Crossover Frequency Bass-management crossover frequency (speaker to subwoofer)

What the crossover frequency is

In a bass-managed system, a speaker set to Small gets a high-pass filter and the subwoofer gets the matching low-pass. The crossover frequency is the frequency where the two meet. Denon's manual states it directly: sounds below the crossover frequency are cut off in the output from the speaker set to Small, and the cut bass is sent to the subwoofer or the front speaker. The menu offers 40, 60, 80, 90, 100, 110, 120, 150, 200 and 250 Hz, with 80 Hz as the default, which the manual says works best with the widest variety of speakers. For small speakers it recommends a higher value.

Wikipedia's bass management entry puts typical values at 40 to 80 Hz. THX's number is 80 Hz: HomeTheaterHiFi confirms that THX prescribes an 80 Hz crossover, then adds that the right setting depends on the quality of the sub and the main speakers. For full-range mains it suggests trying 60 Hz, and for a full-range sub, 100 Hz. The full routing logic is in bass management.

Large vs Small, and when the crossover applies

The Large and Small labels describe bass capability, not cabinet size. Denon defines Large as a speaker that can adequately play back very low bass and Small as one with inadequate playback capacity for it. The crossover only becomes adjustable when the Subwoofer Mode is LFE+Main or a speaker is set to Small. A Large speaker with the mode on plain LFE gets no high-pass, so the sub sees only the LFE channel, and Denon warns that the subwoofer may then produce no sound depending on the signal. On the AVR-X2700H, setting Front to Small while the subwoofer is set to No automatically switches the subwoofer to Yes, and setting any other speaker to Large is blocked once Front is Small. Dolby also reminds installers that Atmos enabled speakers are not full range, so bass management must account for each speaker's low-frequency extension.

How to choose the number

The working rule is to cross over above the speaker's in-room -3 dB point, where its output has fallen to half power. Audyssey follows it: owners in an AVForums thread report that MultEQ finds the -3 dB point of each speaker and sets the crossover from it, and they warn that manually setting a lower value than Audyssey chose is discouraged because the correction filters only extend down to that -3 dB frequency. Raising it is safe. One owner reports 150 Hz for front and center speakers rated to 55 Hz, and 90 Hz for rears rated to 68 Hz. Rated and in-room figures differ, so the receiver's measured number is the one to trust. The same thread notes that a rear speaker near a wall gets boundary reinforcement, and one owner suggests raising the rear crossover to match the fronts. The Denon menu also includes 120 Hz, the same figure as the LFE bandwidth limit, but the two settings act on different signals.

No standards body I could fetch says how far above the -3 dB point to go; the owner reports above are 1.45 octaves (150 Hz over 55 Hz) and 0.4 octaves (90 Hz over 68 Hz). Higher crossovers give the sub more of the work and the speakers less.

Slopes and the LFE low-pass

Wikipedia describes the usual filters: a 12 dB/octave Butterworth high-pass on the main channels and a 24 dB/octave Linkwitz-Riley low-pass on the sub. Natural speaker roll-off adds to that, 12 dB/octave for sealed enclosures and 18 to 24 dB/octave for ported ones. One octave above an 80 Hz crossover, at 160 Hz, the sub's 24 dB/octave low-pass is down about 24 dB. One octave below, at 40 Hz, the 12 dB/octave main high-pass is down about 12 dB. The crossover slope entry covers the trade-off.

Do not confuse this with the LFE low-pass. ITU-R BS.775-4 limits the LFE channel to 120 Hz and warns that low-pass filters add group delay, especially when cascaded. Denon's separate "LPF for LFE" setting offers 80 to 250 Hz with a 120 Hz default and applies only to the LFE track. Its two knobs, crossover and LPF for LFE, feed the same sub. See the LFE channel.

Worked example

Take a pair of towers with an in-room -3 dB point of 55 Hz (the AVForums example). At the 80 Hz Denon default, the crossover sits log2(80/55) = 0.54 octave above that point. The towers reproduce everything from 80 Hz up and the sub covers everything below. At Audyssey's reported 150 Hz, the same speakers hand over 1.45 octaves above the -3 dB point, so the sub also covers the 80 to 150 Hz band.

Drop to 60 Hz on those towers and the margin shrinks to 0.13 octave, so the speaker rolls off right where the filter does, and the Audyssey warning applies, since its correction filters extend down only to the speaker's -3 dB frequency. For measurement checks, the Denon distance and level steps in speaker distance and delay and speaker level calibration should be repeated after any crossover change, since the sub's phase relationship at the handoff shifts with it. Related reading: how to choose a subwoofer.