Audio Crossover
Design a 2-way passive crossover to protect your tweeters and properly route bass to your woofers. Enter your filter order, driver impedances, and target frequency below to calculate the required capacitor and inductor values.
Why You Need a Crossover
Feed a full-range signal straight into a tweeter and you will eventually kill it. Bass frequencies push the tweeter's voice coil far outside its designed excursion, and it either overheats or the coil former physically collides with the magnet structure. A passive crossover exists to prevent exactly that: it splits the incoming signal into a high-frequency band for the tweeter and a low-frequency band for the woofer, using nothing but capacitors and inductors sitting in the speaker cable path between the amplifier and the drivers.
This calculator builds a 2-way passive crossover from four inputs — filter order, tweeter impedance, woofer impedance, and your target crossover frequency — and returns the capacitor and inductor values needed to build it. Everything below explains what those numbers mean, why the formulas behind them are shaped the way they are, and how to translate a calculated value into a real component you can order.
What Is an Audio Crossover?
An audio crossover divides one audio signal into two or more frequency bands and routes each band to the driver built to reproduce it. In a 2-way system, that means a high-pass filter feeding the tweeter (letting highs through, blocking lows) and a low-pass filter feeding the woofer (letting lows through, blocking highs). Run at the same crossover frequency, the two filters overlap enough to sum back into a flat, continuous response across the listening range, rather than leaving a dip or a peak where the drivers hand off to each other.
Crossovers come in two families:
- Active crossovers split the signal at line level, before amplification, using op-amps or DSP. Each driver then gets its own dedicated amplifier channel.
- Passive crossovers split the signal after amplification, using capacitors and inductors placed directly in the speaker wiring. One amplifier channel drives both the tweeter and woofer through the passive network.
This tool designs the passive kind — the type built into nearly every off-the-shelf bookshelf and floor-standing speaker, where a single set of binding posts feeds a crossover board mounted inside the cabinet.
Crossover Types & Formulas
Filter order determines the slope: how aggressively the crossover attenuates frequencies on the wrong side of the crossover point. Slope is measured in dB per octave. A steeper slope means less overlap between drivers, tighter protection for the tweeter, and more components in the circuit.
1st Order (6 dB/octave)
The simplest possible crossover: one component per driver, nothing else. Roll-off is gentle — 6 dB for every doubling of frequency past the crossover point — which means the tweeter still receives a meaningful amount of energy an octave below the crossover frequency.
| Filter | Component | Formula |
|---|---|---|
| Low-pass (woofer) | Inductor (L) | L = Z / (2π × f) |
| High-pass (tweeter) | Capacitor (C) | C = 1 / (2π × Z × f) |
Where Z is the driver's nominal impedance in ohms and f is the crossover frequency in Hz.
Trade-offs: a 1st-order crossover has the flattest phase response of any filter order and, because it uses only one part per driver, the lowest cost and the least insertion loss. Its major weakness is that gentle 6 dB/octave slope — a tweeter rated for use above 3 kHz can still see substantial energy at 1.5 kHz, which limits how low you can safely cross it and how much power you can send through the system.
2nd Order (12 dB/octave)
The most commonly used order in commercial speaker design. Each driver gets two components instead of one, and the steeper 12 dB/octave slope gives real protection to the tweeter well before its resonant frequency. This calculator uses the Linkwitz-Riley alignment (Q = 0.5), which is the standard choice for flat on-axis summing at the crossover point — the two driver outputs add up to unity gain instead of producing a peak or a dip.
| Filter | Component | Formula |
|---|---|---|
| Low-pass (woofer) | Inductor (L) | L = Z / (π × f) |
| Low-pass (woofer) | Capacitor (C) | C = 1 / (4π × Z × f) |
| High-pass (tweeter) | Capacitor (C) | C = 1 / (4π × Z × f) |
| High-pass (tweeter) | Inductor (L) | L = Z / (π × f) |
Trade-offs: better tweeter protection and a cleaner acoustic handoff between drivers, at the cost of two extra components (more insertion loss, more cost) and a 180° phase shift between the two filter outputs — which is why the FAQ below covers reversing tweeter polarity.
| 1st Order | 2nd Order (Linkwitz-Riley) | |
|---|---|---|
| Slope | 6 dB/octave | 12 dB/octave |
| Components per driver | 1 | 2 |
| Tweeter protection | Minimal | Good |
| Phase shift at crossover | 90° | 180° |
| Typical use case | Simple, budget, or vintage-style builds | Most modern hi-fi and car audio designs |
Practical Applications
| Application | Typical setup | Why passive |
|---|---|---|
| Hi-Fi bookshelf/floor-standing speakers | 2-way or 3-way crossover mounted inside the cabinet | Single amp channel per speaker, no external electronics needed |
| Car audio component sets | External crossover box between the amp and the door/dash drivers | Keeps tweeter and woofer on one amp channel while separating the bands physically |
| PA and sound reinforcement | Passive crossover ahead of a horn tweeter | Protects the horn driver from low-frequency overload during high-SPL use, without adding an active crossover and extra amp channel to the signal chain |
| Vintage and DIY restorations | 1st-order networks, often reproducing an original design | Simplicity and a known, well-documented sound signature |
Bookshelf speakers are the clearest example: open almost any two-way design and you'll find a small board with an inductor, a capacitor or two, sometimes a resistor for tweeter level matching, wired directly across the binding post terminals. No power supply, no active components — the crossover runs entirely on the signal already being sent to the drivers.
From Calculated Value to Real Component
The number the calculator returns is a mathematically ideal value. Real components come in standard sizes, and getting from one to the other takes a couple of extra steps:
- Round to the nearest standard value, or combine two components to get closer to the target. Capacitors in particular are often built by paralleling two values (a 10 μF and a 1 μF in parallel gives 11 μF) since exact values aren't always stocked.
- Check the inductor's DC resistance (DCR). Every real inductor has some series resistance, which eats into output level and can shift the effective crossover point slightly. Lower DCR generally means a physically larger (and more expensive) inductor.
- Check the capacitor's voltage rating. For passive speaker crossovers this is rarely a binding constraint at typical listening levels, but it matters more in PA and high-power car audio applications.
- Verify power handling. Both parts need to handle the continuous and peak power the amplifier can deliver at that frequency, not just the driver's rated power.
FAQ
What capacitors should I use?
Non-polarized capacitors are required — audio signal swings in both directions, and a polarized cap will fail almost immediately if wired into a crossover. Within the non-polarized category, film capacitors (polypropylene, often labeled MKP) are the standard choice for audio quality: low dielectric loss, stable value over time, and good high-frequency behavior. Bipolar electrolytic capacitors show up in budget designs and cheaper commercial speakers because they're smaller and less expensive for a given capacitance, but they degrade over time and generally measure worse for audio use than film types of the same value.
Should I reverse the tweeter polarity?
For 2nd-order (12 dB/octave) crossovers, yes. A 2nd-order filter introduces a 180° phase shift between the high-pass and low-pass outputs at the crossover frequency. Left as-is, the tweeter and woofer would be pushing air in opposite directions right at the point where their output overlaps most, which causes a dip in the summed response. Wiring the tweeter in reverse polarity (+ to the amplifier's − terminal, and vice versa) corrects for that shift and restores flat summing at the listener's position. This does not apply to 1st-order crossovers, where the phase shift is only 90° and standard polarity is correct.
Does impedance matter?
Yes, and it's one of the more common mistakes in DIY crossover builds. Every formula on this page has Z — the driver's impedance — sitting directly in it, which means the calculated component values are only correct for the impedance you designed around. Take a crossover built for an 8 Ω driver and connect it to a 4 Ω driver instead, and the effective crossover frequency shifts — in some configurations it roughly doubles or halves, moving the handoff point well away from where you intended. Always use the driver's actual nominal impedance (from its own datasheet, not an assumption), since real drivers frequently deviate from the "standard" 4, 6, or 8 Ω values printed on the box.
Can I use these formulas for a 3-way crossover?
Not directly. A 3-way network needs a band-pass section for the midrange (a high-pass and low-pass combined) in addition to the tweeter's high-pass and the woofer's low-pass, and the interaction between three filters is more involved than three independent 2-way calculations. The 2-way formulas here are a correct starting point for the tweeter/woofer relationship in a 3-way design, but the midrange band-pass section needs its own calculation.
Why does my measured crossover frequency not match the calculated one?
Two common causes. First, real drivers are not purely resistive loads — impedance varies with frequency, often rising sharply near resonance, and the formulas here assume a fixed, resistive Z. Second, component tolerance stacks: a 10% tolerance capacitor paired with a 10% tolerance inductor can shift the actual crossover point by a similar margin from the calculated value. Measuring the finished crossover's response with a microphone and comparing it against the target is the only way to confirm the real-world result matches the design.
Video Reference
For a walkthrough of the calculation process and how the component values map onto a physical crossover build, this video is a useful companion to the formulas above:
Crossover Network | Calculation of Component Values
https://www.youtube.com/watch?v=2PRBlHsn3P0
It covers the same core relationship between impedance, crossover frequency, and component value used in this calculator, worked through step by step for both filter types.