Crossover Frequency Planner - Subwoofer and Speaker Overlap Without Gaps

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Where the subwoofer stops and the speakers start

A crossover is the frequency where bass duties hand over from the subwoofer to the main speakers. Below the crossover, the subwoofer does the work; above it, the speakers do. The planner above shows how that handover behaves: pick a crossover frequency and a filter slope, and it draws both filters, lists their levels at key frequencies, and gives the wavelength at the crossover point. The goal of a good crossover is a sum that stays flat through the handover region, with no hole where neither driver is playing and no bump where both play too loudly together.

The filter curves matter because drivers do not switch off instantly. A low-pass filter feeding the subwoofer rolls off gradually above the crossover, and a high-pass filter feeding the speakers rolls off gradually below it, so around the crossover both drivers play the same frequencies at reduced level. If their outputs add in phase, the sum is flat; if they are out of phase, they cancel and leave a hole. That is why crossover choice is about phase and placement as much as frequency, and why the planner shows both curves together rather than one number alone. At the crossover frequency itself, each side of a first-order Butterworth pair is down 3 decibels, so the two halves sum to full level when they are in phase.

How to use the planner

Enter the crossover frequency you are considering, commonly 60 to 120 Hz for home systems.
Choose the filter slope: first order rolls off gently at 6 decibels per octave, second order at 12, fourth order at 24.
Read the table of levels at a quarter, half, one, two and four times the crossover frequency.
Check the chart: the blue low-pass curve and the red high-pass curve should cross at the crossover frequency and separate cleanly on either side.

A worked example

Take an 80 Hz crossover with a second-order slope. At 40 Hz, one octave below, the subwoofer’s low-pass filter is nearly flat while the speakers’ high-pass filter is down about 12 decibels, so the subwoofer owns the deep bass. At 160 Hz, one octave above, the situation reverses. At 80 Hz itself, both sides sit near minus 3 decibels for a first-order pair, and the planner’s table shows the exact values for your chosen slope. The wavelength at 80 Hz is about 4.29 metres, which is a useful reality check: the crossover region’s wavelengths are longer than most rooms, so moving the subwoofer by a metre changes the phase relationship at the listening seat significantly. That is why subwoofer placement and the phase control on the subwoofer matter as much as the frequency setting.

Choosing the frequency itself is a trade-off. Set it too low and the main speakers must reproduce deep bass they cannot handle cleanly, distorting and robbing headroom. Set it too high and the subwoofer starts playing frequencies where its location becomes audible: above roughly 100 to 120 Hz, listeners can localise the subwoofer’s position, and the bass seems to come from a corner instead of from the music. Small satellite speakers push the crossover up because they cannot play low, while large floor-standing speakers allow it down. The planner does not make this choice for you, but it shows the consequences of each choice in numbers and curves.

Slopes: gentle versus steep

A gentle slope hands over gradually, which forgives placement errors and phase mismatch but leaves a wide region where both drivers play together, and any phase error in that region colours the sound. A steep slope separates the drivers cleanly and shrinks the overlap region, but it demands more precise alignment: steep filters shift phase more, and a small placement error can carve a hole at the crossover. Fourth-order Linkwitz-Riley crossovers are popular in professional systems precisely because their two sides sum flat in phase, but they still need the drivers time-aligned. Start with the slope your equipment provides, usually second or fourth order, and change it only if measurements show a problem.

Common mistakes

Setting the crossover by the subwoofer’s marketing range instead of the main speakers’ real low-frequency limit.
Running both the receiver’s crossover and the subwoofer’s own crossover at once, stacking two filters and carving a hole.
Placing the subwoofer for convenience rather than sound, then trying to fix placement problems with the crossover frequency.
Ignoring the phase control: a 180-degree phase flip can turn a flat handover into a deep notch at the crossover.

Who uses this planner

Home cinema owners use it when adding a first subwoofer, car audio installers use it when splitting door speakers from a sub, and studio engineers use it when checking a new monitoring setup. Set the frequency, check the curves, then listen to a bass sweep and a familiar track: the handover should be inaudible, and when it is, the system disappears and only the music remains.

Crossover Frequency Planner – Subwoofer and Speaker Overlap Without Gaps