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Thread: A question of strength

  1. #1
    hector.murray
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    A question of strength

    I'm building a bass box, as in for a bass player, using the JBL Professional Enclosure Guide as my design guide. I'm using the 10th referance, for a pair of 2225 woofs. Now I have 5 sides of the box built and went to fabricate the baffle board. After drawing the cutouts for the 2225s on the baffle board, I looked at where the four 4" port locations would go and had to pause. I felt that cutting the ports in on the front would weaken the baffle board. Is it acceptable to cut them into the rear of the cabinet? I've seen lesser makes do this, just wondered if this was an acceptable practice.

  2. #2
    aust-ted
    Guest
    Hector

    I am not an expert in this but think you could probably get away with rear ports provided you accepted that you could not place the speakers against a rear wall.

    I recently constructed a pair of boxes for dual 2235s and faced the same dilemma you are now facing. I ended up using dual front facing ports but reinforced the front baffle in a number of ways.

    Firstly I glued & screwed pieces of hardwood on the back of the baffle in places where it looked weakest.

    Secondly I placed a hardwood brace from the back of the back of the baffle to the back of the box which also had hardwood strip bracing applied in an irregular fashion to reduce modes of vibration. I also reinforced all the other inside faces of the box similarly. I then attached top to bottom and side to side braces which were also attached, in the middle of the box, to the front to back brace.

    Thirdly I even included a diagonal brace from the front to one side to further increase rigidity and reduce modes of vibration.

    I used 1 1/2" veneered particle board for the box because it was free from a local office building refurbishment. All in all I was very happy with the outcome which I think passes the 'knock' test pretty well.

    I also applied some carpet underfelt & polyester fibre but this was more in an attempt to increase the effecive Vb.

    The only draw back is the weight which require two men or an industrial trolley to move one of the boxes.]

    Regards
    Ted

  3. #3
    hector.murray
    Guest

    Young Bruce Lee

    My 13 YO styles himself after Bruce Lee. and he is roughly the same size - maybe a fraction taller. I say this b/c he is not Schwartznagger (it's the 13 year olds' bass box) . I'm putting wheels and handles on it, as it is. I think I will try a center brace between the two woofs first. Thanks for the advice.

  4. #4
    Steve Gonzales
    Guest

    All in the FAMILY

    Hector, I see you're raising the boy RIGHT

  5. #5
    hector.murray
    Guest

    get 'em young!

    Yes!
    As I was taught, so will he be ( "son, there's 2 grades of speakers JBL and everything else" )

  6. #6
    Steve Gonzales
    Guest

    A Good Man!!!

    Hector, I lost my dad when I was very young to cancer and always think to myself when I'm alone listening to my system, I wish you could see and hear this Dad

  7. #7
    RIP 2011 Zilch's Avatar
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    Diagonal brace front baffle to rear corners:

    http://audioheritage.org/vbulletin/s...t=4950&p=44780

    Triangle is the strongest geometry....

  8. #8
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    Hi Hector

    - If I was in your place, I'd explore turning those (4) 4"Ø ports into something else. For instance, my software suggests that the 8 cu' box tuned to 40hz wants a "Minumum Vent Area" of 44.1 sq" . Manipulating the software ( keeping 45sq" as a constant ) returns a Vent Length of 4" .

    - This means using 1 , 4" deep duct that is a single rectangle of maybe 15" by 3" ( or 18" x 2.5" ),( or 24" x 1.875" ) etc. . This type of single duct is seen in many MI boxes situated between the two woofers ( I guess , so that they load the port equally ).

    - Point being ; if you were doing this with software you'd see that you aren't obligated to use JBLs' suggested porting scheme .

    - I'd fish around for some more opinions on this matter. ( I'm not aware of any one style of porting scheme being superior to the others ) .





    Oh I forgot , a duct/port that runs the full width of the baffle with those 4" ? deep "lips" - effectively adds strength by bracing the front & the sides ( since they are all tied together ) . Keeping that same 45 sq" total vent area but shortening the duct to 2" , gives an approx. tuning of 45Hz . A 3" deep duct gives @ 42.2Hz .

  9. #9
    Senior Member grumpy's Avatar
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    Slotted port

    I'm with Earl. Full width port, vertical septum for additional strength. Have seen countless
    boxes built this way and taken quite a beating. Can measure a SWR Big Bertha
    (dual 15"... think they used custom PAS drivers) if you like. It's ~24" wide with slot/port
    on the bottom.

    -grumpy

    (21x3x5.25") if that helps you get a ballpark idea.
    Last edited by grumpy; 03-09-2005 at 08:50 AM. Reason: (dimensions added)

  10. #10
    RIP 2011 Zilch's Avatar
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  11. #11
    Senior Member 57BELAIRE's Avatar
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    Thumbs up Bass box

    Hi....over the years I have used a multitude of designs for my bass cabs.
    These have included small 2-12 infinite baffles to refridgerator sized folded horns....I've finally found a deisign that not only produces ample bass it is also easy to transport.

    It utilizes 4-10 in. woofers and a Foster horn (similar to an SWR Goliath III).
    It has one large (tuned) rear port. Not only does this cab accurately reproduce the low B on a five string, when placed in front of a wall the rear port uses the wall as a reflecting surface.

    If I were designing a bass reflex cab from the ground up I would most certainly incorporate a rear port.

    Russ Howard

  12. #12
    hector.murray
    Guest

    Duct

    I think I'm going to try the 15x3x4 port. I'll shift the speakers to one side and center the duct between the two speakers vertically. Need to do something quick, b'day is the 13th.
    Last edited by hector.murray; 03-09-2005 at 11:17 AM. Reason: spelling

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