EQing a sealed sub w/BFD

Discussion in 'Home Theater Projects' started by Dan_M, Aug 30, 2004.

  1. Dan_M

    Dan_M Agent

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    I have recently been reading up on using Linkwitz Transforms to flatten sealed subs. I'm wondering how well a BFD can be used in this way. I know they don't have any filters below 20hz, but can you use the Fine and Bandwidth settings to reach below 20? Also is there anythign I can use to model the effects of the EQ on things such as the excursion and SPL of the setup? I want to be sure I'm not going to be pushing my driver past its limits.

    Right now I have a large(143l) ported box taking up quite a bit of space in my room. If I can get the size down while not loosing too much extension or SPL, both me and my wife would be very happy.
     
  2. GrahamT

    GrahamT Supporting Actor

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    You cant get below 20 Hz with the BFD, not even with the fine settings. Maybe a wide bandwidth at 20, but I dunno. The filters have some effect below 20 but I am not sure by how much. If you can afford a Tumult, it is happy in a small box.
     
  3. Wayne A. Pflughaupt

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    I’ve heard that too, that the BFD brick-walls at 20Hz. But if that were true how to you explain Sonnie Parker’s EQ’d curve?


    [​IMG]


    Obviously a broad filter set low will extend below 20Hz.

    Regards,
    Wayne A. Pflughaupt
     
  4. GrahamT

    GrahamT Supporting Actor

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    Maybe I should have said you can't have a center filter frequency below 20 Hz. My Tempest only goes down to 16 Hz, and between there and 20 they are effected by the 20 Hz wide band filter, but by how much I am not sure.
     
  5. Dan_M

    Dan_M Agent

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    I figured a filter with the widest bandwidth possible at 20hz would get me the extension I'd need. I'm not positive how far this would stress the driver as far as excursion goes though. Is there anyway I can check this?
     
  6. dave alan

    dave alan Second Unit

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    A smaller, sealed box generally protects your driver from over excursion. It's the amp that will be stressed at playback of the lower frequencies.

    It takes 12 dB of boost to move the F3 down 1 octave. For example, if your F3 is 40 Hz, you'll need 16 times the power to equal that at 20 Hz, SPL limited by the amplifier much moreso than the driver's excursion capabilities.

    Post the info on your driver and some numbers can be posted for box size, EQ, amp power, SPL and excursion.
     
  7. Jack Gilvey

    Jack Gilvey Producer

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  8. ThomasW

    ThomasW Cinematographer

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    The filters do not brickwall at 20Hz. Here's some text from Ken Bruce's review
    Obviously one must chose a pretty wide filter to get boost down low. And doing that may have a negative impact on the higher frequencies in the filter's passband

    And no parametric EQ can't not substitute for a LT circuit
     
  9. Dan_M

    Dan_M Agent

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    Here are some T/S params of my Dayton driver:

    500 watts RMS/700 watts max *Le: 3.17 mH *Impedance: 4 ohms *Re: 3.07 ohms *Fs: 22.2 Hz *SPL: 88.6 dB 2.83V/1m *Vas: 2.61 cu. ft. *Qms: 6.12 *Qes: .47 *Qts: .44 *Xmax: 18.7 mm *Sd: .0447 m^2

    I'm using the PE 250W plate amp.

    I played with that spreadsheet linked above quite a bit. It seems odd it uses the driver diameter to calculate Sd and Vd. That doesn't seem to be very accurate.
     
  10. ChristopherD

    ChristopherD Stunt Coordinator

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    ThomasW, can you please clarify?
     
  11. dave alan

    dave alan Second Unit

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    Dan,

    I would try approx 1.7 cu ft.

    I would also e-mail Dr Phil Marchand and ask him about a single board, assembled and tested Bassis circuit with fixed values for:

    Fs = 36 Hz
    Qs = .7
    Boost = 10 dB
    Qb = variable (Purchase a pot and knob. This will give you variable 'Q').
    You won't need a SS filter.

    I'd suggest a 550 watt amp. That should give you an F3 of 20 Hz@ 104 dB, 1W/1M.

    I'll guess you'll lose about 5 dB@ 20 Hz vs your existing setup, but you'll have a 2nd order roll off, which should work well with your room and much lower group delay.

    Of course, the numbers are theoretical, YMMV, but I think they'll be close to reality.

    My 2 cents.

    Dave
     

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