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Porting a 1x15 cab...advice?

Discussion in 'Amps and Cabs [BG]' started by lbwdog, Feb 11, 2006.


  1. I have a small cab which is actually the bottom part of an older Acoustic combo amp. It is 2 cu.ft. internal and presently loaded with JBL K-140. It sounds okay but obviously a little shy of deep bass. I'm thinking of porting it in the rear as there is no place on the front (it's just large enough to accomadate the 15" spkr.) My thoughts are to buy a ready made port tube from one of the supply houses but curious what size to get and what results to expect.
    I'm not expecting it to be a large venue cab but something portable for small clubs and such. Any advice??...Bill??
     
  2. I did the same thing to my Polytone. It's just my practice amp so it was more of an experiment than anything.

    This was way before I had the Internet so I pick up a book on designing and building speakers cabs.. Basically you will have to measure the inside dimensions of the cab and figure out the volume. In this book there was a conversion chart to figure out what diameter and how long the port had to be in order to tune it to the cab.I did mine with no money, I used a thick cardboard tube. I believe you can also use PVC tubing found in plumbing.

    It' seemed to work for me.

    One thing to keep in mind is that some speakers were designed to work in ported cabs and other are designed to work in sealed cabs so you may not have the same results. My Polytone remains my practice amp so I never push the speaker, but it does add some bottom when the rear port is aimed in the corner of the room.
     
  3. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    Download WinISD at linearteam.org and you can figure it out, but you'll need the K140 specs. Do the Beta version, the JBL is in the database, it's not in the Alpha version.
     
  4. The K140 is a high efficiency JBL and doesn't have much bottom, by design. It is designed to make a lot of noise from a small input power.

    That said, for 2.0 cubic feet (SBB4) the tuning frequency is 30 Hz. You will have an F3 around 90 Hz, but this is par for JBL drivers (golf pun intended).

    You can get a little bit more under the curve by putting the same driver into QB3 which is 1.51 cubic feet (net) tuned at 55 Hz for an F3 of 72 Hz.

    JBL recommends an EBS alignment, which is 4.0 cubic feet tuned at 40 Hz.
     
  5. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    Just to put this into some perspective, I modeled your speaker in it's existing sealed 2cu.ft cab, and you're currently -3dB at 108Hz. So it seems porting can give you that little extra bottom end.

    Sticking to 2Cu.ft, tuning to 55Hz gives you -3dB at 63Hz.

    Instead of buying a port, consider modeling the port diameter so that the thickness of the timber becomes the port length. In other words, you might be able to get away with just cutting a hole, provided it's the right diameter. Say for example the timber is 1 inch thick. All you'd have to do is cut a hole that's 3.08in in diameter, and the cab will be tuned to 55Hz.
     
  6. i just ran it through WIN ISD real quick, if you add a 6"x6.5" round port to it (check www.partsexpress.com for it) it will give you more responce in the lower range you didnt have before. you'll lose a little bit from 46htz on down, but everything above 47htz will get a little more boost. so its your call if you want to port it still
     
  7. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    At 2Cu.ft, that equates to a tuning (Fb) of 60Hz. I agree it creates a nice looking frequency response graph but it introduces the problem of speaker excursion. It's a characteristic of vented cabs that they don't behave too well when you feed them frequencies below Fb. Even at 55Hz, we're probably higher than we should be given that low E has a fundamental of 41Hz. I have one commercially made cab that's tuned to 55Hz (Nemisis 410) and it requires caution when playin the low notes with any amount of gusto. I wouldn't tune the cab any higher than that for bass guitar.
     
  8. so basically, as his cab sits now, hes fine, but when introducing a port it allows for unsafe speaker excusrsion?

    this may be a problem i overlooked with the eminenece speaker im using for my 1x15 im building...

    if i understand where your going, you should tune out the cab to not really exceed the Fs given in the theile small of the speaker? the given Fs on my speaker is 40htz, and my cab is designed to be tuned to 37.00htz for optimum output. sound good? sorry for the hijacking too there
     
  9. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    That's the downside of vented cabs, but provided you don't neglect to address it in the design phase, there shouldn't ba a problem.

    If the cab is tuned to 37Hz, you should be able to sit on the low E (41Hz) all day with no problem. You should even be able to achieve acceptable excursion at low B (31.5Hz). But before I say any more, I have 2 questions - Which Eminence 15 is it, and where is your DIY thread? I must have missed it.
     
  10. DIY thread is titled " can someone critique my design" and its an eminence Legend b15, everything is in the thread, my ISD stuff and all the theile small stuff
     
  11. Thanks guys, for your input. Pete, I'm fine with just cutting a hole and not using a port tube. The rear wall of the cab is 3/4 " not 1". I'm thinking about a square port instead of round since I don't have a hole saw but do have jig saw. I would be able to do a cleaner job using right angles. So, if it's not too much trouble, what dimensions would you reccomend?....EW
     
  12. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    If you're more comfortable cutting a square, then that's what we'll use.

    With a thickness of 3/4", make the square hole 2.5" x 2.5". In your 2cu.ft cab, that equates to the 55Hz tuning frequency I mentioned earlier.

    Just remember not to block the port. This means you'll have to be careful where you place your cab on stage.

    And if you try it and decide you don't like it, we can always plug up the hole and go back to sealed. It may not loook pretty so I'm glad you're cutting into the back where it's not visible.