Third Order Low Pass Filters
Third order
low pass
filters consist of a coil in series followed by a parallel capacitor, followed by another
coil in series to a loudspeaker.
The coils and capacitor must be in this order. For a 100 Hz third order low
pass filter for a 4 ohm load, L3 = 9.95 mHy, C3 = 529 µfd, and L4 = 3.18 mHy.
For a 50 Hz low pass filter for a 4 ohm load, L3 = 19.1 mHy, C3 = 1058 µfd,
and L4 = 6.37 mHy. For a 100 Hz third order low pass filter for a 2 ohm load,
a 4.77 mHy coil, 1058 µfd capacitor, and a 1.59 mHy coil are needed. To calculate values,
use this
calculator or the appropriate chart.
Using a 100 Hz third 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 98.44% or 18 dB.
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Third Order High Pass Filters
Third order
high pass
filters consist of a capacitor in series folowed by a parallel coil, followed by another
capacitor in series to a loudspeaker.
The capacitors and coil must be in this order. For a 5000 Hz third order
high pass filter for a 4 ohm load, C4 = 5.31 µfd, L5 = .095 mHy, and C5 = 15.92 µfd.
For a 4000 Hz high pass filter for a 4 ohm load,
C4 = 6.63 µfd, L5 = .12 mHy, and C5 = 19.89 µfd.
For a 5000 Hz third order high pass filter for a 6 ohm load, a 3.54 µfd capacitor, a .14 mHy coil, and
a 10.61 µfd capacitor are needed. To calculate values,
use this
calculator or the appropriate chart.
Using a 5000 Hz third 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 98.44% or 18 dB.
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Third Order Band Pass Filters
Third order
band pass
filters consist of a third order high pass filter followed by a third order low pass filter.
Either the high pass filter or low filter may be first in line.
This combination allows a band of frequencies between the two crossover frequencies
to pass with a steeper rate of cut-off than a second order. The high pass crossover
frequency point must be lower than the low pass crossover frequency point.
As with the third order low pass and high pass filters, power above and below
the respective crossover frequncies will be reduced by 98.44% or 18 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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Third Order Narrow Band Pass Filters
A
3rd Order Narrow band pass filter consist of a capacitor and coil in
series followed by a parallel capacitor and coil, followed by another capacitor and
coil in series 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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