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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