What are the uses for fourth order active filters?

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Loudspeaker crossovers can make use of this type of filter. Most often they are made by connecting two 2nd order Butterworth filters. The attenuation should be twice that of a 2nd order filter, typically 24db/ octave, and the high-pass and low-pass outputs are always in phase with each other.
 

Thanks for the reply,
Thats mean inside the loudspeaker, tweeter is connected to the output of high pass active filter, bass is connected to the output of low pass active filter?

What is the difference between 24dB / octave and 24dB / decade?
What I know for a fourth order active filters have -80dB / decade.
 
An octave is 2:1 in frequency. 1khz and 2khz
A decade is 10:1 in frequency. 1khz to 10khz
If your filter slope is 80db/decade then it is much less than 80db/ octave. I bet you can find the answer with out a calculator.
 
HiTech said:
the high-pass and low-pass outputs are always in phase with each other.
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Nope.
Even-order (2nd, 4th, 6th etc) Butterworth and Linkwitz-Riley highpass and lowpass filters have outputs that are out-of-phase and make a notch at the crossover frequency.

The wires of the tweeter are usually reversed to correct the phase but then a Butterworth filter causes +3dB at the crossover frequency.
 
Well, I thought to create a Linkwitz Riley type crossover, two 2nd order Butterworth sections are cascaded (now a 4th order filter) and from that we get an in-phase output that doesn't require reversing the driver's polarity connections. That's where I was coming from. A Butterworth filter by itself would have an out of phase output, thus requiring reversing the connections to the driver. Am I right or wrong?
 
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2nd, 6th, and 10th order L-R filters are out of phase. 4th and 8th are in phase. It continues like that, but no one really uses more than 6th order acoustic slopes unless they are doing an elliptical crossover or something. Also note that audio designers will frequently use the inherent 2nd order falling response of a driver cascaded with a 2nd order electrical filter to get a 4th order acoustic.
 
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Hi HiTech and Speakerguy,
Thanks, I was mistaken.
Rane company confirms some of what you say about a 4th-order Linkwitz-Riley crossover having zero phase shift.
 

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Yep sound right to me after reading that bit. Point number 1 is something else I remembered from school but I didn't bother to mention that since it seemed to delv away from the topic somewhat.
 
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