Subwoofer path length

Discussion in 'Archived Threads 2001-2004' started by Dave Milne, Sep 27, 2001.

  1. Dave Milne

    Dave Milne Supporting Actor

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    I'm building a sub for our new dedicated HT using JBL's 2245H. The vented, 6th-order alignment yields F3 of 24Hz and ruler-flat 121db(!) output. But it requires a 9+ cu ft box. I'm struggling with where to put it. There's a spot about halfway down the side wall but the path length from the sub to the listener would be about 7' shorter than from the LCRs. Is this a problem? 1/2 wavelength at the crossover frequency is about 7'. Does this mean I should invert the sub polarity? If I use a 2nd order crossover, there is already a 180 deg. phase shift at the crossover frequency. Or I could use 4th orer Linkwitz-Reilly crossover to minimize the overlap and interactions. Thoughts?
     
  2. Jack Gilvey

    Jack Gilvey Producer

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    I don't really have any input on your question, Dave, but I'd love to hear more about your sub. I assume you're using eq to create that F3 and the 6th order rolloff? I modeled some of those big JBL-Pros a while ago, but the Fs was too high for me to get a result I liked.
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  3. ThomasW

    ThomasW Cinematographer

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    This question can't be precisely answered with computer modeling. Take an existing sub, put it in the desired location and give it a listen. Only experimentation will let you hear how the location will work. You might consider getting a phase adjustable XO to fine tune placement. The more expensive Paradigm's have this feature.
     
  4. Greg Monfort

    Greg Monfort Supporting Actor

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    I've used Keele's 6th order formulas for this driver, and based on them you must not have factored in VC heating, which requires a much larger cab/lower tuning to keep from having a severely underdamped alignment. FWIW, ~14.6ft^3 net/17Hz Fb/F3 = ~20Hz gets you a real world max flat sub with ~118dB peak capability without exceeding Xmax on 300W. Two of these fills a small movie house with Jurassic Park impact so one should be more than ample for HT.
    WRT your location situation, yes, in theory you would swap polarity, but as TW noted, the room dominates so who knows? With an 80Hz XO (4th L-R preferred IMO) and an implied ~14ft listening distance, can the sub be located directly behind, or as I've done before build the sub as a platform?
    GM
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  5. Dave Milne

    Dave Milne Supporting Actor

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    Thanks for the tips. JG: FS on the 2245H is 20 Hz --it's made for high fidelity woofer/subwoofer applications. TW: Adjustable phase is a good idea. I generally design and build my own active filters; I haven't done a circuit like this, but if I think about it for a bit I'm sure I can come up with something. As for the "show me" part, I'm not sure how to paste a graph in here but BassBox 6.0 shows it flat at 121 dB to 24 Hz. According to JBL, the 1W/1M efficiency is 95.4 dB. With rated 300 watts (24.7 dBW), it should be actually 120.1 dB. However as GM pointed out, it's Xmax limited in the bottom octave to about 118 dB. GM: Where do I get the info on Keele's equations and VC heating? And the sub platform idea is interesting. Are there special issues with a long flat box?
     
  6. Jack Gilvey

    Jack Gilvey Producer

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    Thanks, Dave, I hadn't found that one, the others were around 35Hz. That looks very impressive! What's the price on that one?
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  7. Greg Monfort

    Greg Monfort Supporting Actor

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    >According to JBL, the 1W/1M efficiency is 95.4 dB. With rated 300 watts (24.7 dBW), it should be actually 120.1 dB. However as GM pointed out, it's Xmax limited in the bottom octave to about 118 dB.
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    Actually, it's not Xmax limited in the discussed alignments/300W, though it exceeds it by up to 6dB in the 20-50Hz BW when driven up to its 600W program material limit. The real world Vs calculated SPL I listed is due to thermal compression from VC heat rise and the attendant rise in Qes/Qts at 300W in the ones I built.
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    > GM: Where do I get the info on Keele's equations and VC heating?
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    I don't know unless he published an AES paper on it. I have the info in an Altec engineering notes tech letter.
    Vb6 = 4.1*Vas*Qts^2
    Fb6 = 0.3*Fs/Qts
    Fp = 1.07*Fb6
    F3 = Fp
    I don't know all the math to calculate Qts rise due to VC heating (and even if I did I doubt they would be useful since I wouldn't have access to all the material parameters), only some approximations from testing over the years. In general, the Altec, JBL woofers I've used lose ~3dB at rated power. Pretty good compared to most LF drivers which lose up to >6dB.
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    > And the sub platform idea is interesting. Are there special issues with a long flat box?
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    Width/length can be an issue as the standing waves can be in the sub's BW (though at ~18ft^3 only the length is a big deal in this case) so building for extreme rigidity is a must.
    GM
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  8. Dave Milne

    Dave Milne Supporting Actor

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    Jack,
    The 2245H is not listed in the current catalog, but I think they're available through selected dealers as a replacement part for around $400 --which is not bad, they were about that new 10 years ago. The "new" 2242H ($811 list) is more efficient and has better power handling, but its 35Hz FS makes it less friendly for deep-deep bass. I've had my 2245H for years; it served brief duty as a sub for a two-channel audio system. It was overkill, even in my wimpy 3/4 MDF box. I tossed the cabinet, stored the driver, and built a smaller sub with the 15" 2235. Now that I have a new theater, I'm bringing out the big guns!
     

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