How are first-order crossover values calculated?

A first-order high-pass uses a series capacitor of C = 1 ÷ (2π · fc · R), and a first-order low-pass uses a series inductor of L = R ÷ (2π · fc), where fc is the crossover frequency and R is the driver impedance.

What is the difference between filter orders?

The order sets the slope: 1st order rolls off 6 dB per octave, 2nd order 12 dB, 3rd order 18 dB. Steeper slopes keep frequencies out of a driver's danger zone better but use more components and shift phase more.

What does Butterworth alignment mean?

Butterworth is a filter alignment chosen for a maximally flat passband. The 2nd and 3rd order values here come from standard Butterworth tables, which give a smooth summed response when the high-pass and low-pass meet at the crossover point.

Why does driver impedance matter so much?

Every formula scales with R. A crossover designed for 8 ohm drivers uses different parts than one for 4 ohm drivers at the same frequency. Use the driver's nominal impedance, and remember real impedance varies with frequency — an impedance-corrected (Zobel) network helps.

Can I use the nearest available component value?

Yes, within reason. Capacitors and inductors come in standard values, so pick the closest. Small deviations shift the crossover point slightly; for critical builds, combine parts in series or parallel to hit the target more precisely.

Crossover Frequency Calculator

A component-value calculator for DIY speaker builders, hi-fi tinkerers and car-audio installers designing passive crossover networks. Enter the crossover frequency, driver impedance and slope, and get the exact capacitor and inductor values.

How it works

A passive crossover uses reactive components — capacitors and inductors — whose impedance changes with frequency to route highs to the tweeter and lows to the woofer.

For a first-order filter (6 dB/octave):

high-pass capacitor:  C = 1 / (2π · fc · R)
low-pass inductor:    L = R / (2π · fc)

where fc is the crossover frequency and R is the driver impedance. At 3000 Hz into 8 Ω the high-pass cap is about 6.6 µF and the low-pass inductor about 0.42 mH.

Second- and third-order filters add components and follow standard Butterworth alignment tables (for example the 2nd-order coefficients C = 0.1125 / (R · fc) and L = 0.2251 · R / fc). These give a maximally flat summed response when the two sections cross.

Choosing a slope

Order	Slope	Trade-off
1st	6 dB/oct	Simplest, minimal phase shift, but lots of overlap
2nd	12 dB/oct	Common, good driver protection, 180° phase shift
3rd	18 dB/oct	Steep, excellent isolation, more parts and phase rotation

Tips and notes

Use the driver’s nominal impedance, but be aware that real impedance rises at resonance and high frequencies; a Zobel (impedance-correction) network flattens it so the crossover behaves as designed.
Air-core inductors avoid saturation distortion; use low-DCR wire on the woofer to preserve damping.
Use the closest standard component values, or combine parts in series/parallel to hit the target. A small error shifts the crossover point only slightly.

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