Good advice
Be aware, if you haven't measured your driver's impedance, you don't know your Q factor. 2nd order filter Q is dependent on the impedance. You can make a ballpark estimate, but any discussion about precise values is only speculative..
The point I hope to convey is that these theoretical exertions are just a crude foundation when you have a sub not precisely coaxial with your mains.
Even then - if you give it a moment's thought - from the point of view of the listener's ear, you have wrangled very little benefit. Just think about the phase angles coming out of the crossover box (which are just plain unhelpful, eh), the response of the drivers to the signal they get, depth of mounting drivers, the locations, etc.
You're dealing with a Black Swan.
B.
presscot suggested a 90 degree phase gap. Not that there's anything wrong with that but it isn't the approach needed in this case. We're all correct whether we say its phase and response, or it's only about response, and we know why both can be true.. I think it's just a matter of who needs to hear what when?
In case of active subwoofers, I discovered some models offer a 90 degrees phase-shift function, a switch attached on their active crossovers. Back to passive subwoofers, are there any ways to obtain a 90 degrees phase-shift from 2nd-order passive filters? In my case, I found there was a bit tough to decide since both 0 and 180 degrees phase offered their own good characteristics, an unintentional equalizer, dip and peak response at crossing frequency, respectively. The average of these two types may be obtained by the midway between them, I guess.
Once upon a time, I had to redesign an automotive passive sub/satellite for the $3B audio company I was working for. What I found was
IT DOESN'T WORK
worth a darn. Why not? Well, because both the sub and the satellites have big variations in impedance (peaks and dips) around those frequencies. So all the passive crossover theory-based on resistors-goes out the window. And you could possibly end up with a very bad load to the amplifier.
--> Get an active crossover, don't waste time with this.
🙁
Some conjecture there. The phase issue will persist even with an active solution unless some control can be exerted, eg measurement.
AllenB, the passive crossover can "degrade" not only the acoustic but also the electrical phase (of the impeadance response), this is sometimes a bad thing from the amplifier point if view. The acoustical peaking which caused by a passive crossover in close proximity around a resonance (such as the low-end resonance of a normal driver) will modify the acoustical phase and can be a pain too, the active xo don't have that problem. Although all this can be correctable somewhat with additional passive elements, but we soon ending with a lot of physically large and costy passive elements which even can cost more than a flexible and more effective active solution.
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YSDR, have we had this conversation before? So, if a Voltage source amplifier with near zero output impedance can have that effect.. do you conjugate your drivers' raw impedances before connecting them to an amp?