Designing and building a cab. Need help!

Discussion in 'Amps and Cabs [BG]' started by cfsporn, Jan 12, 2015.

  1. cfsporn


    Aug 20, 2011
    New York City
    I want to design, engineer, spec, and then contract out the building of my own bass cab. Unfortunately, I have never done this, so I'm asking you all for help! There are a couple of these types of threads going on, and while I've learned a lot in them, I don't think they're 100% pertinent to my musical needs.

    While none of the specs are set in stone, they have been churning around in my mind for quite some time, and I would appreciate any advice given.

    I'm thinking a two way design involving a 15" or 18" (sub?)woofer transducer crossed over to a JBL E-120 that I have sitting around for the mids and highs at approximately 600Hz (whatever the 20th overtone of my low B is). I may put a tweeter in, but since I've never liked the sound of a tweeter in a bass cab, I'm saving that for v2.0. Plus, the E120 has plenty of high end for me.

    I think my first order of business should be a woofer. To that end, I have a couple of requirements.

    1) I want at least 15" of woofer. While I know Sd is far less important than Vd, with today's high xmax drivers, I see no reason not to go >= 15" for maximum volume, and since I'm running it crossed over, beaming shouldn't be a problem.

    2) I'd prefer a ceramic magnet. While I know that magnet material has little to no bearing on the tone, I'm not a fan of the ecological ramifications of neodymium mining/extraction. Plus, I'm a young and relatively strong 24 year old man. Not going to pass judgement on people who use Neo speakers, just trying to lessen the white liberal guilt.

    3) 4 ohms. I have not studied crossover design yet, so this is the one I'm so far least sure of as my mid is an 8 ohm transducer. However, if I can pair a 4 ohm woofer with an 8 ohm mid without any deleterious effects, then I think I'd like to go that route.

    After perusing USSpeaker, this driver seems to tick a lot of boxes. Has anyone used one of these before? It has the Vd, the ceramic magnet, and has the Greenboy seal of approval.

    I have a lot of questions about cabinet and crossover design among many other things, but this post is getting rambly enough, and I don't want to go on before I choose my drivers.

    Thanks again for any and all help!
  2. DukeLeJeune

    DukeLeJeune rational romantic mystic cynical idealist Gold Supporting Member Commercial User

    Nov 24, 2008
    Princeton, Texas
    Owner & designer, AudioKinesis; Auth. mfg, Big E (Home Audio only)
    Dom Oatman at Low Dow Sound specializes in custom cabs. I don't see any major hurdles to pairing a good 4-ohm woofer with your 8-ohm mid, but the Kilomax you mentioned is an 8-ohm woofer.

    Unless you really need subwoofer-class bottom end extension, you might consider the Sigma. The Sigma will give you more crossover options - imo you want to cross over no lower than 800 Hz or so, for the sake of power handling (cross over too low and it will be hard on the E-120).
    Last edited: Jan 12, 2015
    Arjank likes this.
  3. cfsporn


    Aug 20, 2011
    New York City
    Thank you for your reply.

    LDS has crossed my mind for the building portion. Part of the fun would be building my own crossover though. Since I'm resuming my long dormant college education in Electrical Engineering, I'd imagine it would be rewarding to have a project that I can use for recreation.

    While I don't need subwoofer deep lows, they would be nice to have available and they are readily tamable by an external HPF.

    I just looked at the Sigma and it seems better on just about all fronts! Thank you very much Duke!

    Also, I've read that you have an open-format semi-open back midrange enclosure design. Can you give me a some more info on it?
  4. DukeLeJeune

    DukeLeJeune rational romantic mystic cynical idealist Gold Supporting Member Commercial User

    Nov 24, 2008
    Princeton, Texas
    Owner & designer, AudioKinesis; Auth. mfg, Big E (Home Audio only)
    I tip my virtual hat to you on resuming your pursuit of a degree in EE!

    As for crossover design... well, I tell people that it's easy to design a crossover, but it's hard to design a really good one.

    At a minimum, use a good guide like The Loudspeaker Design Cookbook. Take as much into account as you can, and pay attention to power handling and your impedance curve. It's easier than you'd think to end up with a nasty impedance dip if you're using anything steeper than first order slopes.

    If you want to make your own measurements and really get into it, let me know and I'll do a brief overview of what you need.

    Yes! The open-format mid chamber works when you can cross over high enough that you don't need the enclosure to protect the midrange driver from over-excursion. You can think of it as a dipole, but with the openings for the backwave on the sides and top instead of directly behind the cone. You want to keep the mid-chamber's cross-section (as seen from the side) from being a square or close to it, so in practice this may mean making the chamber as shallow as possible and a little bit taller than needed. Allow space to install corner protectors if you're going to use them. You can eyeball pictures of my open-format cabs for a starting point for your designs. On mine, the chamber runs the width of the cab so it's not the most efficient use of space, but it does have some upsides: You can hear the overtones better when right on top of the cab on a tight stage; a bit better dispersion to the sides as well; a more "open" sound (surprise surprise) even though it doesn't measure any better (and may actually measure worse); and better cooling because the hot magnet isn't heating up a tiny closed box.
  5. cfsporn


    Aug 20, 2011
    New York City
    Thank you for the hat tip! I'd be lying if I said that some of the things I have read on this forum from you and others inspired me to resume.

    I figured that the crossover would be the hard part. While it is a science, it is a science pertaining to art which makes it much more difficult. Luckily, I only have my own tonal goals to worry about!

    I think I actually used to own a copy of that book! I asked for it as a birthday present for several years in a row and got it for my 15th. In the past 9 years, it got lost/thrown out/destroyed, so I may have to get a new copy.

    Do you have any pictures or diagrams of your design?
  6. Passinwind

    Passinwind I know nothing. Commercial User

    Dec 3, 2003
    Columbia River Gorge, WA.
    Owner/Designer &Toaster Tech Passinwind Electronics
  7. DukeLeJeune

    DukeLeJeune rational romantic mystic cynical idealist Gold Supporting Member Commercial User

    Nov 24, 2008
    Princeton, Texas
    Owner & designer, AudioKinesis; Auth. mfg, Big E (Home Audio only)
    No diagrams, but here are some pics. This is my first cab to use the open-format mid chamber:


    You can see the openings on the top and sides. The chamber itself is just a shallow rectangle (barely deep enough for the magnets), the same width as the enclosure, and tall enough for both mids to just barely fit. The front baffle is two layers of 1/2" plywood, and the little midrange cones are "recessed" a half-inch (one board thickness).

    This next one is a prototype that ended up not going into production. What you see in front is a 12" guitar speaker, and behind it is a 15" woofer:


    The format worked fine with the right filtering; it was the concept behind the driver choice that was a "learning experience".

    Here's a smaller variation on the theme presented in the first photo (sorry I don't have a photo of just one all by itself):


    This is probably the simplest implementation: One cone mid, nothing fancy.

    And speaking of fancy, here we have two cross-firing mids:


    The gentle cross-firing gives a wider and more uniform pattern than we'd get with the vertical stack seen in the first photo, along with a bit better vertical dispersion so you can hear your overtones better.

    I don't use any damping material in the midrange chambers.

    An impedance sweep will reveal that the open-format mid chambers have a Helmholtz resonance of several hundred Hertz. I use damped second-order or steeper highpass filter topologies, which are relatively immune to impedance peaks compared with a simple first-order filter.

    Feel free to copy or modify any or all as you see fit.
    Last edited: Jan 14, 2015