?)

    i'm looking to squeeze three gnx12's into my mustang (small trunk) and get some mad bass too but i need a box that won't use all of my trunk space...if this push/pull box produces some kind of really loud sound i might just use two subs...which would be grand because of my limited space....

    but anyway, if i could maybe get those plans emailed as an attachment that'd be fantastic.....
    Steevo a.k.a. DJ Nostalgia

  9. #9

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    Um, about port sizes. For one thing, if you can find a "port" that is 5 inches in diameter, Home Depot or Lowes probably has a PVC pipe wide enough. You can always get a mounting flange and sand it down and roll the top of teh flange. That will give you your diameter port that you need and rolling the edge and giving a slight bevel to the inside diameter of the tube, near the end, will help reduce port noise. The whole problem with port noise is that it is caused by an uneven and turbulent transfer from the internal box pressure to the external atmosphere. The whistling and whooshing of a port is caused by pockets of high pressure air in eddies around the edge of a typical port. Making a flange and rolling the edge smooths out the air flow and ransfer and makes those high pressure eddie much smaller. Small eddies means less port noise.

    Also, about port diameters. While the port size affects frequency response, the major proponent of SPL is volume of air moved. If you want loud, using a pair of 2.5 inch ports will still give you roughly the same amount of volume even if it truncates the frequency range a bit. It's a trade off but, you may be able to gain some of that range back by playing with the lengths of the port tubes and polyfil and/or baffling.

  10. #10

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    pbd,
    you using a band pass?????
    -Cody

  11. #11

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    Originally posted by sntnsupermen131
    pbd,
    you using a band pass?????
    -Cody
    last summer when i still had my DX's i was so displeased with how the sound quality turned out that I was tossing around 100 difft ideas of what to do with them...

    i even considered the bandpass for a while -- looked over the plans and then decided against it. (not that the plans were bad -- i just decided BP was def. not the way for me to go).

    in the end i think i just had too many subs in too small of a space, not enough power handling for my deaf ear's taste, and in fact a difficult truck to work in.

    The guys at ID have been tellin me that after working on several late model dodge full size trucks they've noticed that the cabin is just too "Square" and really causes audio havoc on their install attempts... anything but downfiring with high power subs is a waste of time unless you want to do a small single sub on one side of the car... anything else causes cancellation, phase differentials, poor imaging, and a million other issues.
    "With your own attitude it is hard to survive here... But who gives a damn, we are here to change the world, and we dont need a password for that."
    - Anurag

  12. #12

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    i see....
    one of the great downfalls or car audio...
    -Cody

  13. #13
    Colin_S
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    Just wondering what the difference between 2 sealed .88cf boxes would be and the plans Kim attche to her post woud be?

  14. #14

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    Well first off, I do believe that Kim is a guy but I could be mistaken.

    As far as what the difference is between the two plans, I'm not sure what you are asking. They seem to be similar. Can you clarify or elaborate please?

  15. #15
    Colin_S
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    Sorry Kim.

    Anyway the options i have is having 2 sealed .88cf boxes OR the dual band pass options that Kim attached the plans for. What differences would there be?

  16. #16

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    stupid internet....sorry
    Last edited by sntnsupermen131; 03-18-2003 at 12:53 AM.

  17. #17

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    a sealed box will have better sound quality
    a bandpass box will not sound good(well to me) but it will be louder
    i have never liked band pass but polk as started to endorse them
    you can argue over it all day but really you just got to hear it for yourself
    sometimes thats not an option so go to your local best buy/circuit city or even better a more dedicated car audio place that has both types of boxes and hear it
    truthfully, the momos are an sq sub...and i dont really understand why you would want to put a sq sub in a box that is made for spl
    but whatever floats your boat
    and kim IS a guy
    -Cody
    Last edited by sntnsupermen131; 03-20-2003 at 05:07 PM.

  18. #18

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    CHeers, just going to out them in sealed boxes

  19. #19

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    Originally posted by Colin_S

    Anyway the options i have is having 2 sealed .88cf boxes OR the dual band pass options that Kim attached the plans for. What differences would there be?
    I realize that this is a bit late for you but I have been meaning to reply and I've just been crazy busy or it slipped my mind.

    Anyway, you have the two options here. You have to ask yourself what your goal is. Do you just want noise to wake the neighbors at 2 am or do you want a quality sound with a large and accurate range?

    If you want noise, the bandpass will do it, very well. Quality suffers and you only have a frequency response of about 30Hz to 50Hz typically, some bandpass enclosured can exceed 80Hz but the frequency drop off is a straight line down after 80Hz. You will get sharp drop off from 35Hz and lower and after 30Hz, it's like gravity decided to work extra good there because the drop-off is almost as steep and the higher end. However, the bandpass does not need a whole lot of power to boom. It may not sound great but if you want to rattle fillings, windows and foundations, a bandpass will do it. You can usually get away with 50-75% of the RMS rating of your speaker as the peak power of the amp you choose so it can be cheap but I wouldn't recommend it because you can get into trouble with clipping. The bandpass enclosure is very efficient in that respect. But as always, more power = more LOUD! The drawbacks to a bandpass box though are muddled sound, inaccurate response and truncated frequency range. But bandpass is meant to make noise and provide ample low-end reinforcement. If you want something more musical, don't look here. A bandpass is not musical.

    If you want accurate, tight response with a wide frequency range for musical purposes then you want to go with the sealed box. Since you have two subs, I would seperate them in the enclosure with a baffle board making 2 seperate boxes in the single box. There are many reasons for this and I don't think I should explain them in any detail beyond mentioning them. If you want more info, let me know, I'd be glad to discuss it. The drawback to having a shared enclosure with 1 chamber for 2 subs is alot more complicated than it seems. Things like standing waves, cone and/or enclosure resonance and frequency response are affected, usually adversly. However, if the enclosure is ported, you won't have too many of those problems. Since you need a sealed enclosure, we will keep those problems as bad things for you.

    The benefit to seperating the two speakers is that they cannot affect each other in the way they respond which is very important. So, with that in mind, the benefits to a sealed enclosure are many. They are not very efficient power-wise but in construction and design they are extremely efficient. A sealed box can actually extend the range of a speaker a tad. My Polk dB sub is rated down to something like 10Hz I think it was but it was measured as low as 8Hz (if it was 20Hz instead of 10Hz, just add 10 to the numbers. It's been a while since I saw the readout from the test machine). The enclosure has alot to do with that ability. The sealed enclosure will provide you with strong and very tight bass. If you have a good amp, it will be clean too. The drawback to a sealed enclosure is power. It may seem like they are weak but they will out-BOOM any other type of enclosure, IF you have enough power behind them. However, a sub in a sealed enclosure will be more likely to be able to exceed it's power ratings than if it were in a ported enclosure. Sealed enclsoures will kick harder than any other enclosure because of the frequency range they are capable of producing. In addition, a sealed enclosure will be more likely to produce the frequencies that you can't hear but you feel. All this makes a sub in a sealed enclosure very accurate and very efficient in all respects except power. This makes it an ideal choice for anyone who wants more than a boom-boom sound for thier stereo. So if you want a musically accurate stereo then your ideal choice is sealed.

    So what about ported boxes? Well, they serve a purpose too. Since they are not sealed, they will not have as wide a frequency range as the sealed enclosure. The truncation is only about 10% at most from what the sealed box would be. The benefits to the ported box are that they are very efficient power users and they have the same wide range as the sealed boxes even if you are missing a few Hz at the top and bottom of the range. Ported boxes can play loud like a bandpass but they retain a good deal of the accuracy of the sealed box.

    The drawbacks are quite a few though. Ported boxes are large. They need space and that makes them big. Usually about 35% bigger than a sealed box for an equivalent sub. Eventhough they have the same accuracy as the sealed box, at the extreme ends of the frequency response range, they start to lose steam because there isn't any stiff air pockets for them to push against like in a sealed or bandpass enclosure. The frequency range is just as large but, when you get near the ends of the range, frequency falloff gets sharp, very quickly. Also, in a ported enclosure (same goes for bandpass here) roughly 75% of the volume is coming from the ports so a ported box needs to have a vent into the listening compartement otherwise, they sound like they are stuffed with socks. Also, in a ported enclosure, it can be very easy to over-power and over-extend a speaker because they are almost like an infinite baffle design. They have no sealed air pocket behind them which can limit movement through air pressure. So a ported enclosure is good if you like big boom but want it cheap because the box will be cheap, the drivers can be cheap and the amp doesn't have to be real big so it'll be cheap too. But I do not think that it is the best bet for you.

    I think you went with the best bet with the dual sealed enclosures. Good choice, sorry I didn't respond with this sooner. I'm sure it could have helped you out more but hey, now you know for future use!

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