Crossover 101 - Page 2

Below is a chart to represent the filter order or number of poles, the rate of attenuation and the maximum amount of phase shift in degrees (+/-):

Filter Order Attenuation per Octave Phase Shift
1st -6 dB 90 degrees
2nd -12 dB 180
3rd - 18 dB 270
4th -24 dB 360
5th -30 dB 450
6th -36 dB 540
7th -42 dB 630
8th -48 dB 720

Note: It is possible to build 5th through 7th order but they are not generally used in audio. Most passive crossovers do not go beyond 3rd or 4th order filters, and some passive designs may chose different orders for the low pass than the high pass, (example 2nd order low pass & 3rd order high pass).

The following graphics will depict the roll-off and phase shift apparent in typical filter orders. I will use 800 Hz as the crossover frequency to keep the graph more symmetrical:


Graphic 1: 1st Order Low Pass Filter
Note: -3 dB point = 800 Hz
Phase shift = +90 degrees
Graphic 2: 1st Order High Pass Filter
-3 dB point = 800 Hz
Phase shift = -90 degrees
Graphic 3: 2nd Order Low Pass Filter
Phase shift = -180 degrees
Graphic 4: 2nd Order High Pass Filter
Phase shift = +180 degrees


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