DIY mini 1x12 report

Discussion in 'Amps, Mics & Pickups [DB]' started by fdeck, Oct 4, 2006.

  1. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    It's at my website. Click on my sig.

    A while back I embarked on designing a bass cab, and found that there was very little guidance available for the DIY'er on what I call the "small box" design style. Yet small bass boxes are widespread and commercially successful. So after arriving at a design that I am happy with, I decided to go a bit overboard on describing the technical background and design method. Some existing materials at my site provide background to the design report, and I will upload the design spreadsheet soon.

    Happy reading. Questions and criticism are mandatory.
  2. OK I'll bite with a few questions - Hope I am coherent - I have not slept yet

    1. Have you devised a methodology to check how well your general calculatons about the lower end response match the cab you have built.? Hard to do in a real room, a good enough reference mic is fairly cheap at least.... I'll also be one to contend that a noise spectrum probably has as much or more real use than a swept sine for this purpose. Both have their place.

    2. Without a way to measure actual mechanical displacement versus time, anything to do with response in the time domain - step or impulse response - will be somewhat speculative. Some lab somewhere has this gear but I bet they are designing drivers and not cabs. I think I am in agreement - unless a cab designer goes very far astray, audible transient response issues will not crop up within the intended range of operating frequencies as long as the drivers are being applied to their intended use.

    3. Have you A/B tested your single driver cab versus a cab with a tweeter? If so what were your subjective impressions? Have you measured the top end output from the Deltalite?

    4. Are you willing to share dimensional/construction details of your current prototype cab with any of us potential home builders either publically or privately?

    I gotta get some sleep...
  3. thanks for the report. i particularly enjoyed the qualitative write up at the end, which i think even those not building their own speakers might enjoy. there are some nice things to think about when looking at the specs of potential amp choices (of course, your ears are ultimately your guide).

    one line i'm fascinated by: "You can see it’s a toss up between the higher power of the BL600 at 180 W and the effective cone area of two 12" speakers driven at 75 W apiece."

    what about the school of thought that amp power is needed to "control the cone" on the low end -- that is, to make sure low end response is fast enough by being able to drive the coil with enough power?

    i routinely bring two powered speakers (pub 280's) to loud gigs. would i be fine with just an extension cab? my concern with just an extension cab is that it might be a little muddy.
  4. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    I am glad my article is good bedtime reading. ;)

    1. I don't have an anechoic chamber, so I have to be very careful about what conclusions I draw from my data. At the same time, I assume that the basic electromechanical model is accurate. This assumption lets me draw broader conclusions from measured data, but of course leaves me incapable of actually testing the validity of the model.

    I measure the response curve of the driver in near-field using a cheap electret mic element, and then compute the overall response curve of the ported system. That gives me a relative curve, to which I apply an absolute sensitivity scale by comparing with the model curve. My program generates a mixture of frequencies, then takes a FFT and reports the amplitude at those frequencies.

    2. It is true that my step response curve is speculative, but again, I am treating the model as a given and drawing some conclusions directly from it without testing.

    3. I first plan to try a cone midrange... any day now I will get around to ordering it. The top end of the Deltalite seems to agree qualitatively with the datasheet, with the proviso that I don't entirely trust a near-field measurement above a few hundred Hz.

    4. Yes...

    Internal dimensions = 14 x 16 x 9.75 inches
    External dimensions = 15 x 17 x 12
    Port = 8.375 x 1.375 x 10 deep

    If you draw a cross section, you note that the shelf port goes almost all the way to the back. A challenge with small boxes is finding space for a port of large enough frontal area to avoid "chuffing." I expected to treat this as a port going around a corner, but misjudged the end effect a bit, so the tuning is around 37 Hz instead of whatever I had originally hoped for. The lesson is: Either make the port adjustable, or test it carefully before gluing everything together. The other thing to notice from the dimensions is that I am condemned to rebuilding the box anyway if I decide that I want the midrange.

    Plywood is garden variety birch veneered stuff from Home Depot, 1/2". Judging from the edges, the material seems reasonably void free. I just butt-jointed it with wood glue, then glued in cleats around all the joints to reinforce them. Then I glued in some rather minimal bracing. I am not too worried about bracing since it is such a small box.

    The whole thing is slathered in a few coats of poly. Note that the 2512-ii driver came with a rear gasket already installed, so I don't have to install gasketing. Also, I know that I have to get some padding onto the inner walls of the cab... soon.
  5. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    What controls the cone at the low end is a high damping factor (low output impedance) but practically any contemporary amp is satisfactory in this regard. Once the output impedance of the amp is down into the couple hundred milli-Ohm ballpark, then the limit to controlling the cone is the DC resistance of the coil, which is fixed by the driver design.

    But in terms of your toss-up situation, there might be an advantage to keeping the pair of powered speakers. Adding an extension means that you are running the one amp at a much higher output power. Granted, it is supposed to be capable of it, but running the two amps will be easier on both.
  6. fdeck,
    Thanks for sharing the info
  7. gmarcus

    gmarcus Supporting Member

    Apr 4, 2003
    Hi Fdeck are you still around? I have an Aguilar GS112 that I could never really get a good sound out of in a live setting. I have heard that some people replace the woofer with 1 Eminence Deltalite 2512-II speaker with good results. The internal volume of this box is ~2.3 cuft. After reading your excellent small 12" speaker paper, I started to wonder if it might be a good idea to put two Deltalites into this box. one direct replacement and one in the side of the box.

    Do you still like the sound of your mini 12? how has it held up in live playing situations. I play a P-bass in a roots rock type band. Medium volume

    What should the vent lengths be with 2 deltalites in the GS112? I think the vents are 3" diameter and there are two of them. What are your thoughts on running this as an acoustic suspension cabinet and eqing a little bit of bass back in.

    Thanks for any info you can provide

  8. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    Still alive and kicking :)

    I'm still happy with my speaker. Originally, I built it for my "big rig" to be used with electric bass and louder gigs. What's happened since then is that I've done very little electric bass work, but I like the tone of the speaker and am satisfied with its portability.

    Today, I'd consider comparing the 2512-ii with the newer 3012-HO driver. The 3012-HO wasn't available when I built my speaker. It has a longer excursion length, and could probably handle more power in a bigger box. It might be worth seeing if the 3012-HO will work in the Aguilar box, as that would be a simpler and cheaper upgrade than cramming two 12's in there.

    About the ports, their tuning frequency is independent of the number and type of drivers, so you could probably leave them alone. The only caveat is that they aren't huge, so you might get some noticeable "chuffing" if you switch to a driver capable of delivering more output.
  9. Greg Clinkingbeard

    Greg Clinkingbeard Commercial User

    Apr 4, 2005
    Kansas City area
    Black Dog Bass Works
    Francis, although I don't have your knowledge of speaker box design the newer driver doesn't look like an improvement for the little box. Aside from the greater excursion and power handling I don't see it.
    I wish they would make a 4 ohm version of the 2512.
  10. gmarcus

    gmarcus Supporting Member

    Apr 4, 2003
    Thanks fdeck
  11. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    Let me look into it a bit closer. Like you say, the new driver probably won't outperform the old in my tiny box, but the additional cubic foot of the Aggie might be a different story.
  12. How much benefit is the port? I run a Bag End 12" in a DIY small box with no port and it's great for double bass. I wondered about what benefit if any I would get by porting it.
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