Like Gregm said: a speaker xover IS a filter! Hence people say "butterworth xover", linkwitz -riley xover", etc, etc.
A butterworth filter/xover is a maximally flat amplitude response filter. IOW, the filter pays more attention to the amplitude characteristic of the signal @ the expense of the phase. It is usually used in places where the screwing up the phase is not much of an issue. The roll-off of this filter is very smooth thereby yielding very little perturbation to the amplitude characteristic of the music signal. This is achieved by placing all the poles of the filter on a unit circle - there is no in-band ripple. (Roll-off of is area of the filter where the signal starts to get attenuated - it's the commencement of the what is called the transition region of the filter. In the pass-band of the filter, the filter response is such that the music signal passes thru largely unhindered).
A bessel filter is a maximally flat phase response filter. It pays more attention to the phase of the music signal rather than the amplitude. If I remember correctly, the Bessel response rollds off/attenuates the music signal even more gradually compared to a Butterworth. The Butterworth has a shallow roll-off. Well, the Bessel filter is even shallower! That is why you do not see Bessel filters used much - you need a higher order Bessel filter vs. a Butterworth. However, the Bessel is better @ preserving phase than the Butterworth.
A Linkwitz-Riley filter is an active filter (the Butterworth & Bessel xover used in the industry as 80-90% passive i.e. simply using R, L & C) i.e. it uses op amps to do its job. Thus, most speakers made by Linkwitz Lab use external active xovers. It seems that the Linkwitz-Riley xover is a modified Salen-Key filter. It is then followed by an active all-pass filter to smooth out the radiation patterns of the drivers. This all-pass network, as its name suggests, passes all frequencies un-hindered but provides a delay. You might know that the acoustical plane of a tweeter, a midrange & woofer are staggered in that the acoustical center of a tweeter is the most in front of the driver plane while the woofer's is at the plane. That is why you see the baffle of a speaker is sloped (like a Thiel or Meadowlark). This active all-pass network provides an active delay so that the drivers can be put on a non-sloped baffle.
ANyway, I hope that this helps. FWIW.
A butterworth filter/xover is a maximally flat amplitude response filter. IOW, the filter pays more attention to the amplitude characteristic of the signal @ the expense of the phase. It is usually used in places where the screwing up the phase is not much of an issue. The roll-off of this filter is very smooth thereby yielding very little perturbation to the amplitude characteristic of the music signal. This is achieved by placing all the poles of the filter on a unit circle - there is no in-band ripple. (Roll-off of is area of the filter where the signal starts to get attenuated - it's the commencement of the what is called the transition region of the filter. In the pass-band of the filter, the filter response is such that the music signal passes thru largely unhindered).
A bessel filter is a maximally flat phase response filter. It pays more attention to the phase of the music signal rather than the amplitude. If I remember correctly, the Bessel response rollds off/attenuates the music signal even more gradually compared to a Butterworth. The Butterworth has a shallow roll-off. Well, the Bessel filter is even shallower! That is why you do not see Bessel filters used much - you need a higher order Bessel filter vs. a Butterworth. However, the Bessel is better @ preserving phase than the Butterworth.
A Linkwitz-Riley filter is an active filter (the Butterworth & Bessel xover used in the industry as 80-90% passive i.e. simply using R, L & C) i.e. it uses op amps to do its job. Thus, most speakers made by Linkwitz Lab use external active xovers. It seems that the Linkwitz-Riley xover is a modified Salen-Key filter. It is then followed by an active all-pass filter to smooth out the radiation patterns of the drivers. This all-pass network, as its name suggests, passes all frequencies un-hindered but provides a delay. You might know that the acoustical plane of a tweeter, a midrange & woofer are staggered in that the acoustical center of a tweeter is the most in front of the driver plane while the woofer's is at the plane. That is why you see the baffle of a speaker is sloped (like a Thiel or Meadowlark). This active all-pass network provides an active delay so that the drivers can be put on a non-sloped baffle.
ANyway, I hope that this helps. FWIW.