From: wkb@cbnews.cb.att.com (wm.keith.brummett)
Subject: Re: Amplified SubWoofer
Organization: AT&T
Distribution: usa
Date: Wed, 18 Nov 1992 03:24:46 GMT


    In article <TONYB.92Nov17165451@oberon.juliet.ll.mit.edu>,
    tonyb@juliet.ll.mit.edu ( Tony Berke ) writes:
    > 
    > Isn't the AC Sub-1 the "synthesized bandpass" (Floor-firing, funky
    > crossover) thingy?
    
    Yup.  AC has all sorts of marketing appellations scattered throughout
    the Sub-1's literature.  From what I gather, the "Synthesized Bandpass"
    (tm) stuff is a combination of the crossover, and six of what AC calls
    "aperiodic vents" (I call them 3/8" holes) in the cabinet.  And, BTW,
    they don't *have* to be floor-firing, though mine are for convenience.

    > I thought the passive crossover had more to do than just low-pass
    > filtering the output.

    The Sub-1 crossover has a low-pass section for the sub driver, a high-
    pass section for a satellite ("main") speaker, and a terminal that can
    be strapped to the input terminal for "upper bass adjustment".  The
    cabinet plans I got show component placements on the crossover board,
    but no values are given.  It looks like the high-pass is just four caps
    in parallel, giving a 6dB/oct slope.  The low-pass section is a 12dB/oct
    setup with three coils wired in series followed by four more parallel
    caps.  The adjustment strap appears to merely short out the first coil.
    This apparently raises the cutoff frequency of the low-pass section,
    causing an overlap with the satellite's response and, thus, giving a
    bulge in the upper bass response of the total system.  Must be there
    for the rock-n-roll folks.

    Now an interesting thing is that AC cautions us in the plans to *not*
    defeat the passive crossover in favor of an active crossover box.  Gary
    Galo, a contributing editor of SPEAKER BUILDER, repeats this caution in
    a article he wrote on the Sub-1's.  But nowhere does anyone explain why
    this might be a bad idea.  A friend called AC and asked this -- the
    answer was more or less a meaningless mumble.  So, I assume the reason
    is that AC wouldn't make money on the very expensive passives otherwise.
    They do mention something somewhere about the roll-off going from 2nd
    order to 4th order at some point, but I've got to believe that it's more
    to do with the driver's mechanical response, or maybe the 3/8" holes,
    than with some sort of crossover magic.  I mean, the crossover just
    doesn't look complex enough to be doing anything unusual.
    
    > Have you been able to measure the results?  I've read some difficult-
    > to-believe figures for the in-room flatness of the SB-1.  Do yours
    > perform this well?

    Well, the subs do sound pretty good.  But, you gotta' read the AC
    literature to get the whole story.  They claim: "The average domestic
    living room has 8-10dB of boost at 20Hz relative to 100Hz ... The Sub-1
    has been designed to take these room effects into account".  What this
    probably means is that they start rolling off below 30Hz, like a lot of
    other good subs.  The only questionable practice involved is in that AC
    seems to assume this room response when they claim 20-90Hz, +/-3dB.

    I have measured my subs using the Radio Shack SPL meter (and we all know
    what a precision instrument that is :-) ) and the Stereophile test CDs.
    What I got was (with a 90dB nominal level):

	100 Hz   +2dB          40 Hz    0dB
	 80 Hz  +10db          32 Hz   +1dB
	 63 Hz   -1dB          25 Hz   -6dB
	 50 Hz   +1dB          20 Hz  -13dB

    The big hump at 80Hz is because my family room is 13 feet wide.  I gotta
    do something about that.  But you'll notice that the subs are pretty
    flat down to 30Hz or so.  And, if you add AC's supposed 10dB of boost,
    guess what? -- you get -3dB at 20Hz.  Now, this of course means that
    either my room doesn't have this boost, or that it does and the subs are
    really down -23dB at 20Hz.  I dunno.  Oh wait, I know what it is!  AC
    says "living room", but I've got them in the *family* room!  Maybe if I
    moved them into ... no ... sorry. :-)  The subs are sitting a couple of
    feet from corners, and the long dimension of the room is effectively 40
    feet or more.

    Oh yeah, when a buddy gives me back the SPL meter, I'm going to measure
    the boxes while plugging up the aperiodic vents one at a time, just for
    grins.  If there's anything un-subtle about them, I might drill a few
    more.  I'll let the world know if anything exciting happens.

    > This sounds pretty promising...

    Yeah, it ain't a bad way to go.  Of course, things take on much greater
    meaning for me when I get to play around with the guts.  I'd always
    choose a DIY job that's, say, a few dB's off of perfection over a
    slightly better piece that you just buy and set on the floor.

    If you pursue this any farther, and have any more questions, let me
    know.  I'd be glad to help where I can.

    -- Keith

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