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Thread: Acoustic (Air) Suspension

  1. #1
    RIP 2011 Zilch's Avatar
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    Acoustic (Air) Suspension

    It's been a very long time since I've worked with or listened to small closed-box speakers, a design approach pioneered by "East Coast Sound" proponents of the '60's - AR, Advent, KLH, etc.. Building DIY Minis recently led me to trying that approach with JBL drivers, with good result.

    A little research in Dickason's Loudspeaker Design Cookbook serves as a refresher on the essentials:

    1) A good starting DIY project - easy to build and easy to succeed; the alignments are not critical.

    2) Excellent extended bass, as the rolloff is just 12 dB/octave.


    3) Driver parameters:
    EBP = Fs/Qes < 50
    Qts > .3
    Low Fs
    High Xmax
    High MMS
    High Vas



    4) Box size - for true air suspension, < 1/3 Vas.

    5) Easy to measure (test) and understand.

    There's not many JBL drivers that qualify, but BB6P EBP meter identifies these potential candidates:

    116A, H, H-1
    123A
    125A
    127A, H
    2145A
    2213

    Another group, more intermediate between closed and vented designs includes:

    2245
    508G
    LE10A
    LE8T-8
    Sub1500


    [Lists subject to change without notice.... ]


    I have been using a $15 yard sale pair of AR4x as test boxes (18.45 l, 0.652 cuft gross internal volume,) but recently discovered that the OASR "Dr. Seuss" horn would just fit. Ut-oh:

    3" hole saw and small Sawzall make the "clover-leaf" cutout for the OASR horn with 2407H neodymium tweeter.

    708G-1 8" woofer fits perfectly. I'm trying others, too.

    Control 128W crossover (2.5 kHz) plus notch filter on the woofer and L-pad + compensation cap on the tweeter gives immediately satisfying results, bottom.

    Off to critical listening tests, soon....
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    Zilch,

    be so kind and explain the difference between the orange and green graphs.
    ___________
    Peter

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    Glad you feel the 2245 (sorta) qualifies Zilch, as I'm playing with some in A-S mode.

    Another characteristic;

    Excursion at resonance is minimized. Therefore, unlike BR designs, you don't have to quit at Fs (yay). Naturally, there's a catch, small boxes raise the system resonance and introduce a parameter not used in BR design, Fb. Suddenly, your 20Hz Fs driver has an effective resonance of 40Hz or more (boo). No problem, just apply 12dB-sloped EQ, and take it back down, way down (yay), as long as you can supply enough juice and your driver can take it (boo). You also no longer have a port to integrate with its' chuffing, one-note bass (yay). Critical damping via stuffing density can be tricky to achieve, but when you get it, it's smooth as a baby's butt.

    My favorite A-S design was Roger Russell's (McIntosh) ML SYSTEM. I emphasized system, because it needed the active EQ unit to work as intended, something many owners never understood, so they didn't buy the EQ with the speakers (big mistake). The EQ gave 3, 6, or 12dB boost at 20Hz, which allowed one to compensate for placement and room gain, and run flat to 20Hz without blinking.

    Great thread topic, and something new to DIY. I know you had a big grin taking the sawzall to an AR box.

    And don't worry, we won't make you move to Boston.

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    Zilch - that's got to be the ultimate "bookshelf" system.

    John

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    Quote Originally Posted by Hoerninger
    Zilch, be so kind and explain the difference between the orange and green graphs.
    Hi, Peter.

    The orange is the nearfield response of the woofer at 1" or less, running through the filter, showing what's happening at the very lowest frequencies, particularly the rolloff. I don't even bother turning off the tweeter for that measurement; you can see it still playing down 30 dB there. You can also see I've notched down ~400 - 1 kHz in the woofer response (perhaps too much for this one, actually,) to get rid of the hump in response common to several 8" drivers in this alignment.

    The green is the full system response at ~1 M. I've shown both RTA and non-windowed MLS, separately. There's boundary and room response effects inculded in those, of course. A purist would stitch groundplane or nearfield LF and windowed MLS HF response together to show a composite pseudo-anechoic response curve instead, but these separate curves pretty much show what's going on.

    Quote Originally Posted by moldyoldy
    Great thread topic, and something new to DIY. I know you had a big grin taking the sawzall to an AR box.
    Heh, heh. I jus' KNEW there were others out there, tho some members might desire I show less of the gore.

    I sure wish Giskard were here to participate. He likely knows the full skinny on this inside and out.

    A/S bass certainly has a different character from that of the vented systems we're used to, as much different as bass horns, I would imagine, though in an other way. Transients have something to do with it, and the tonality seems more "natural." Anyone running Sub1500 in a small closed box, for example, knows what it's like.

    I'm thinking there's a way to match the inherent power compression and system "Q" to equal loudness contours for an expanded SPL range without EQ. It's certainly easy to get sweet-sounding bass at moderate levels in a modest-size package with this.

    Quote Originally Posted by johnaec
    Zilch - that's got to be the ultimate "bookshelf" system.
    Well, there's precedents.

    In addition to the entire "other" industry, JBL themselves makes a couple of products configured this way:

    http://www.jblpro.com/pub/cspels/CSP18.pdf

    http://www.jblpro.com/ae/am4200_95.html

    Those're both vented, tho.

    Performance Series is closed box.

    I'm starting to consider more what the cone material is and how well it plays the mids, too. The woofer does the lion's share of the musical content work in these....

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    Cool idea Zilch. Even though the idea of a 2235H in a sealed box may not seem to make sense, I tried it with a 5cu.ft (4505 box) with medium fill and with EQ it was pretty darned good, somewhat along the lines of the old McIntosh ML/MQ systems from the 70's, someone mentioned above.

    The MAC's were damned good in their day if correctly set up.

    Even without EQ, the 2235H was impressive.

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    My favorite box of all time is the sealed bandpass. I have built quite a few of them and always enjoyed the performance of any driver in this configuration. The transient response of a sealed system will usually be superior and accurate than a vented box because of the gradual rolloff and better phase characteristics. Amplifiers also prefer the impedance curve of a sealed system over a vented one.

    A/S bass certainly has a different character from that of the vented systems we're used to, as much different as bass horns, I would imagine, though in an other way. Transients have something to do with it, and the tonality seems more "natural." Anyone running Sub1500 in a small closed box, for example, knows what it's like.


    The difference is in the low end rolloff. As a transient signal, such as a bass drum, decays it's fundamentals are very low. The vented box cuts them off while the sealed box responds to the note. The vent in a ported system is a strict component in how it operates while the air in a sealed box offers variable damping in both frequency and amplitude. With big enough drivers and large boxes sealed systems are truly spectacular. The only drawbacks are lower efficiency ( -3 db less) and greater cone movement ( higher distortion compared to a vented box operating at the port frequency) although Qtc has a LOT to do with the cone movement, distortion and ringing performance. I prefer .707, others like .5 or even 1.2

    I have built more sealed systems than vented, although it was with non-jbl drivers that I worked with. JBL has always been a ported box company it seems.

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    How about the "Plenty available from L100 partouts" 123A-1?

    How high do they play well? Clearly not 2 kHz, e.g., in L88, but if 1 kHz, that would open up a bunch of good DIY options for HF, maybe....

    Edit: L100 woofer only 1" nearfield, port closed. Fc = 56.19 Hz, Qtc (as Qts) 1.25. Still a boomer, just a little less so:

    [1 kHz looks "workable," at least insofar as it BEING there is concerned.... ]
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    Talking L100 Fans:

    Take NO prisoners!

    Two-way @ 1.2 kHz.

    [Windowed HF, cyan, bottom.]
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    I'm not about to take the SawZall to this L100, of course, but since it's on the test stand, here's a look at performance of this one in comparison to the experimental two-way above.

    I'm providing RTA and unwindowed MLS results for the same 4 conditions:

    1) Presence and Brilliance controls at "0", red.

    2) My best attempt at dialing it flat, LE5-2 (Presence) virtually off, and LE20-1 (Brilliance) at -2, green.

    3) Same, but LE20-1 full on, to see VHF response, orange.

    4) Two-way with PT-F95 waveguide from above, Presence and Brilliance full off, external crossover, cyan.

    The "problems" with this L100, if I may call them that, are:

    A) Boomy bass.

    B) Brilliance and Presence both operate in the HF ~10 kHz. It's not possible to adjust mids and highs independently to achieve good balance.

    C) LE20-1 dies at ~12 kHz.

    D) Sounds terrible - can't mitigate the "forward" rising midrange successfully.

    Anyone else have L100s they can measure similarly? It'd be interesting to see if it's just mine behaves like this....
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    From the start, this L88 is certainly better balanced (orchid) than my L100, above. If this is chacteristic of the genre, I think I'd forgo the "plus" process and just do the factory recommended replacement of the tweeter with 035TiA to fix the VHF.

    http://manuals.harman.com/JBL/HOM/Te...0Nova%20ts.pdf

    However, I do also hear the midrange muddiness rotormotor speaks of:

    http://audioheritage.org/vbulletin/s...ad.php?t=12077

    Upgrading the HF to a mylar-diaphragm re-entrant annular-ring compression driver on a progressive transition waveguide with a lower (~1.2 kHz) crossover frequency improves the situation substantially (cyan). Getting a low-pass filter on the woofer is part of this, also (nearfield LF, orange). Stock, it runs full range.

    Still a bit "boxy" sounding, I'll adjust the damping with more fill.

    Note port closed for acoustic suspension bass using Zilch's proprietary box tuning device.

    Fc = 60.94 Hz, Qtc (as Qts) = 1.24
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    Quote Originally Posted by Zilch
    Note port closed for acoustic suspension bass using Zilch's proprietary box tuning device.
    Great tool.
    ___________
    Peter

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    Re: #10, above:

    In fairness to L100, per Mr. Widget:

    http://audioheritage.org/vbulletin/s...04&postcount=3

    I measured on tweeter axis. See his observations with respect to this there.

    So, "Blown Away" orientation is correct, then.

    [Or, lying in bed.... ]

  14. #14
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    An interesting case, L88.

    I was puzzled by the bass response measurements I was getting, closed box versus ported.

    BB6P tells the story (below).

    Ported L88 LF rolloff is already ~12 dB/octave (red) in this alignment.

    Closing the port apparently gets rid of the 85 Hz bump very nicely, but only slightly extends the bass response. Typical fill (grey) vs. heavy fill (black).

    Group delay is substantially improved with port closed (bottom).

    [Where's GISKARD, dammit! ]
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    121a's

    I use 2 121a's in separate sealed enclosures as stereo subs. I absolutely love the way they sound.

    Allen

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