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Recommendations for mid-bass suplementation of planars...

Discussion in 'Home Theater Projects' started by RichardHOS, Apr 24, 2003.

  1. RichardHOS

    RichardHOS Second Unit

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    Just toying around with ideas, as usual, to get a feel for what each alternative for the new HT would involve.

    For the moment, the idea I'm toying with is magnepan's all around in a 7.1 setup. Assuming I already have a really nice sub system in place to handle L/C/R lows + LFE information...

    I'd be using smaller magnepans for side and rear surrounds - MMG's or SMGc's most likely. These speakers certainly need low bass suplementation, and can actually use some reinforcement up to around 120Hz or even higher. For HT I'd just set them as small, and live with the minor directional cues being sent to front L/R or sub at these mid-bass frequencies, but I'm more concerned about multichannel audio.

    I'd hate to have a trombone that should be in the rear corner sound like it's coming from up front. OK... so, supposing I wanted to add mid-bass reinforcement to each surround channel, to keep directional cues intact, what would you recommend?

    Assume I already have amplifier power to spare (I actually have four spare channels at 130w/8ohms, so that part is taken of), and suggest something for the actual driver and enclosure.

    I've never really been a fan of ported enclosures, and I don't think they would be the best match for magnepans, but if you disagree tell me why. If you think sealed is best, what size/brand driver? I could still cross over the lower frequencies and pass them to the sub system (say, somewhere between 40Hz and 60Hz), so low end extension wouldn't be a primary concern. I would be looking for something that is accurate, flat within the ~40Hz to 150+Hz range, and clean. Good phase characteristics (active crossover suggestions are welcome), etc.
     
  2. Greg Monfort

    Greg Monfort Supporting Actor

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    Blending to planars/ribbons/horns all dictate a matching farfield acoustic sensitivity and ~flat phase/transient perfect response in the passband (this includes the XO's overlapping BW also), ergo you ideally need a bigger version of what you've already got and high order active L-R XOs. Once you move away from the ideal you have to decide where you're willing to compromise. Most folks who want to keep the mismatch to a minimum just ditch the phase response, limiting its match to the XO point (time aligned/phase aligned).

    This leaves us with a wide BW Q = 0.5 sealed, or open baffle (dipole) with a true 1st order or 4th/8th active L-R XOs. In this BW, of these two, the open baffle/active is the better choice, but is larger/more expensive.

    I prefer prosound drivers which I assume would be much bigger/more expensive than you're willing to tolerate so others will have to chime in with any potentially suitable consumer grade midbass drivers.

    GM
     
  3. RichardHOS

    RichardHOS Second Unit

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    I think there is a lot of good information in your post, if I can get my brain cells running fast enough to properly digest it! [​IMG]

    OK, first off, what is "L-R" crossovers? Left/right?

    Second, (and I'm trying to get my knowledge of acoustic theory up to speed at the same time, so be patient), I'm guessing you recommend 1st/4th/8th order crossover because it doesn't screw up the phase in the XO region, correct? Would that include that linkwitz-raleigh 24db/octave design (BTW, what order is that?).

    Third, sealed with Q ~ .5 sounds a lot like what I had in mind. If others could offer some suggestions for appropriate driver/enclosure configurations, I would be most apprciative. I think I would know where to start with a sealed "sub" design, but having a higher frequency response I'm left a bit uninformed as to driver requirements. I'd also like to keep cost down, seeing as I would need four of these... probably avoid $200 drivers if possible.

    Fourth, anyone have specific recommendations for a four channel active crossover with the requisite 1/4/8th order slope that could handle both low pass (for the mid-bass driver) and high pass (for the planar driver)? I would also consider sending the lowest information back up front to the sub (IB), so that low pass might actually work better as a mid pass band. Prosumer gear? I could see spending as much here as for all the drivers and materials required.
     
  4. Greg Monfort

    Greg Monfort Supporting Actor

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    >OK, first off, what is "L-R" crossovers? Left/right?
    ====
    Linkwitz-Riley, named for the guys who invented the L-R XO design.
    ====
    >Second, (and I'm trying to get my knowledge of acoustic theory up to speed at the same time, so be patient), I'm guessing you recommend 1st/4th/8th order crossover because it doesn't screw up the phase in the XO region, correct?
    ====
    For 1st order. 4th/8th are 360/720deg off, but can be rotated back to zero by moving the HF back in relation to the LF.
    ====
    > Would that include that linkwitz-raleigh 24db/octave design (BTW, what order is that?).
    ====
    Yep, except for the name error. [​IMG] 1st = 6dB/octave, 2nd = 12dB, 3rd = 18dB, 4th = 24dB, 6th = 36dB, 8th = 48dB and each represents a pole or 90deg phase shift, ergo 4th = 4p/360deg.
    ====
    >Third, sealed with Q ~ .5 sounds a lot like what I had in mind. If others could offer some suggestions for appropriate driver/enclosure configurations, I would be most apprciative. I think I would know where to start with a sealed "sub" design, but having a higher frequency response I'm left a bit uninformed as to driver requirements.
    > I'd also like to keep cost down, seeing as I would need four of these... probably avoid $200 drivers if possible.
    ====
    Ideally you want a driver that will be flat in the desired passband (this includes the XO overlap) with a well damped HF roll off, so if 4th order is used on both ends you'll need a driver that's flat for half octave and preferably one octave below/above the passband minimum (-12dB/-24dB). Octaves are 2:1 ratios so that would be either 0.707x or 0.5x Flm, and 1.414x or 2.0x Hfm.

    Browse the specs/FR plots at Madisound for likely candidates. Flat from 40-150Hz plus the XO BW = 28.28Hz-300hz. This dictates EQ'd JBL, etc., dual 15" cinema sub drivers or at least four consumer midbass drivers, so time to start thinking about compromising the sub/midbass XO point/slope.
    ====
    >Fourth, anyone have specific recommendations for a four channel active crossover with the requisite 1/4/8th order slope that could handle both low pass (for the mid-bass driver) and high pass (for the planar driver)?
    ====
    None I know of. Check the JBL, Rane, and dbx sites for what they offer.
    ====
    > I would also consider sending the lowest information back up front to the sub (IB), so that low pass might actually work better as a mid pass band.
    ====
    FWIW, at 80Hz you're limited to a pathlength difference of ~1130/80 = ~14.12ft. and ideally further divided by pi, or ~4.5ft.
    ====
    > Prosumer gear? I could see spending as much here as for all the drivers and materials required.
    ====
    More. [​IMG] Quality midbass/lower mids performance ain't cheap. The old performance adage 'speed costs money, how fast can you afford to go?' applies to audio also. New replacements for my 20-1kHz 15" drivers currently are $656 plus shipping and I need two/chnl minimum to ~keep up with my 500Hz horns. Ideally, they either need to be horn loaded or four/chnl. Just getting them reconed/rezapped is $250 plus ins/shipping, and at 26lbs plus packaging, figure another $60 for a round trip. [​IMG] Since they don't have grills, no unleashed kids/pets allowed in the room.

    GM
     
  5. RichardHOS

    RichardHOS Second Unit

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    Thanks... definitely a lot of good information there. After seeing how you approach the numbers, I think increasing the midbass/subbass crossover point would be an easy way to reduce driver requirements. I have no problem with redirecting frequencies of around 60Hz or even a bit higher to the front IB system, I was just concerned about the 100Hz+ information.

    When I indicated "flat from 40Hz to 150Hz+" that was really just giving myself an allowance below the crossover point (and, actually, above as well) to ensure the driver remained flat in the passband range. I'm glad you put some numbers to my instinct - an octave below XO minimum.

    Using the information you provided I'll start a search for appropriate drivers. Looks like the XO's are going to be a pain. A high pass for each surround channel to send highs to the planars, a low pass for each surround channel to send mid/lows to the sealed enclosure, and an additional low pass for each channel to send sub/lows to the front IB. Throw in the possibility of another high pass per channel to keep sub frequencies out of the mid/low sealed enclosure. [​IMG] If I could find an XO that has a high pass and low pass for a single channel (or, even a common crossover point that separates the channel into a high and low pass output), then I'd need 8 channels worth - just for the surrounds. Another three channels for the front.



    One more question... how difficult would it be to forgo the high pass XO to keep sub frequencies out of the midbass sealed inclosure, and just use an appropriate driver/enclosure design that naturally rolls off at the correct point? That would simplify things somewhat.
     
  6. ThomasW

    ThomasW Cinematographer

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    Mixing line source and point source drivers is always problematic.

    Most midbass drivers will play well up into the midrange if not rolled off with an XO.

    I suggest using an active XO between the maggies and the cone drivers. That of course means a ton of XO's and amps.

    Passive line-level XOs are a neat and relatively inexpensive way to obtain your goal. The drawback with them is that they must be custom configured for each specific amp and speaker.
    Another option is to add the midwoofers to the L-C-R and forget about the effects speakers.

    As Greg stated dipole mounted midwoofers will give the 'best' match for the maggies

    BTW, I use MMG's for rears and IMO they don't need midwoofers when the L-C-R has a good sub
     
  7. RichardHOS

    RichardHOS Second Unit

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    Thanks for the input Thomas. I wouldn't even consider doing this without active crossovers (or, perhaps, the line level passive option you pointed out). That means crossing over the MMG's for high pass, and enclosure for low pass or mid pass.

    You're right... that will require a significant number of XO's in the system. Take a look at the dbx XO I found in this thread in the amps/separates forum. It looks perfect for the configuration I'm toying with.

    One stereo three-way XO per side... one channel handling side surround, the other handling rear surround. High pass to the MMG's, mid pass to the sealed enclosure, and low pass summed mono between side/rear and sent to the appropriate side of a stereo IB up front. That's $360 for surround XO's. Up front I only need two-way XO's, since I will have the IB's crossed over a bit higher than normal. There is a cheaper two-way dbx XO that looks appropriate. One for center, another for L/R. That's $300 for front XO's.

    $680 for entire system crossovers; four channels of three-way, and four (one unused) channels of two-way. Considering that some form of bass management was probably going to be required anyway, at a minimum price of $250 (ICBM) and likely more due to keeping the main sub system stereo, it doesn't really seem that exhorbitant. I already have the amplifiers, and four drivers looks like another $500 or so.

    But, I'm still just toying with the idea. For HT I think it's completely unnecessary. For multichannel music, well... hard to decide without having any way to listen to the system config and decide.


    [edit - misspelled "Thomas" [​IMG]]
     
  8. RichardHOS

    RichardHOS Second Unit

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    Wanted to add that Behringer has an active stereo 2/3-way XO with practically the same flexibility as the dbx option, and streets for only $80. Similarly underwhelming ~91db to ~93db S/N, but still looks very appealing.
     

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