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Sub question for BGavin

Discussion in 'Amps and Cabs [BG]' started by ESP-LTD, Jan 10, 2003.

  1. ESP-LTD


    Sep 9, 2001
    I always thought that cloth accordian surrounds were common for MI drivers for durability reasons to survive transients. In your Rockford subs, have you had any problems with holding surrounds together?

    How did you decide on the crossover point?

    I was considering a small/light system with car audio subs under a B-102 (or a pair of B-102's) and there are plenty of choices in 10"/12" that would be easy to carry. Have you tried this?
  2. Accordian surrounds are for lower excursion drivers. (Nearly) all the high excursion drivers use the rolled surrounds. Also, an accordian surround is indicative of a higher efficiency driver that is more than likely designed for use in a vented box.

    My RFR-2215 subs have a long Xmax of 0.53", and the Xmech maximum travel is unspecified. By design, these subs are engineered for very long cone excursions that are typical in a sealed box at low frequencies. The surrounds are half-roll Santoprene which is fine for subs, but less than ideal for high frequencies.

    A crossover point is dictated in multiple ways. First, by the upper response limit of the subwoofer. If the driver is all done at 100 Hz, it is futile crossing over at 325 Hz.

    The other factor is the frequency where the driver becomes directional and begins "beaming". Worst-case is the 18" driver, which is fully spherical (omni-directional) at 221 Hz, and severely beaming at 884 Hz and higher. This is a function of piston diameter and wave length.

    I have a real time analyzer that shows me the rolloff of the sub as the frequency goes up. Set the active crossover so the sub gets the full spectrum and note where it rolls off. That point (or lower) is the crossover point. Do the same for the Highs portion. After the crossover point is identified, the Lows and Highs are volume balanced with the analyzer, to produce the same loudness. This is the beauty of active crossovers and bi-amping.

    The new Magnum 18LF I'm testing has the accordian surround. It also has a much higher sensitivity, a tad more than an Acme B4. It does not go down nearly as low as the Rockfords. The Magnum is a perfect match for a single B102, as the sensitivity of both are identical.

    A B102 won't take the full 200 watts of rated input power below 160 Hz due to thermal/displacement limitations. In SBB4 tuning, 100 Hz is the lowest input point at just 72 watts. This is where the cone excursion is highest. A D2 sealed box doesn't improve things for this driver either, so I'd stick with the SBB4 vented arrangement. What this means is the B102 must cross over higher up.

    There are many 10" to 12" subs available, but understand they are just awful for power consumption and low efficiency. Do not expect to match a single sub to a B102. The B102 has an effective SPL of 96.4, where most subs are in the mid to high 80's. If you play quiet gigs, you can pump 200 watts into a sub, and 20 watts into the B102 and get them to balance. But.. 200 watts into the B102 will require 2000 watts into multiple subs. So much for small and light weight.

    Advice: don't get into subwoofers unless you have a genuine requirement to go down that low. If low E is the bottom limit, you have lots of choices. If you need 23 Hz, you're stuck with subs and their inherent limitations.
  3. ESP-LTD


    Sep 9, 2001
    Thank you for the very complete reply; as always, I learned something useful. The alignment tips on the B102 are very helpful; I could have fried one at 100hz pretty easily.

    I love your idea for an 18/10 arrangement, but I am too old/fat/slow to be packing 18 cabs around... I will continue exploring something in smaller/lighter pieces.
  4. I also qualify for old/fat/slow, and hauling around my regular two 1x15 subs is a chore. My current pair of Rockfords require more cabinet than the Magnum 18LF does, so the 18 will be a size reduction for me. My current cabs are 7.9 cubic feet external volume at 21 x 21 x 31 inches. The 31 inch height makes it possible to lever the cab over the tailgate of my pickup without assistance. A shorter cab will require me to pick it up to get it into the truck.

    Output-wise, it takes (4) Rockfords at 1 watt each to produce the same small signal output as (1) Magnum at 1w/1m at 100 Hz.

    I'm running a DBX 120A sub-synth in my rack, and this extends down to 26 Hz, which makes the Magnum all but useless this far down. I already have the amp power, so I'll probably rebuild the Rockfords into smaller vented boxes of about 3.2 cubic feet, and stuff them to increase the apparent volume.
  5. ESP-LTD


    Sep 9, 2001
    I have a hard time spending $265 for a driver although I could buy 8 $30 drivers without blinking; I guess it's an intellectual thing I have to get past.

    So, as I understand it, you will have a 4' ID box tuned to 32hz or so for a 5 string, and it will weigh 80-100#. On top you will have a single B102 crossed over at 200hz.

    What do you figure the max power to be and SPL?

    My only experience with 18's is Peavey 18 BW subs (about 6-8' unknown tuning) that were pretty mushy to me, but it could have been that they were only working to 120 and I couldn't hear them well.

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