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Car Audio / Passive Crossovers, Second Order Filters, 12dB per Octave
Second Order Low Pass Filters |
Second order low pass
filters consist of a coil in series followed by a capacitor in parallel to a loudspeaker.
The coil must come before the capacitor. For a 100 Hz second order low
pass filter for a 4 ohm load, L2 = 9.00 mHy and
C2 = 281 µfd. For a 50 Hz low pass filter for a 4 ohm load,
L2 = 18.00 mHy and C2 = 562 µfd. For a 100 Hz second order low pass filter for a
2 ohm load, a 4.5 mHy coil and a 562 µfd
capacitor are needed. To calculate values, use this
calculator or the appropriate chart.
Using a 100 Hz second order low pass filter on a woofer or woofers, at 200 Hz or one octave
above the crossover frequency, power to the woofer(s) will be reduced by 93.75.% or 12 dB.
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Second Order High Pass Filters |
Second order high pass
filters consist of a capacitor in series followed by a parallel coil to a loudspeaker.
The capacitor must come before the coil. For a 5000 Hz second order
high pass filter for a 4 ohm load, C2 = 5.62 µfd
and L2 = .18 mHy. For a 4000 Hz high pass filter for a 4 ohm load,
C2 = 7.03 µfd and L2 = .23 mHy. For a 5000 Hz second order high pass filter for a
6 ohm load, an 11.25 µfd capacitor and a .09 mHy coil are needed. To calculate values,
use this calculator or the appropriate chart.
Using a 5000 Hz second order high pass filter on a tweeter or tweeters, at 2500 Hz or one octave
below the crossover frequency, power to the tweeter(s) will be reduced by 93.75.% or 12 dB.
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Second Order Band Pass Filters |
Second order band pass
filters consist of a 12 dB high pass filter followed by a 12 dB low pass filter.
Either the high pass or low pass filter may be first in line.
This combination of high pass and low pass filters allow a band of frequencies between
the two crossover frequencies to pass with a steeper rate of cut-off than a first order filter.
The high pass crossover frequency point must be lower than the low pass crossover
frequency point.
As with the second order low pass and high pass filters, power above and below
the respective crossover frequncies will be reduced by 93.75.% or 12 dB.
Remember if the low pass frequency is less than 20 times the high pass frequency
use this calculator for
narrow band pass filters. For all others, use
this calculator or the appropriate chart. |
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Second Order Narrow Band Pass Filters |
A 2nd Order Narrow band pass filter consist of a capacitor and coil in
series followed by a parallel capacitor and coil to the loudspeaker.
The capacitor and coil values and configuration are not the same as a standard band
pass filter. Use this calculator or the appropriate chart.
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