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How would you go about finding out what the xover points are in a network? Is there a formula of something?
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A few manufacturers (like Wilson) are hush-hush top secret about their slopes and crosspoints (like anyone could possibly market an exact clone!), but most will tell you at least generally that it's 4th order acoustic at 2.2kHz or whatever. The authorized repair techs for any given brand know exactly what's in the crossover and can probably tell you, if they're not sworn to secrecy.
Otherwise, as others have pointed out, you're going to do a lot of measuring and calculating. Once you know the impedance and frequency response characteristics of the raw drivers IN THE ENCLOSURE, you can determine the values for all the crossover parts and their positions in the circuit layout, and reverse engineer the whole thing according to the established math for capacative and inductive reactance. (There are LOTS of formulas "or what" involved here, most of them full of logs and roots and square pies). Or just measure the electrical transfer functions of the filters -- voltage vs. frequency -- across the terminals of each driver and superimpose this on the measured unfiltered acoustical responses of the drivers.
If your eyes haven't yet glazed over, and you're really interested in learning this stuff, start with Ray Alden's "Loudspeaker Design 201" for basic principles. If you like lots of equations, supplement this with Vance Dickason's "Loudspeaker Design Cookbook." The "Woofer Tester" and "Omnimic" software/hardware packages from Parts Express, combined with a suitable laptop, will give you serious measurement capabilities. And then you can purchase some really advanced speaker design software, or download for free some altogether decent software (Jeff Bagby's Passive Crossover Designer, and various Excel-based designware from the FRD Consortium), and get serious about learning to measure and design speakers. Expect a fairly long learning curve, especially if your math and engineering background is weak.
It is not hard. Set up your amp with the signal generator at its input. Set your calibrated 'oscope up across one of the drivers; you could start with the woofer. Turn on the sig gen so that you have a comfortable level at, say, 100Hz out of the woofer; that will represent 0dB. Record the p-p voltage as it will be your reference for each driver. Now sweep the sig gen from maybe 20 hz on up while you chart on graph paper the peak to peak voltage on the scope versus frequency on the sig gen. Repeat the proceedure for the other drivers and you are done. You will have a graphical presentation of each driver's output relative to your 0dB level. Note you will obviously need to use the same voltage for 0dB for each driver so make sure that the frequency is in the midband of the driver and not in the filter slope. The frequency on the sig gen may not be exact but you will get a good idea of the slope of the filter. More accurate results will be obtained if you use a frequency counter.
Since the actual X-over points and the slope of the X-over will be determined by the reactance of each driver, as well as by the values of any resistors, capacitors, or inductors in the network, it is rather difficult to do this by solving a mathematical formula; there are too many unknowns. One method would be to isolate each driver and then derive an impedance vs frequency curve for that driver alone. At the x-over point (hi-pass or lo-pass), the impedance will begin to rise dramatically; that's how you'll know you are at the high or low x-over point for that driver. To do this, you need a calibrated audio signal generator, a variable resistor, and a very good quality AC voltmeter, at minimum.
Well, you can disconnect drivers and measure each driver and it's crossover. You would need a calibrated mic and freeware like REW. You could measure all the crossover components and draw a schematic and use a internet crossover formulas to get a ballpark idea. Crossovers are designed around the performance specs of specific drivers.
Can be very complicated advanced math and physics applications.
Or... you do what I did and just copy what other people did with the particular drivers and cabinets in hand.
The question itself implies that you should take the "don't reinvent the wheel" tactic.
why do you ask? The crossover points are usually determined by sound engineers that work for a speaker company and are determined by what is best for the driver and also consider other factors. Of importance too is the type of crossover used (how "fast" they crossover). Subs have adjustable crossovers and I believe some speakers do to.
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