There's no easy answer to that one, just a variety of compromises. Basically, the filter should be wherever it needs to be to achieve your desired goals with the drivers you will be using and enclosure they will be used on / in. There is no fixed point; assuming you want to make a proper job of it, you work with what you're using, unless you're happy sticking a cap + coil on it & hoping for the best. The telephone bandwidth (midrange) as defined by Bell Labs 80-odd years ago is 200Hz - 4KHz. Some prefer not to cross in this region. The major power-demands for orchestral music occur < ~500Hz, so that is also a popular frequency. Neither account for diffraction effects etc., so some consideration needs to be given to that also, along with acoustic slopes &c.
So would you say something around 200 Hz is in the range of optimization. I plan on porting the enclosure for the woofer.
The problem with 200Hz is it's not very practical from standpoint of a reasonable size inductor for the low pass and capacitor for high pass. Both huge and expensive. You also will get increased distortion from the full range driver running it that low. I have used 400Hz to 600Hz and they work well for what I would still consider a FAST system.
Take for example this recent system I made for testing the FF85WK in a FAST: 4mH coil and 60uF cap gets a 600Hz XO. Letting the woofer handle a little more above 200Hz helps increase mid bass punch too. More air is moved by a bigger membrane.
http://www.diyaudio.com/forums/full-range/292130-ff85wk-rs225-8-passive-fast.html
With active systems you are not constrained by size and cost of parts but I still prefer a 350Hz XO to reduce distortion on the full range driver.
Take for example this recent system I made for testing the FF85WK in a FAST: 4mH coil and 60uF cap gets a 600Hz XO. Letting the woofer handle a little more above 200Hz helps increase mid bass punch too. More air is moved by a bigger membrane.
http://www.diyaudio.com/forums/full-range/292130-ff85wk-rs225-8-passive-fast.html
With active systems you are not constrained by size and cost of parts but I still prefer a 350Hz XO to reduce distortion on the full range driver.
No. Because I don't know what your design goals are, what the drivers are, and what the enclosure is (along with its dimensions). That was the point I was making in my post: there is no optimal per se, just a variety of compromises. You select what is likely to work best for what you are trying to achieve. Without knowing any of those, we can't really offer much in the way of advice.
Choosing an optimum XO point in a FAST requires weighing off a number of factors. Choosing to do a passive XO puts some constraints on the woofer & the FR, and once getting below 200-250 hz becomes next to impractical as the parts get so large that you can just get another amp & biamp for similar or less money.
One of the typical starting points is to put the XO in or around the baffle step F3 (i usually generalize to somewhere between the BS F3 or 0.707 x that frequency). If you are trying to do passive then the woofer should be 2-6 dB more sensitive than the FR (how much is needed depends on the speaker design, the room, and where in the room you put the speakers).
That said i will startout with the XO as low as i think i can get away with (i am usually bi-amping).
With the above we started out at 160 Hz (with a PLLXO), which sounded fine, but was a bit thin in the mid bass, so we moved it up to 240 Hz, where they were judged outstanding. That is still quite a bit below the BS F3 but it seems to work.
dave
One of the typical starting points is to put the XO in or around the baffle step F3 (i usually generalize to somewhere between the BS F3 or 0.707 x that frequency). If you are trying to do passive then the woofer should be 2-6 dB more sensitive than the FR (how much is needed depends on the speaker design, the room, and where in the room you put the speakers).
That said i will startout with the XO as low as i think i can get away with (i am usually bi-amping).
With the above we started out at 160 Hz (with a PLLXO), which sounded fine, but was a bit thin in the mid bass, so we moved it up to 240 Hz, where they were judged outstanding. That is still quite a bit below the BS F3 but it seems to work.
dave
Elaborating further to Dave's post above, on several FAST builds other than the little walnut towers shown - both with the FF85WK, and several Mark Audio units - we've found that the 240 - 330 HZ range seems to be a sweet spot. With the improving quality and ever decreasing prices of class D amps, we've found it almost makes sense to design these systems with bi-amping in mind from the outset.
When I can source a used chassis for re-purposing, my next project will be a 4x"100" TK2050 amp with selectable 1st order PLLXO for our most common frequencies . So far the amp board, power supply and mechanical parts have amounted to $151. USD - a pretty reasonable figure to my mind.
When I can source a used chassis for re-purposing, my next project will be a 4x"100" TK2050 amp with selectable 1st order PLLXO for our most common frequencies . So far the amp board, power supply and mechanical parts have amounted to $151. USD - a pretty reasonable figure to my mind.
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