Some times refer to as "audio filters" or simply "crossovers / x-overs"
The primary objective is to protect the speakers from frequencies it cannot handle. PERIOD.
Secondary objective may be used to gain advanced "control" over the sound that it is "allowed" to produce.
Please note that if the loudspeaker does not have a particular frequency (sound). The x-over will not be able to force it out.
A practical example of how to use it correctly is shown on Jwo Shong's Car Audio
Human creativities are limitless and therefore may come up with other methods to use the crossovers, however only the two objectives above are "true" and will not cause "unacceptable" results.
Again, it is to be stressed that many DIYers do not have all the information required to adjust / setup a system properly (manufacturer's fault), therefore will misuse the x-overs more than getting good results. Please remember factors in Flat Frequency Response.
One "wrong" example is manufacturers' "high-passing" the tweeter at a much higher frequency and publish it's power handling as 10 times more. Although the objective is achieved, but sacrificing much sound. To be describe in Jwo Shong's Car Audio.
Another "wrong" example is users (be it installers or DIYers) setting the x-over frequency, where there are no frequency to "protect" or "control". This is a common error. Again, mainly manufacturer's fault for the case of genuine technical people, but some are user error, not the manufacturer's.
Why manufacturer's fault, because they publish in-accurate specifications / in-complete specifications.
Some are intentional, while allot are not, but why are they still wrong? because the specifications may be published by a "businessman" 98% of the time not the engineers. While the last 1.9% is by the few lucky engineer but un-qualified in reality. The last 0.1% of true? I haven't found it yet. Except on this website. :)
Please consider, there are over 10,000 times more E&E engineers compared to Electroacoustics engineer or Acoustics engineer. Even not every Acoustics engineer is capable for loudspeakers / Electroacoustics devices. How can so many E&E engineers know what is correct or not?
Please read MLSSA Operation Guide to get a picture of how easy it is for any engineer to do things wrongly without realizing it until an extremely long long time later. Unless the engineer is extremely meticulous and is constantly searching for more accurate means of doing things and constantly infinitely checking all previous work done.
Before proceeding, please realize that the information provided are a journey of searching for audio nirvana from car-audio to home hi-fi to diploma in recording studios + PA systems to many car-audio literature from USA to special car audio courses in USA, to university degree in acoustics, to working in R&D environment, to breaking R&D measurements technology and science and round and round and round.
I think that is one extremely big circle of infinite directions therefore I hope you'll appreciate this.
next to Jwo Shong's Car Audio
back to Flat Frequency Response
Oh, by the way, in reality, I also don't follow 100% flat frequency response :) I always let the bass play louder :). After that everything from around 100Hz to 20kHz have about near 100% flat frequency response in Anechoic Conditions only. Still not 100% flat, since no loudspeakers are flat and I never push the product from what it naturally cannot do. Not real environment though, it is simply not possible :( as you have read from Flat Frequency Response.
Think of it this way, in order to make a good sound system, find parts that match to each other perfectly and naturally, don't force them to do what they cannot in the 1st place.
Finally as an engineer, I stress, good engineer don't force things to work, they simply find things that works together in harmony.
Like the olden days saying - there are plenty of fast running long traveling horses, but there are not many who knows how to pick one.
So, are you the one that knows how to pick a good horse from a bunch of bad horses?
If you need me to explain to the last words, then here it is: -
All the products out there are horses, not all of them are good. Branding a horse or selling it expensive doesn't make the horse run any faster nor travel any further.
Feeding them gold, also does not make them any better, good horses are natural phenomenon. Also, good parent horses does not necessary produce good offspring.
Another example is: -
Good engine put in a Ferrari shell will naturally run extremely well. That same engine put in a k-car, will not get very fast.
This goes with loudspeaker construction / design / manufacture. The shape of the loudspeaker affects the sound quality more than the material used.
As same with the shape of the engine's internal affect the power output more than what material it is made of.
Please proceed to Jwo Shong's Car Audio