Subwoofer Crossover Frequency Optimizer

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 80 Hz setting is also the default LFE crossover reference in most home theatre receivers — even though the dedicated LFE (.1) channel bypasses bass management and goes directly to the sub, the bass-management crossover for the remaining channels still follows the same 80 Hz standard. It's a great fall-back if you don't know your mains' f3.

Why 24 dB/oct Linkwitz-Riley?

The crossover slope controls how sharply the sub's lowpass and the mains' highpass roll off around fc. A 24 dB/oct Linkwitz-Riley slope is 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.

Gain trim and level matching

Setting the crossover frequency is only half the job. You also need to set the subwoofer's gain (volume) trim so the sub's output level matches the mains at the handoff point. Most AVRs handle this during auto-calibration (Audyssey MultEQ, Yamaha YPAO, Pioneer MCACC), but the calibration microphone is placed at the primary listening position only — trim the sub to −10 dBFS from there, or use a 80 Hz test tone and a SPL meter to match within ±1 dB. For music-only stereo systems without bass management, a real-time audio spectrum analyzer at the listening seat is the most practical level-matching tool.

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). Rooms with digital signal processing (DSP) from automatic room correction systems — Dirac Live, Audyssey XT32, or miniDSP — can also shift the apparent crossover behavior after correction. Use the recommendation as a target, then fine-tune by ear or by measurement.