My AVR only offers fixed crossover steps. Which do I pick?

Round the recommendation up to the nearest available step. Rounding up gives the sub more bass and protects the mains from frequencies they roll off below. If the tool says 65 Hz and your AVR offers 60 or 80, pick 80.

What about LFE from movies?

LFE is a separate 0–120 Hz channel routed directly to the sub regardless of bass-management crossover. The crossover you set here is bass-management — what's stripped from the mains and added to the sub. LFE bypasses it.

Why does the recommendation jump when I change room size?

Your room's axial modes shifted. If a mode lands within ±6 Hz of the recommended fc, the tool nudges up 10 Hz to avoid the boom peak. Try to keep the yellow fc marker away from the red dashed mode lines.

The sum curve isn't flat. What does that mean?

Butterworth crossovers sum with a +3 dB bump; the mains' natural f3 rolloff isn't a perfect Butterworth. LR4 with fc well above mains f3 gives the flattest blend. Deviations of ±1–2 dB are inaudible.

What about non-rectangular or open-plan rooms?

Axial-mode math assumes a closed box. Open-plan rooms have weaker, less predictable modes. The mode-flag is a heuristic; final tuning needs in-room measurement.

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Subwoofer Crossover Frequency Optimizer

Tell the tool your mains' −3 dB point, your room's longest dimension, and the crossover slope — it returns the recommended sub crossover frequency, flags any nearby room-mode booms, and plots the live sub + mains blend. Sweep the fc slider to feel the trade-off in real time.

Input

Mains' −3 dB low-frequency point (f3, Hz)

Bookshelf: 50–80 Hz · Floorstander: 30–45 Hz · Mini-monitor: 70–100 Hz. Look this up in your speaker's spec sheet.

Room longest dimension

Used to flag axial room-mode booms near your crossover. Use the longest of length / width / height.

Crossover slope & alignment

24 dB/oct Linkwitz-Riley is the industry-standard active subwoofer crossover.

Crossover frequency fc 80 Hz

Slide to override the recommendation and see how the blend changes. Changing any of the inputs above resets fc to the new recommendation — use the button to snap back.

Result

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Reasoning & room modes

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Sub + Mains blend (20–500 Hz) — red dashes mark axial room modes

How to Set a Subwoofer Crossover

A subwoofer crossover hands off low-frequency duty from your main speakers to the sub. Set it too high and you'll hear the sub (it'll sound boomy and "localizable" — your ear can tell where it's coming from). Set it too low and there's a hole in the bass where the mains have rolled off but the sub hasn't taken over yet. The art is finding the smooth handoff point.

The 1.3× rule

A safe starting point is 1.3 × your mains' −3 dB point, rounded to a convenient value (5 Hz steps work well — most AVRs let you pick 40, 60, 80, 90, 100, 110, 120 Hz). A speaker spec'd at 60 Hz (−3 dB) suggests a sub crossover around 80 Hz; a 40 Hz floorstander suggests 60 Hz.

The THX/SMPTE 80 Hz standard

Movie theatres and the THX cinema standard use 80 Hz with a 24 dB/oct Linkwitz-Riley crossover. Below 80 Hz, human ears struggle to localize sound, so the sub can sit anywhere in the room and still sound integrated. This is also the default crossover in most home theatre receivers, and it's a great fall-back if you don't know your mains' f3.

Why 24 dB/oct Linkwitz-Riley?

It's steep enough to prevent the sub bleeding into the localisable region, and gentle enough not to mangle phase too violently. Critically, two Linkwitz-Riley filters of the same order summing flat (at −6 dB each at fc, same polarity for LR4) means no peak or dip at the crossover — the response is smooth. Most active subs implement an LR4 24 dB/oct lowpass internally.

Room modes — the silent saboteur

Every room has resonance frequencies (modes) where standing waves boost certain bass notes. For a rectangular room, the strongest mode along the longest dimension is at f = c / (2L) where c = 343 m/s. A 5-metre-long room has a first axial mode at 34 Hz, second at 69 Hz, third at 103 Hz. If your subwoofer crossover lands near one of these, that frequency will boom — sometimes 6–10 dB louder than the rest. The optimizer flags collisions and nudges the recommendation upward by 10 Hz to dodge them.

Phase & polarity

At the crossover frequency, the sub LP and mains HP each have phase shifts. With a 24 dB/oct Linkwitz-Riley, both are 360° at fc and add in phase with normal polarity — no inversion needed. With 12 dB/oct LR2 you typically need to invert the sub (or the mains, equivalently) for a flat sum. The tool picks the polarity automatically.

What the tool can't do for you

This tool gives you a theoretically-optimal starting point. The real final crossover depends on your room's response (which a measurement mic like a UMIK-1 + REW will reveal), your sub's placement (corner = boundary gain), and your mains' actual in-room response (which is rarely flat to f3). Use the recommendation as a target, then fine-tune by ear or by measurement.

Frequently Asked Questions

My AVR only offers fixed crossover steps (40, 60, 80…). Which do I pick?

Round the recommendation up to the nearest available step. Rounding up is safer than rounding down — it gives the sub more of the bass and protects the mains from being asked to produce frequencies they roll off below. So if the tool says 65 Hz and your AVR offers 60 or 80, pick 80.

What about LFE / .1 content from movies?

The LFE channel is a separate stream containing low-frequency effects (explosions, rumble) at 0–120 Hz with up to +10 dB extra headroom. Most receivers route LFE directly to the sub regardless of the music-bass crossover for the mains. The crossover you set with this tool is the bass-management crossover — what gets stripped from the mains and added to the sub. LFE bypasses that.

My sub has its own crossover knob. Use it or bypass?

If your AVR has bass management, set the sub's crossover knob to bypass (often labelled "LFE" or "Direct"), or to its maximum value. The AVR's crossover is more accurate and consistent. Only use the sub's knob if your source (a stereo preamp without bass management) doesn't have its own crossover.

Why does the recommendation jump 10 Hz when I change room size?

Because your room's axial modes shifted. If the new mode-1 frequency lands within ±6 Hz of where the recommendation was about to land, the tool nudges up by 10 Hz to put fc off the boom peak. You can see the modes as red dashed lines on the plot — try to keep your fc marker (yellow) away from them.

Can I trust the f3 number on my speaker's spec sheet?

Mostly yes — most reputable manufacturers spec −3 dB anechoic, which is a fair approximation of half-space (against-a-wall) response. Boundary gain from room placement extends low-frequency reach by 3–10 dB below 80 Hz, so your in-room f3 may be lower than the spec. If unsure, measure with a mic; otherwise the spec is a reasonable input here.

The plot's "sum" curve isn't flat. What does that mean?

It means the sub's LP and mains' HP don't add to a perfectly flat response in this configuration — usually because the chosen slope/alignment isn't perfectly complementary (Butterworth bumps +3 dB, the mains' natural f3 rolloff isn't a perfect Butterworth, etc.). LR4 with fc well above mains f3 gives the flattest blend. Small deviations of ±1–2 dB are normal and inaudible.

What about non-rectangular rooms or open-plan spaces?

Axial mode math assumes a closed box. Open-plan or odd-shaped rooms have weaker, less predictable modes (sometimes lower Q, sometimes shifted). The room-mode flag is a heuristic — for a non-rectangular space the recommendation without nudging is usually fine, and final tuning needs in-room measurement.

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