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Thread: Positioning of ports and driver?

  1. #16
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    Well those boxes says that if you want to put the slot at the bottom of the box like this one:



    you should place the driver in the center of the box. Also it says that there will be midrange frequencies coming out of the port, so the box only be used to about 250 Hz.

    Hornresp is very limited when it comes to reflex boxes, you can only place the port at the end of the box. But it can simulate what happens inside the box (limited to one dimension). It is also useful because you can export an akabak script from such a design so you don't have to make one from start.

  2. #17
    Member Greg_M's Avatar
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    Perhaps I will build according to my original plan with the thought in mind that I may have to divide the enclosure into 2 parts for the addition of a mid-bass speaker should it not work out according to my ears.....?

  3. #18
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    Get the plans for the 4430

    that will give You enough bass. Somebody here in the forum should be able to give us a pointer to these plans.

    If You want even lower bass You should consider a so-called B6 alignment. That is a 4th order box combined with a 2nd order elctronic filter. You will need an active crossover with provisions for that required 2nd order filter, such as the EV DX38 I can help You with the 4th order box.

    But 1st try the 4430.

    Regards
    Ruediger

  4. #19
    Administrator Robh3606's Avatar
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    As Widget already pointed out the best general purpose box for a 2235 is 5 cubic ft tuned to 30 or so Hz.

    The 4430 is tuned to high. The B6 box is the B380 at 4.5 cubic ft tuned to 26Hz.

    Rob
    "I could be arguing in my spare time"

  5. #20
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    My modified Akabak script. I have tried to move the driver and port placements, but could not improve the response. This is the Akabak simulation (floor placement):



    It will work in a large room, but in a small room there is too much output below 50 Hz or so. Midrange output through the port will be less of a problem with a smaller box.

    Akabak will simulate only in one dimension, that is from top to bottom of box. Interference from driver to back of box must also be considered. The depth of the cabinet is 22" which is about 56 cm. 344/0.56 = full wave of 614Hz. The interfence is at a quarter wavelength, about 150 Hz. The box is a bit too deep.

    The Akabak script has a few constants at the start, L12, L23, S1, ... Modify these constants and rerun the simulation. This drawing shows the constants in a box:


  6. #21
    Member Greg_M's Avatar
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    If you get a minute try a 6.7 cu-ft enclosure and see how your numbers look.

  7. #22
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    If you give me the dimensions of the 6.7 cu-ft box (width, depth, height) and port (area, length) I will do a simulation tonight. Prefferably in cm.

    Do you want traditional round port(s) or a slot port?

  8. #23
    Member Greg_M's Avatar
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    Internal dimensions
    W 17"
    D 23.25"
    H 30"

    Two round 4" ID ports 8.08" long ea

    I also deducted some internal bracing to get to 6.7 cu-ft

    Thanks!
    G

  9. #24
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    New akabak record



    Driver in middle of the box and center of ports 10 cm from bottom. I believe it is tuned a bit too high.

    Compared to the larger box you have moved the problem frequency from 350 Hz to 450 Hz. If you make the box even lower, and maybe wider, you will push this problem even higher in frequency.

    I have also simulated moving the port to the middle of the box, you will get a slightly larger dip/peak at 450 Hz.

  10. #25
    Member Greg_M's Avatar
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    Neat!
    What would you recommend the tuning at?

    Thanks for the quick answer!

  11. #26
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    You better trust Mr Widget and the others for tuning recommendations. I have never constructed with this driver.

    Do some more WinISD simulations, and when you are happy I'll do another Akabak simulation for a "second opinion". Don't forget to check port velocity in WinISD, if too high you will hear the port. When you do the velocity thing, add enough power to reach xmax somewhere, then you can plot port air velocity.

    I still believe that the box is too deep. If you make it wider, and put the midrange in a equally wide box on top, you will push down the baffle step frequency as well. If you can get the baffle step frequency to the same frequency where you plan to divide, you can use that in you filter design.

  12. #27
    Member Greg_M's Avatar
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    I do appreciate the help.....
    Thanks
    G

  13. #28
    Member Greg_M's Avatar
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    I'm still alive.
    Has it really been 7 years?
    Still got the woofers and my boxes (shells).
    A lot has happened since the beginning of this project.
    Built a house in New Mexico and moved in. Everything else got put on hold during that time.
    Just about ready to pick this project up again.
    Glad I was able to find it.
    G

  14. #29
    Senior Member RMC's Avatar
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    Hi Greg,

    Nice to see you're back in business.

    I haven't followed the thread from the beginning, but here's a few notes that should help you decide.

    RE "you should place the driver in the center of the box." (post # 16)

    Weems (note 1) recommends that one does not place the woofer at the center of the box to reduce promoting standing waves in the cabinet. A little off center is recommended. Mounting a woofer a little higher or lower than box center should do the off center trick, plus it doesn't cost a penny more and its the same work baffle hole cutting wise, so why not if it helps. Though you say having your boxes (shells).

    As for vent placement its usually not critical, more important is to avoid extreme vent proportions for slot/rectangular shape vent, and making sure the port has sufficient space to "breathe" inside the cab.

    JBL's rule of thumb for port size (dia.) is one-third the woofer's diameter, so a 15" driver means 5" vent if only one is used. Two 4" dia. vents are equivalent to a 5.7" vent so that's OK. Two 3" ones = 4.25" so a little short. But then vent size also depends on the amount of power the speaker will be fed with. Practicalities usually determine if you'll use one, two or more (e.g. space on baffle or cost).

    With regards to vent placement vs woofer location, Dickason (note 2) has looked into the driver/vent coupling/interactions with simulations for a 12" driver using 6", 4", 3" and double 3" vents. His conclusion:

    "In the case of the 6" and 4" diameter vents, very close proximity to the driver seems to cause the least amount of disturbance. The 3" vent showed less problems when located at a distance from the driver. The dual 3" ports, Fig. 2.31 produce fewer problems when the two ports are separated by a reasonable distance."

    In addition note the duo 3" vents were placed at the same distance from the woofer (so, for double 3" at some distance from each other but both being the same distance from woofer).

    I think what Dickason refers to with "disturbance" may be vent air turbulence. However there's another aspect mentioned by Weems when vents are not 3"+ from woofer. He says that a port too close to driver is more susceptible to reflected sound from upper bass or mid frequencies. So you'll have to play this by ear...

    Hopefully the above will be of some help to you. Regards,

    Richard

    (1) David B. Weems, Designing, Building, and Testing Your Own Speaker System, 4 th Edition, P.26 & al, 54.
    (2) Vance Dickason, The Loudspeaker Design Cookbook, 5th Edition, P. 55

  15. #30
    Member Greg_M's Avatar
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    I really appreciate your thoughts on the topic.
    Thanks!

    I decided before I go any further to make sure the Thiele/Small parameters are correct on these "recones", so I bought a Dayton Audio DATS V3 Test System gizmo.
    It's on the way and should be here at the end of the week.
    Downloaded WinISD again, so I'm getting ready to go.

    It's good to have projects.

    Thanks again.
    G

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