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Car Audio / Passive Crossovers, First Order Filters, 6dB per Octave
First Order Low Pass Filters |
First order low pass
filters consist of a coil in series with a loudspeaker.
The larger the coil size (mHy), the lower the crossover frequency. For a 100 Hz first order low
pass filter for a 4 ohm load, L1 = 6.36 mHy.
For a 50 Hz low pass filter for a 4 ohm load,
L1 = 12.73 mHy. For a 100 Hz first order low pass filter for a 2
ohm load, a 3.18 mHy coil is needed. To calculate values, use this
calculator or the appropriate chart.
Using a 100 Hz first 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 75% or 6 dB.
At 400 Hz, power is reduced by 93.75% or 12dB, and so on.
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First Order High Pass Filters |
First order high pass
filters consist of a capacitor in series with a loudspeaker.
The larger the capacitor size (µfd or mfd) is, the lower the crossover frequency. For a 5000 Hz first order
high pass filter for a 4 ohm load, C1 = 7.96 µfd.
For a 4000 Hz high pass filter for a 4 ohm load,
C1 = 9.95 µfd. For a 5000 Hz first order high pass filter for a 6
ohm load, a 5.31 µfd capacitor is needed. To calculate values, use this
calculator or the appropriate chart.
Using a 5000 Hz first 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 75% or 6 dB, and
by 93.75 % at two octaves and so on.
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First Order Band Pass Filters |
First order band pass
filters consist of a capacitor in series
followed by a coil in series with a loudspeaker. Either the capacitor or coil may be first in line.
This is a combination of a high pass filter and
a low pass filter which allow the band of frequencies between the two crossover frequencies
to pass. The high pass crossover frequency point must be lower than the low pass crossover
frequency point.
As with the first order low pass and high pass filters, power above and below
the respective crossover frequncies will be reduced by 75% or 6 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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First Order Narrow Band Pass Filters |
First order narrow band pass filters
also consist of a capacitor in series followed by a coil in series with a loudspeaker.
The capacitor and coil names are the same as a standard band pass filter, but use different
values. Use this calculator or the appropriate chart.
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