Speaker Crossover Calculator

Frequently Asked Questions

How do I size a passive crossover capacitor and inductor?

First-order high-pass cap C = 1 ÷ (2π × Fc × R) and low-pass inductor L = R ÷ (2π × Fc), where R is driver impedance. For Fc = 2,500 Hz and 8 Ω: C ≈ 8 µF, L ≈ 0.51 mH.

What crossover frequency should I pick?

Set it within both drivers' usable range and at least one octave away from a tweeter's resonance. Typical two-way home speakers cross around 2,000–3,500 Hz; subwoofers around 80 Hz.

What does crossover order/slope mean?

Order sets the rolloff steepness: first-order = 6 dB/octave, second = 12, third = 18, fourth = 24. Steeper slopes reduce driver overlap but add components and phase complexity.

Why does driver impedance matter in the formula?

Crossover component values depend on the load impedance (R) the filter sees. Using 4 Ω instead of 8 Ω roughly halves the inductor and doubles the capacitor for the same crossover frequency.

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How This Calculator Works

A passive crossover splits the amplifier's full-range signal so the woofer gets the lows and the tweeter gets the highs, protecting the tweeter and keeping each driver in its comfortable band. This calculator computes the capacitor and inductor values for both first-order (6 dB/octave) and second-order (12 dB/octave) Butterworth crossovers from your chosen crossover frequency and the driver impedance. First-order networks use a single component per driver and have the gentlest, most phase-coherent roll-off; second-order networks roll off twice as steeply and keep more out-of-band energy off each driver at the cost of an extra component and a phase flip. For anyone building or repairing a speaker, these two numbers - a capacitor value in microfarads and an inductor value in millihenries - are exactly what you take to the parts shop. Getting them from a formula rather than guessing means the tweeter is protected from damaging low-frequency energy and the woofer is not asked to reproduce highs it cannot handle cleanly, which is the whole point of a crossover in the first place.

Formula / Method

Let ω = 2πf_c and Z the driver impedance. A first-order (Butterworth) high-pass uses a series capacitor C = 1 ÷ (ωZ) on the tweeter; the matching low-pass uses a series inductor L = Z ÷ ω on the woofer. A second-order Butterworth uses C = 1 ÷ (√2 · ωZ) and L = (√2 · Z) ÷ ω in the appropriate series/shunt arrangement. Capacitance comes out in farads (shown here scaled to microfarads, µF) and inductance in henries (shown in millihenries, mH).

Worked Example

For a 2500 Hz crossover into an 8Ω driver, ω = 2π × 2500 ≈ 15,708 rad/s. First-order capacitor C = 1 ÷ (15,708 × 8) ≈ 7.96×10⁻⁶ F ≈ 7.96 µF; first-order inductor L = 8 ÷ 15,708 ≈ 5.09×10⁻⁴ H ≈ 0.51 mH. The second-order values are C ≈ 5.63 µF and L ≈ 0.72 mH. Halving the impedance to 4Ω doubles the capacitor and halves the inductor, because C scales inversely and L directly with Z.

Common Mistakes

Using the amplifier's nominal 8Ω when the driver's impedance at the crossover frequency is actually higher or lower.

Ignoring the driver's voice-coil inductance and rising impedance, which shift the real acoustic crossover point.

Forgetting that a second-order crossover inverts tweeter phase - many designs reverse tweeter polarity to compensate.

Designing around the electrical crossover only, not the combined acoustic response with the drivers' own roll-offs.

Picking the nearest stock component without checking how far it moves the corner frequency.

Tips & FAQ

First or second order? First-order is simplest and most phase-linear but lets more out-of-band energy through - risky for fragile tweeters at high power. Second-order protects better and is the most common DIY choice. Do I need impedance compensation? Often yes - a Zobel network flattens the woofer's rising impedance so the inductor behaves as intended. Why Butterworth? It has a maximally flat passband; Linkwitz-Riley (a different alignment) is preferred for its better summed response at the crossover and is worth exploring next. Will the textbook values just work? They are an excellent starting point, but real driver impedance curves and baffle effects mean measurement and tweaking are part of any serious crossover design; treat these as nominal targets.

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