Speaker Crossover Frequency Calculator
First-order passive crossover: solve for fc, capacitance, or inductance given driver impedance and the other two values.
Result
- Driver impedance R8 Ω
- Capacitor C8 μF
- Inductor L0.5 mH
- fc from C (high-pass: tweeter)2,487 Hz
- fc from L (low-pass: woofer)2,546 Hz
Step-by-step
- 1st-order high-pass: fc = 1 / (2π·R·C) = 1 / (2π·8·8μF) = 2,487 Hz.
- 1st-order low-pass: fc = R / (2π·L) = 8 / (2π·0.5mH) = 2,546 Hz.
How to use this calculator
- Pick the variable you want to solve for.
- Enter driver impedance (8 Ω is most common, 4 Ω for some woofers).
- Read the result and pick the nearest standard component value.
About this calculator
A first-order passive crossover uses one capacitor (in series with the tweeter, for high-pass) or one inductor (in series with the woofer, for low-pass). The corner frequency where the response is down 3 dB is fc = 1 / (2π·R·C) for an RC high-pass and fc = R / (2π·L) for an RL low-pass. First-order networks have a gentle 6 dB/octave slope; many real designs use 2nd-order (Butterworth, Linkwitz-Riley) for steeper rolloff but the first-order math here is the foundation.
What this calculator does
This calculator solves the three quantities in a first-order passive crossover: corner frequency fc, required capacitance C, or required inductance L, given the driver impedance R and one of the other two. The high-pass form (capacitor in series with a tweeter) uses fc = 1/(2π·R·C). The low-pass form (inductor in series with a woofer) uses fc = R/(2π·L). Both are textbook RC and RL filter formulas applied directly to loudspeaker design. The "fc" mode also reports both formulas side-by-side so you can sanity-check that the chosen C and L target the same crossover region.
How it works — the formula
High-pass (C in series with tweeter): fc = 1 / (2π·R·C)
Low-pass (L in series with woofer): fc = R / (2π·L)
C = 1 / (2π·R·fc) L = R / (2π·fc)A 1st-order RC high-pass blocks DC and attenuates everything below fc at 6 dB/octave. A 1st-order RL low-pass passes DC and attenuates everything above fc at 6 dB/octave. The driver impedance R closes the loop in both cases. Higher-order crossovers (Butterworth 12 dB/oct, Linkwitz-Riley 24 dB/oct) cascade reactive elements; the first-order corner equation here is the building block they all derive from.
Worked examples
- Inputs:
- mode=cap, R=8, fc=2500
- Output:
- C ≈ 7.96 μF — pick standard 8.2 μF (fc shifts to ~2.43 kHz)
Always round to the nearest standard value; a few percent shift is inaudible.
- Inputs:
- mode=ind, R=8, fc=800
- Output:
- L ≈ 1.59 mH — pick standard 1.5 mH (fc shifts to ~848 Hz)
Standard inductor values: 0.1, 0.22, 0.33, 0.47, 0.68, 1.0, 1.5, 2.2, 3.3, 4.7 mH.
- Inputs:
- mode=fc, R=4, C=10 μF, L=0.4 mH
- Output:
- fc_HP ≈ 3979 Hz, fc_LP ≈ 1592 Hz
For a 2-way design, both should land near the same target; large mismatch indicates wrong R for one driver.
When to use this vs other tools
Use this for first-order passive crossovers. Active and higher-order crossovers need different tools.
- Audio Impedance Matching
Use to check the driver impedance at the chosen crossover frequency — voice-coil inductance shifts impedance from the nominal rating.
- Decibel Calculator
Use to compute slope attenuation in dB at any frequency relative to fc (6 dB/octave for first-order).
- Wavelength Frequency
Use to convert fc to acoustic wavelength — relevant when placing the crossover near a room mode or driver spacing.
Authority note
The 1st-order corner-frequency equations are canonical RC/RL filter mathematics — present in every electrical-engineering text. AES is the professional society that codifies their application to loudspeaker design.
Limitations
- Assumes the driver impedance R is constant at fc. Real voice-coils rise with frequency; use a Zobel network to flatten R before applying the formula.
- First-order only. Butterworth, Linkwitz-Riley, and Bessel topologies need 2+ reactive components and different equations.
- Phase shift at fc is ±45°; aligning tweeter and woofer phase requires careful driver placement or polarity inversion.
- Power handling: capacitors and inductors used in passive crossovers must be rated for the full amplifier output — bipolar electrolytics OK below ~1 kHz, film caps preferred above.
Real crossover design also accounts for driver sensitivity, baffle step, and off-axis behavior. This calculator gives the textbook first-order target — refine in measurement software for production designs.