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Car Audio / Passive Crossovers, Third Order Filters, 18dB per Octave

Third Order Filters:
Third Order Low Pass
Third Order High Pass
Third Order Band Pass
Third Order Narrow Band Pass

top of page 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 fd, and L4 = 3.18 mHy. For a 50 Hz low pass filter for a 4 ohm load, L3 = 19.1 mHy, C3 = 1058 fd, 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 fd capacitor, and a 1.59 mHy coil are needed. To calculate values, use this calculator or the appropriate chart.

3rd Order Low Pass Filter (18 dB per Octave)

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.

3rd Order 100 Hz Low Pass Filter Curve

top of page 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 fd, L5 = .095 mHy, and C5 = 15.92 fd. For a 4000 Hz high pass filter for a 4 ohm load, C4 = 6.63 fd, L5 = .12 mHy, and C5 = 19.89 fd. For a 5000 Hz third order high pass filter for a 6 ohm load, a 3.54 fd capacitor, a .14 mHy coil, and a 10.61 fd capacitor are needed. To calculate values, use this calculator or the appropriate chart.

3rd Order High Pass Filter (18 dB per Octave)

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.

3rd Order 5000 Hz High Pass Filter Curve

top of page 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.

3rd Order Band Pass Filter (18 dB per Octave)

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.

top of page 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.

18 dB Narrow Band Pass Capacitor and Coil Indentification
<< Previous - Second Order Filters Next - Passive Crossover Formulas and Calculators >>

Passive Crossovers:
Low Pass Filters
High Pass Filters
Band Pass Filters
Narrow Band Pass Filters
Zobel Filter
Acoustical Output & Power
First Order Filters
Second Order Filters
Third Order Filters
Formulas & Calculators
2 Ohm Reference Charts
4 Ohm Reference Charts
8 Ohm Reference Charts
Car Audio and Mobile Video:
Glossary of Terms
Passive Crossovers
Passive Crossover Calculators
Recommended Wire Sizes
Subwoofer Enclosures
Subwoofer Enclosure Calculators
Thiele - Small Parameters
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