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Calculating cross-frequency & filter/slope

Uncle GeorgeUncle George Posts: 32
I m wondering if there is any reliable way to calculate the "right" cross-frequency and filter/slope (db/octave) of any woofer, based on thiele/small parameters (or even box parameters)...

If so, it would be great for cases as custom-made sub-woofers, e.t.c.

Comments

  • DraDra Posts: 3,844
    The filter needs to be suitable for the system as a whole. In other words, is it wise to cross a sub over at 200hz just because it extends to 300hz? I don't like vocals and guitar in the subs, but if the top cabs can't go below 200hz, you really have no choice.

    Speaker prediction software will only get you so far.
    Another thing to remember is that it is best to apply filters (at x-over) at frequencies where the speaker / box is still flat at possible, so that the applied filter isn't being affected by any naturally occurring decline. That just complicates things.

    Filter freq and slope calculations are too variable and would become too complex to predict, in my opinion. I know of no "if this and that, then this" database.

    DRA
  • To be honest, I have read couple of times "rules" about it... Some are already known, such as:

    1. A common cross-frequency for subs is 100Hz (±20%).
    2. By using Linkwitz-Riley filters (instead of Butterworth), you don't increase (+3db) the level around the crossover frequency (flat-crossing).

    But, there are some other "rules" too, such as:

    3. Rule of thumb says that a setting is usually "correct" when crosses a driver (woofer) over at double its resonant frequency (Fs).
    4. Another rule of thumb for subs says that a Low Pass Filter (LPF) is necessary, exactly under resonant frequency (Fs) of a sub-woofer.

    That's why I asked about thiele/small parameters and cross-settings based on them...
  • DraDra Posts: 3,844
    To be honest, I have read couple of times "rules" about it... Some are already known, such as:

    1. A common cross-frequency for subs is 100Hz (±20%). True
    2. By using Linkwitz-Riley filters (instead of Butterworth), you don't increase (+3db) the level around the crossover frequency (flat-crossing). True, but not always the best choice. FYI I use LR except for the sub HPF (BW).

    But, there are some other "rules" too, such as:

    3. Rule of thumb says that a setting is usually "correct" when crosses a driver (woofer) over at double its resonant frequency (Fs). I am pretty sure that the Fs is free air. Box tuning is probably a better indicator, but truly I don't know.
    4. Another rule of thumb for subs says that a Low Pass Filter (LPF) is necessary, exactly under resonant frequency (Fs) of a sub-woofer. I think you are flipping the definition of Low and High pass filters. The Low pass filter of the sub is where it intersects with the top box. The High pass filter is where the top box intersects with the sub (and where the sub cuts off the lower, out of range frequencies. I remember the difference by this: "High pass = Low cut. Low pass = High cut." When I first got into audio, my first electronic x-overs either had a "Sub-sonic filter" or a "Lo-cut" button. I still have to do the mental conversion when I hear "____ pass filter"

    That's why I asked about thiele/small parameters and cross-settings based on them... Putting a BW18 filter at the -3db spec point is my go to. Some folks like to use the -10db spec point. Others may use the half-way point between. The higher the point, the more protection from over-excursion and the more amp power reserves you'll have.
  • Thanks for the notice about LPF & HPF Dra. Sometimes I still flip them... :)

    So, the conclusion till now is that there are no "rules" for calculating cross-frequency based on thiele/small parameters... :(

    By the way, why are you using a BW filter (instead of LR) at the -3db spec point (or any other "lower" point)? And why 18db/octave?
  • DraDra Posts: 3,844
    I couldn't remember why, so I looked up this in the READ ME FIRST SECTION here.
    viewforum.php?f=60

    Lots of good stuff in the many articles. Driverack related and general sound questions answered as well.

    Here is the direct link about why BW18.
    viewtopic.php?f=60&t=1204
    DRA
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