Originally Posted by Sweetmeat
I would have NO idea where to even begin this process. If I hired someone to calibrate my system, is determining the correct slope usually a part of that? I'm considering having someone manually calibrate my system which is the reason fit the question.
Ummmm. No offense, but I get the impression that you are an enthusiastic audiophile, without an engineering background and with limited or no speaker/crossover design experience, yes? And you are asking questions and some folks are responding with some pretty highfalootin' technical answers.
Start with what a crossover does: it separates high an low frequency power signals and sends the appropriate signal along to the appropriate (woofer, mid, or tweeter) speaker. What most crossovers consist of are passive high- and low-pass filters. These are combinations of capacitors (which allow higher frequency signals, but block low frequency and DC) and inductors (or coils, which block high frequencies but let low frequencies and DC pass). It may be a little more sophisticated than this: the filters may have multiple capacitors, inductors, and resistors to achieve design goals.
Two goals which come to mind are phasing, and proper roll-off (or slope). One often graphs the percent of incoming signal to the speaker terminals that gets transmitted to the specific driver. These graphs are often done using log scales for frequency and percent transmitted. That is, there is no "Zero" frequency on the graph, and the distance between 1 Hz and 10 Hz is the same as that between 10 Hz and 100 Hz. Each of these distances is a called a decade. Likewise, instead of graphing 0 to 100 % transmitted, that percentage is translated to dB, which is a log unit describing the ratio of input to output. In this case, 0 dB means no attenuation of power (100% transmitted). -3dB is 50% power transmitted, -10dB is 10%, and -20dB is 1% of power transmitted.
Here is an example showing attenuation through the woofer, mid, and tweeter channels:
Filters aren't sharp. If you have a low pass filter that is nominally 100Hz, that means that 100Hz signals are 50% attenuated, lower frequencies are less so, and higher frequencies are more so. Different crossover designs are sharper (attenuation increases faster with changing frequency) or less sharp (the opposite). Filter slope (or rolloff) is the slope of the lines shown on the graph, above. It might be -10dB/decade, meaning every time I increase the frequency be a factor of 10 (the "decade" part), I drop the power transmission by the same factor (that is, 10% transmission, from the -10dB part).
In general, you want the signal to go smoothly from one speaker to the other as frequency changes. So the filter slope that decreases the woofer signal as frequency increases must increase the signal that the tweeter sees in a precisely coupled way. And changing the slope of either the low pass or high pass sections in the crossover also would require adjustment of the nominal frequency of those filters.
Crossovers also affects the phase of the signal. For a signal right in the "notch" between speakers (when both tweeter and woofer get 50% of the input signal), you want both drivers pushing out and pulling in at the same time. Otherwise, they kind of cancel, and you don't hear that frequency as loudly as intended. So this is a design consideration, and speaker designers take into account the electrical and physical characteristics of their drivers, the box the drivers are in, porting, fill, and other factors when designing their crossovers. It's pretty complex. Or at least, there are a lot of factors to consider.
Confused? Well, if you study this intently (or find better explanations) you won't be. But (IMHO) to get to the point where you are changing crossover filter slope productively will require a lot of effort. Either studying the engineering of the matter, or careful listening. Until you put in a lot of effort, listen carefully to your system as it is, and identify specific problems that suggest this type of action, I'd relax and not worry about changing filter slope. The default value is probably set to a very good level for almost all setups.
In sum, I would suggest your position is like that of a guy who buys a very nice Corvette, and dives in discussing his new car with a bunch of really experienced, sharp, well-healed Vette gearheads and experts. He hears about aftermarket steering mods, transmission replacements, engine rebuilds, modified hoods with air scoops, replacement suspension systems "for improved handling at 186mph!", and... Well, you get the drift. The optimal first step might not be to bore our the engine and replace the pistons and camshaft. He should gas up the Vette, and enjoy driving it for heaven's sake!
I'd hook your system up, see if you like the sound, and enjoy it a bit. If you hear things you don't like, or if you have friends whose systems sound better to you, ask questions. Start playing around. Try different speakers. Try biamping (in fact, that's one easy change you could make to see if you hear any difference at all when tweaking this stuff - if you don't hear any difference with biamping, filter slope shouldn't never trouble you ever again!).
Paraphrasing a political/religious commentator: Don't Immanentize the Eschaton! (roughly, but not exactly, don't expect that you need to get everything perfect right away).
Plug the thing in, hook it it, and enjoy some music. Good luck.