Subwoofer Crossover Calculator for Passive Crossover Parts
Overview
Subwoofer Crossover Calculator. Pick your speaker type or enter your main speakers' low-frequency limit in Hz and the Subwoofer Crossover Calculator returns a recommended crossover range plus the correct large/small receiver setting. Below that, the passive crossover parts calculator turns each speaker's impedance, the filter order and your crossover frequency into coil, capacitor and resistor values, with nearest standard parts. The custom frequency tone generator online runs on the Web Audio API — nothing is recorded or uploaded.
Bass sounds best when each driver only plays the notes it was built for, and a passive subwoofer crossover calculator tells you exactly which coils and capacitors split the signal that way. You enter the impedance of your sub and your main speakers, pick a filter order and a crossover frequency, and get back the component values for a working crossover. It saves you hours of formula work and helps your audio system sound even and clean.
How the Subwoofer Crossover Calculator Works
Every speaker crossover calculator runs the same textbook equations: each capacitor and inductor value depends only on the load impedance, the frequency where you want the split, and the filter alignment you choose. The tool simply does that arithmetic for every part at once and labels the results to match a standard schematic. A 2-way crossover calculator handles one split (sub and mains, or woofer and tweeter); a 3-way crossover calculator adds a midrange band; a 2nd order crossover calculator is the most common preset because it balances part count against protection. Try the channel level matching for a quick, no-install way to check this yourself.
- Choose the number of drivers: two for a sub-plus-satellite or woofer-plus-tweeter split, three if a midrange sits between them.
- Select the filter order and alignment from the list (for example, second-order Linkwitz-Riley).
- Enter the impedance of each driver in ohms.
- Type the crossover frequency in hertz.
- Read the capacitor and inductor values, then wire them in the order the parts table lists (series or parallel, counted from the amplifier), the same layout a standard circuit diagram shows.
Results come back in microfarads (sometimes typed uF) for capacitors and millihenries for coils. The calculated split is the cutoff frequency of each filter, the point where its output is down a set amount from full level.
Woofer Impedance, Tweeter Impedance and Crossover Point
The tool names its inputs after a classic two-driver box, so in a subwoofer build the woofer impedance box takes your sub's rating and the tweeter impedance box takes the satellite speakers or whatever driver plays above the split. Use the nominal impedance printed on each spec sheet (usually 4 or 8 ohms) as a starting point. That figure is an average, though, and real speaker impedance swings with frequency, so a measured value near your chosen crossover point gives more accurate parts.
Why Speakers Need a Crossover Filter
No single driver reproduces the whole audible range well. A large cone moves enough air for deep bass but beams and breaks up at high frequencies; a small dome handles treble cleanly but can be damaged by low frequencies it was never designed to move. Feed both the full signal from one amplifier output and you get distortion, uneven sound and, at high volume, a burnt voice coil. The part values the calculator returns are what keep a sub's cone below the split and the small satellites away from the deepest notes. Open the lufs loudness meter online in another tab any time you need a quick live reading.
A crossover filter fixes that by steering each slice of the spectrum to the driver that handles it best. That is why every hi-fi loudspeaker with more than one driver contains a speaker crossover, and why a subwoofer added to a stereo or home theater system needs one too, so the sub only plays the bass and the mains are spared from the deepest notes in your music.
Low-Pass Filter, High-Pass Filter and Band-Pass Filter Roles
- Low-pass filter: passes everything below the split and applies attenuation to what is above it. This is the branch that feeds the woofer or subwoofer, built around a series inductor.
- High-pass filter: the mirror image, passing content above the split to the tweeter or satellites through a series capacitor.
- Band-pass filter: a high-pass and low-pass combined, used for the midrange driver in a three-driver design so it only sees its own frequency range.
Active Crossover or Passive Parts for a Subwoofer
A passive crossover sits between the amplifier and the drivers and needs no power. An active crossover splits the line-level signal before amplification, so each driver gets its own amplifier channel. Most powered subwoofers, AV receivers and DSP units do this already: the receiver's bass management sends everything below the set point, plus the LFE channel, to the sub. Active filtering is cheap, adjustable and immune to impedance changes, which is why it dominates subwoofer setups. Passive parts still make sense when you drive a passive sub from a single amplifier channel, in some car builds, or in electronics projects where you want no extra amplification stage. As the worked example below shows, the parts get physically large at bass frequencies.
Filter Order and Crossover Slope
The filter order is the number of reactive parts in each branch, and it sets how quickly the unwanted range is cut. Higher orders give more attenuation outside the passband but need more parts, waste more power in coil resistance and are harder to tune. Most DIY builds settle on a 2nd order crossover design; a 3rd order network suits drivers that need extra protection.
1st Order to 4th Order Roll-Off in dB/Octave
| Order | Slope | Parts per branch | Typical use |
|---|---|---|---|
| First | 6 dB/octave | 1 | Simple, low loss, little tweeter protection |
| Second | 12 dB/octave | 2 | The everyday choice for two-driver designs |
| Third | 18 dB/octave | 3 | Better driver protection, more tuning |
| Fourth | 24 dB/octave | 4 | Steep, many parts, higher insertion loss |
A 1st order filter has a gentle roll-off, so a lot of energy still leaks into the wrong driver an octave away. A 4th order filter has the steepest rolloff, which protects the drivers well but stacks up parts whose tolerances interact.
Butterworth, Linkwitz-Riley and Other Alignments
The alignment decides the shape of the curve near the split, which changes how the two outputs add up:
- Butterworth (Q of 0.707) is maximally flat in the passband, with each output 3 dB down at the split. The two outputs overlap, so the summed response shows a small bump.
- Linkwitz-Riley puts each output 6 dB down at the split, so the acoustic outputs add up to a flat response with no peak, which is why it is so popular.
- Bessel has the most even group delay, keeping the phase response smooth at the cost of a softer knee.
- Chebyshev trades a little passband ripple for a quicker initial drop.
For most people the second-order L-R alignment is the safe default, since its flat sum protects sound quality without extra tuning.
Crossover Calculator Formulas for a 2-Way Crossover
A crossover design (also written x-over or cross-over) for two drivers uses one formula per part. For a second-order Linkwitz-Riley 2-way crossover, the series capacitor and shunt inductor on the high side, and the series inductor and shunt capacitor on the low side, are:
$$C = \frac{0.0796}{R \times f} \qquad L = \frac{0.3183 \times R}{f}$$
Here \(R\) is the driver impedance in ohms, \(f\) is the split in hertz, \(C\) comes out in farads and \(L\) in henries. Each alignment just swaps the constants:
| Alignment | Slope | Capacitor constant | Inductor constant | High-pass driver wiring |
|---|---|---|---|---|
| 1st-order Butterworth | 6 dB | 0.1590 | 0.1592 | Normal |
| 2nd-order Butterworth | 12 dB | 0.1125 | 0.2251 | Reverse polarity |
| 2nd-order Linkwitz-Riley | 12 dB | 0.0796 | 0.3183 | Reverse polarity |
| 2nd-order Bessel | 12 dB | 0.0912 | 0.2756 | Reverse polarity |
| 2nd-order Chebyshev | 12 dB | 0.1592 | 0.1592 | Reverse polarity |
The constants come from \(1/2\pi\) and the alignment's Q, which is why the same capacitor and inductor equations apply at any frequency.
Worked Example: Capacitor Value and Coil Size at 110 Hz
Say you have a passive 4 ohm sub in its own speaker box and a pair of 8 ohm satellites, and you want a second-order L-R split at 110 Hz. Entering those numbers into the subwoofer crossover calculator gives:
- Sub series inductor: \(0.3183 \times 4 \div 110\) = 11.57 mH
- Sub shunt capacitor: \(0.0796 \div (4 \times 110)\) = 180.9 µF
- Satellite series capacitor: \(0.0796 \div (8 \times 110)\) = 90.5 microfarads
- Satellite shunt inductor: \(0.3183 \times 8 \div 110\) = 23.15 millihenries
The coil sits in series with the sub and the capacitor sits in parallel with it. The striking part is the capacitor value and coil size: the same alignment for an 8 ohm tweeter at 2,700 hertz needs only 3.69 microfarads and 0.94 millihenries, so the sub's coil is about 12 times larger. Big coils carry their own DC resistance, which is the practical reason most subwoofer crossover work is done actively.
Picking the Crossover Frequency for a Subwoofer
The right crossover frequency depends on how low your main speakers play cleanly. Check the frequency response of your mains, set the split about 10 hertz above the point where they start to fall off, then fine-tune by ear. The THX reference for home theater is 80 Hz, and it works well for most systems because bass below that is hard to localize, so a sub can sit almost anywhere in the room without the sound drawing attention to it.
- Small satellites and on-wall speakers usually need a higher split, often well above 100 hertz.
- Bookshelf speakers with 5 to 6.5 inch woofers commonly cross near 80 to 100 hertz.
- Large tower speakers can cross lower, or run full-range, with the sub covering only the deepest octave.
Settle the split by ear before you enter it into the calculator, because a passive filter can't be dialled in later: changing the crossover frequency means recalculating and swapping every part. If your receiver has an auto EQ routine, borrow its pick as a starting point. Then run listening tests: a bass bump near the split usually means overlap or room gain, and a thin spot means a gap. Check the sub's volume before blaming the frequency, since level errors are the most common cause of poor bass response.
Reading Frequency Response and the Impedance Curve
Two plots on a driver's data sheet tell you whether the calculated parts will behave. The frequency response shows where the driver is flat and where it peaks, so you can place the split away from rough regions. The impedance curve shows how far the load drifts from its nominal rating; a sub driver can read well above nominal near its resonance, and a passive filter built for the nominal value will shift. Check the driver's sensitivity too, because two drivers with different acoustic output need level matching, not just a matched frequency.
Sizing a Passive Crossover Coil for a Bench-Test Sub
The decision on my bench: can a single coil, with no capacitor, keep an 8-inch passive sub out of the midrange of a pair of bookshelf speakers? The answer turned out to be yes, but only with a 9.86 mH coil sized from the measured impedance, not the 6.92 mH the nominal rating suggests.
The bookshelf pair's spec sheet puts their -3 dB point at 82 hertz, so I follow the "about 10 above" rule and aim for a 92 hertz split. The sub is labelled 4 ohms, but my impedance sweep reads 5.7 ohms at 92 hertz, because the driver's resonance peak sits just below that.
- First pass, 1st-order Butterworth, 4 ohms, 92 hertz: the calculator returns 6.92 mH (\(L = R \div 2\pi f\)).
- Cross-check: that coil on the real 5.7 ohm load puts the corner at \(5.7 \div (2\pi imes 0.00692)\), about 131 hertz. That is almost 40 hertz too high, so the sub would still be playing male vocals.
- Second pass with 5.7 ohms entered: 9.86 mH, and the corner lands back on 92 hertz.
So the passive subwoofer crossover here is one coil, sized at 9.86 mH. A first-order slope still lets through about -7 dB one octave up at 184 hertz, which I can accept for a test bench but would not for a finished system.
| Load used for sizing | Coil | Real corner | Level change |
|---|---|---|---|
| 4 ohms (nominal) | 6.92 mH | about 131 hertz | 0 dB |
| 5.7 ohms (measured) | 9.86 mH | 92 hertz | 0 dB |
| 5.7 ohms plus a 0.6 ohm coil resistance | 9.86 mH | about 102 hertz | -0.87 dB |
The last row is the catch that the formula alone hides. A coil this size carries DC resistance, and if it reads 0.6 ohms the corner drifts up to about 102 hertz and the sub loses 0.87 dB. Correcting for it means a 10.9 mH coil, or simply choosing a heavier gauge. In my judgement the number to watch is the impedance you enter, not the order you pick: a wrong load moved the corner by 39 hertz, while the coil's own resistance moved it by about 10. If the sub's impedance sweep isn't available, I would skip the passive coil entirely and use the receiver's bass management instead.
Phase Shift, Polarity and the 3-Way Crossover
Each reactive part adds phase shift, and a second-order filter puts the two outputs 180 degree apart at the split. Wired normally, the drivers are out of phase and cancel around the crossover. The fix is to reverse polarity on the tweeter or satellite, the high-pass side (or on the midrange in a three-driver design) so the outputs sum in phase again. First- and third-order designs have different phase behaviour, and the calculator's results assume you follow the wiring note for the alignment you picked.
A 3-way crossover is not just two 2-way filters stacked together. Well-behaved designs use an all-pass topology and keep the low and high split points a set frequency spread apart, typically a ratio of 8 (3 octaves) or 10 (about 3.4 octaves). Splits closer than about three octaves interfere with each other through the band-pass section. With a 3-way design, pick the lower split for the woofer first, then multiply by the ratio to find the upper one; one octave either way changes how the midrange sounds.
Crossover Component Values, Tolerance and Ratings
The calculator prints values to several decimals, but you don't need to match them exactly. Common parts carry a tolerance of 5 to 20 percent, so the nearest standard value is usually fine, and two parts can be combined (capacitors in parallel, coils in series) to get closer. If a value is awkward, nudging the crossover frequency a little often lands on parts you already have.
- Air core coils do not saturate, so they keep their value at high current, but large values get big and resistive.
- Iron core coils are smaller and have lower resistance for the same inductance, but can saturate at high power, which suits a low-power sub better than a large one.
- Film capacitors such as polypropylene types are stable and last longer.
- Electrolytic capacitors must be the non-polarized (bipolar) kind; they are cheaper and smaller for the big values a sub filter needs.
- Choose a voltage rating at or above the amplifier's peak output voltage.
In DIY speaker design the coil's DC resistance matters as much as its value in millihenries: it adds to the load the calculator assumed, so the corner of a passive sub filter drifts upward and the sub loses level, as the bench example shows. When a big coil is unavoidable, a heavier wire gauge keeps that error small, which is the main cost-versus-size trade in loudspeaker design for a sub's crossover design.
Zobel Network and L-Pad Add-Ons for a Speaker Crossover
The formulas assume a constant, resistive load, and real drivers are not. A driver's voice coil has inductance, so its impedance rises with frequency, and a resonant frequency peak sits near its lowest usable note. A Zobel network (a resistor and capacitor wired in parallel with the driver) flattens that rise so the filter sees something close to its design load in ohms; a notch filter can tame the resonance peak on a midrange or tweeter.
An L-pad is two resistors that add attenuation to a driver while keeping the load roughly constant, useful when the tweeter is louder than the woofer. A plain series resistor also cuts level but changes the load the filter sees. In car audio and home audio builds alike, these add-ons fix load and level problems so the measured frequency response, and the sound you hear, match what the calculator predicts.
Frequently Asked Questions
- What crossover frequency should I use for a subwoofer?
- Start from how low your main speakers play cleanly and set the split roughly 10 Hz above the point where they begin to roll off. 80 Hz is the THX reference and suits most systems; small satellites usually need a higher split and large towers a lower one. Fine-tune by ear before you buy passive parts, because changing the frequency later means new capacitors and inductors.
- What are the main types of crossovers?
- Passive crossovers use capacitors and inductors between the amplifier and the drivers and need no power. Active crossovers split the line-level signal before amplification, so each driver needs its own amplifier channel. Digital crossovers are active crossovers built on DSP, which is what most AV receivers and powered subwoofers use.
- Why do I have to reverse the tweeter polarity on a 2nd order crossover?
- Each reactive part adds phase shift, and a second-order high-pass and low-pass end up 180 degrees apart at the crossover frequency. Wired normally, the drivers cancel around the split. Reversing the tweeter (or the midrange in a 3-way design) brings the outputs back in phase.
- Why is Linkwitz-Riley so often recommended?
- Each Linkwitz-Riley output is 6 dB down at the split, so the two acoustic outputs add up to a flat response with no peak at the crossover point. Butterworth outputs are only 3 dB down there and overlap, which gives a small bump in the summed response.
- How precise do the capacitor and inductor values need to be?
- Close is good enough. Typical crossover parts have a tolerance of 5 to 20 percent, so pick the nearest standard value, or combine two parts (capacitors in parallel, inductors in series) to get closer. Nudging the crossover frequency slightly can also land on parts you already own.
- How do I calculate a Zobel network?
- Read the speaker's DC resistance (Re) and voice coil inductance (Le) from its spec sheet. The Zobel resistor is 1.25 times Re, and the Zobel capacitor is Le divided by the resistor value squared. The network sits in parallel with the driver and flattens its rising impedance so the crossover sees a steadier load.
- What voltage rating do crossover capacitors need?
- Use a voltage rating at or above the amplifier's peak output voltage: the square root of 2 times the speaker impedance times the amplifier's continuous power per channel (enter the amplifier power and the calculator shows it). Electrolytic capacitors must be the non-polarized (bipolar) type for audio.