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Thread: Question regarding rear ported JBL speakers

  1. #1
    Senior Member Eric M.'s Avatar
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    Question regarding rear ported JBL speakers

    I have 2 pairs of custom build cabinets from the 60's / 70's. One is the model C34 (folded horn) and the other is about the same size but has rear ports after passing by several interior mounted baffles . It is very well built and looks to be one of JBLs designs offered for customers to have built but I can not find any literature on it.

    Both cabinets are loaded with identical drivers and crossovers:
    -Rebuilt LX13 and N7000 crossovers
    -2225, 8 ohm woofers, reconed
    -2425 compression driver mids, 8 ohm, new diaphragms (C34 use potato masher horns, the other uses H91 horns and L91 lenses)
    -075 tweeters, 8 ohm, new diaphragms

    The C34 sound great. The others are lacking in that deep bass. My placement of the "not C34" speakers while testing is not ideal as I'm not able to get them in front of a wall to see how much bass that rear reflex port adds.

    My plan was to add L-Pads (fixed, not variable) to both the mids and tweeters. I figure if I can get it close I can use the 3 position switch on the LX13 and the variable L-Pad on the N700 to dial in.

    My question:
    Should I take the time to move these 100 # speakers in front of a wall to have a listen in their proper position before I start playing around with L-Pads? Will it really add that much more bass? If yes, what's the proper distance from a wall? I take it I should be looking for a flat, reflective surface, drywall or wood, no curtins or corners?


    Thanks,

    Eric M.

  2. #2
    Senior Member Eric M.'s Avatar
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    Wellll, never mind! I got off my lazy butt and moved them up against the wall and it sounded like I added a subwoofer. 6" away from the wall, the rest of the area sort of a mess of furniture but it was immediately noticeable. I was really shocked, I didn't think it would make that much of a difference. The mid could probably use a bit more attenuation (it is set on min. now) but definitely workable and the bass is now there as before it was really missing.

    Out of curiosity, has anyone experimented with various distances from wall with rear ports with varying results?

    Thanks,
    Eric M.

  3. #3
    Senior Member BMWCCA's Avatar
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    Glad it worked out.

    My only experience with rear-ported JBLs is with the L7/5/3/1 series. I usually give them 12-inches or so but they're not that picky.

    Pictures?
    ". . . as you have no doubt noticed, no one told the 4345 that it can't work correctly so it does anyway."—Greg Timbers

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    Senior Member Eric M.'s Avatar
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    I will post some pictures shortly. I'll experiment with 12" spacing too. I think I'm just being lazy since they are so heavy.

    Something else I noticed, and I really should start another thread for this, the mids in these "Non C34" are 2426 and I forgot to change the diaphragms to 8 ohms, they are still the original 16 ohms. I think the extra mids I'm hearing is the overlapping crossover curves at the 800 hz crossover point of the LX13. I had to really turn down the pot on the N7000 and that was again probably due to overlapping curves at the 7000 Hz crossover point. I know there have been many conversations (and even graphs) here about crossovers - 8 ohms, 16 ohms, can you mix, what did JBL originally intend?? I don't know if anyone has ever come up with a solid answer but based on what I know about crossover design and what I'm hearing, I think I'll swap the diaphragms before fooling around with L-pads.

    I'll add that moving these speakers against the wall has been my easiest fix ever to date!
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  5. #5
    Senior Member BMWCCA's Avatar
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    Those are both very interesting designs. More pictures, please, as you find the time.

    The "component" label on the front of the one cabinet was normally supplied in the JBL Speaker Construction Kit back in the '70s.
    ". . . as you have no doubt noticed, no one told the 4345 that it can't work correctly so it does anyway."—Greg Timbers

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    Administrator Mr. Widget's Avatar
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    Quote Originally Posted by BMWCCA View Post
    Those are both very interesting designs. More pictures, please, as you find the time.
    +1

    I would definitely look at crossover options. Using the generic JBL networks will almost never deliver the best result available from a system.


    Widget

  7. #7
    Senior Member Eric M.'s Avatar
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    Quote Originally Posted by Mr. Widget View Post
    +1

    Using the generic JBL networks will almost never deliver the best result available from a system.

    Widget
    LOL, sad, but true. I'm at the tinker, tweak, tune and listen stage. I think the first thing I want to do is get all the diaphragms to 8 ohms. That was the original plan, I'm not sure how I skipped that with these.

    More pictures of both at my site:

    http://mciaudio.com/listing.php?id=203

    http://mciaudio.com/listing.php?id=62

    The plan is to sell one pair and keep which ever one does not sell (I hope it's the #34, I'm really liking the folded horn bass). Both cabinets were given to me with other JBL purchases with the agreement I'd take them to the dump. I couldn't bring myself to do it. I listed them for free for some time with no bites. With all the "spare" time I've had during the pandemic (USA), I finally put them together. I have no idea how I'm going to show them until we all get vaccinated. I can't have people over to listen since my 83 year old mother, who has lost a lung already, is living with us. Anyways, the 2 projects were fun.

  8. #8
    Senior Member BMWCCA's Avatar
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    Quote Originally Posted by Eric M. View Post
    Ahh, so you're the Ebay armadillo guy!

    What makes you think the C34 is a home-built? No grille?
    ". . . as you have no doubt noticed, no one told the 4345 that it can't work correctly so it does anyway."—Greg Timbers

  9. #9
    Senior Member Eric M.'s Avatar
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    Quote Originally Posted by BMWCCA View Post
    Ahh, so you're the Ebay armadillo guy!

    What makes you think the C34 is a home-built? No grille?
    I'm not sure. When I got them, they did have the grill and the mounts for the mids in the horn opening but no drivers. The guy had a massive 2350 horn on a 375 just sitting on top of the cabinet. The only reason I think it was made by a shop is because I often see "JBL" burned into the face frame at the top of other C34. But like so many other things JBL ... maybe some had the emblem and some didn't?? Someone did message me on e-Bay claiming it was probably original based on the cabinet joint work.

    The armadillo - a few years back when I started selling, my daughter, then 9, wanted to put something in the picture so she could see it on-line. Needless to say, I've taken over the armadillo. I had to replace it with some scary looking, split personality, stuffed bear from one of the shows she watches. What ever happened to shows like Scooby Doo and the Brady Bunch?

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    Quote Originally Posted by Eric M. View Post
    Wellll, never mind! I got off my lazy butt and moved them up against the wall and it sounded like I added a subwoofer. 6" away from the wall, the rest of the area sort of a mess of furniture but it was immediately noticeable. I was really shocked, I didn't think it would make that much of a difference. The mid could probably use a bit more attenuation (it is set on min. now) but definitely workable and the bass is now there as before it was really missing.

    Out of curiosity, has anyone experimented with various distances from wall with rear ports with varying results?
    What you experienced was increased bass via the wall boundary effect.
    Thanks,
    Eric M.
    Hi Eric. To answer, yes. However, learning about my own rear ported rig and setup led full circle on a few points perhaps not quite as expected.
    First was Speaker Boundary Interference Response (SBIR) -which I was aware of. This site explains it very well, (simplified to 'rear wall only..) but does not get into port location.
    http://arqen.com/acoustics-101/speak...-interference/

    I began considering my rig -rear ports, the reflections from them -and what about frequencies from the drivers fronts?
    a) The ports provide only the lowest frequencies
    b) The lower frequencies radiate omni directionally (independent of port placement
    c) Higher frequencies get more directional -with less SBIR energy

    "As you move your speakers closer to the wall, the cancellation notch in your frequency response moves to higher frequencies.
    That’s great news because higher frequencies are more directional (they radiate less energy backward), and they are easier to manage using acoustic treatment."


    What you experienced was increased bass via the wall boundary's effect, with a new SBIR's dip in play.
    :>)
    Wayne

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    I Really love those backloaded horns! They look pretty good! Nice work

  12. #12
    Senior Member Eric M.'s Avatar
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    Quote Originally Posted by Kreativlos View Post
    I Really love those backloaded horns! They look pretty good! Nice work
    Thanks. This was my first experience with folded horn bass speakers. I am impressed. The efficiency boost at the low end is such that it's actually matching that of the compression driver / horn assemble and the bullet tweeter. That's always been an issue with other horn / cone JBL configurations I've had, attenuating the highs enough to match the lows.

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