What a speaker crossover does, and why it's hard to get right

Why one driver can't cover the band

A cone big enough to move the air that deep bass needs is too heavy and too wide for the treble. As frequency rises it beams its sound into a narrow cone in front of it, and eventually the cone stops moving as one piece and breaks up into ripples that color the sound. A dome small and light enough to reach 20 kHz, on the other hand, can't make the long movements bass asks for and would be damaged trying.

So a speaker splits the job. Each driver gets only the part of the band it handles cleanly, and the network that divides the signal is the crossover. The frequency where one driver hands over to the next is the crossover point.

Where that point can sit depends on the drivers. Triangle redesigned the midrange cone in the Triangle Borea BR09 to raise its breakup frequency, and its review notes that this lets the company cross it over higher than before, which should improve the match with the tweeter.

Passive and active crossovers

A passive crossover sits inside the cabinet, between the amplifier and the drivers, and works at full power. It uses capacitors, which pass high frequencies, and inductors, which pass low ones. The parts have to be large to carry that current, some of the amplifier's power is lost in them as heat, and the drivers' own impedance curves become part of the filter. Its big advantage is that a passive speaker needs one amplifier channel and one pair of cables.

An active crossover works on the small signal before amplification, with one amplifier channel per driver. It can be built far more precisely, and a digital one can also delay each driver's signal so that drivers mounted at different depths on the baffle arrive at your ear in step. The costs are more amplifier channels and a design fixed to one set of drivers.

A good passive network does more than split the band. It usually also pads down the tweeter, which is typically more sensitive than the woofer beside it. It compensates for baffle step, the loss of up to 6 dB of low-frequency output as the sound stops being reinforced by the cabinet's front panel. It flattens the impedance the filter sees and can notch out a driver's breakup peak. One set of parts has to meet all of those requirements at once, which is why crossover work separates good speakers from ordinary ones.

Slopes, and what each one trades

A filter's order sets how fast the level falls past the crossover point: 6 dB per octave for first order, 12 dB for second, 18 dB for third and 24 dB for fourth.

A shallow slope leaves a wide band where both drivers play at once. Because the two sit apart on the baffle, their outputs interfere through that band, so the response changes with your listening height and angle. A gentle slope also sends the tweeter a good deal of energy below the crossover point, which it has to survive.

A steep slope narrows the overlap and protects the tweeter. In exchange it rotates the phase more and needs more parts, and in a passive network every extra part adds cost and loss. Neither is right in general. What decides the result is the acoustic output, the drivers' own roll-off combined with the electrical filter, and whether that sums the way the designer intended.

Why Linkwitz-Riley filters sum flat

Filters shift phase as well as level, and the two halves of a crossover have to add back up to a flat response through the region where both drivers contribute.

Second- and fourth-order Butterworth filters are each 3 dB down at the crossover point, so two of them with their outputs in phase add up to a 3 dB bump. A Linkwitz-Riley filter, made by cascading two Butterworth sections, is 6 dB down at that point, so its two halves sum flat. The fourth-order version, falling at 24 dB per octave, does it with both drivers wired in the same polarity, one reason it became a standard choice.

Siegfried Linkwitz's 1976 paper showed a second benefit. With conventional filters, the spacing between two drivers makes the main lobe of their combined output tilt away from straight ahead and grow louder in the crossover region. The filter it proposed, now called Linkwitz-Riley, removes that problem, and the paper added delay networks to line up drivers whose acoustic centers sit at different depths.

Where the handover lands, and how to hear it

A two-way speaker usually hands over from its mid-woofer to its tweeter somewhere around 2 to 3 kHz, close to the 3 to 4 kHz region where hearing is most sensitive. The Taga Harmony TAV-500B v.2 crosses at 3.5 kHz, and its review lists that as a con, higher than some two-ways. A higher crossover asks the mid-woofer to play further up, and a 6.5-inch cone is already starting to beam its sound forward by then.

The handover happens between two drivers at different heights, so the combined sound changes with vertical angle. You can hear this at home. Play a solo voice and slowly raise and lower your head. A speaker with a well-managed crossover region keeps its tone through that movement; a difficult one changes audibly. Then listen from well off to the side. What you hear there is the speaker's off-axis sound, the same sound the walls reflect back to your seat, which is part of why a speaker that measures well straight ahead can still sound different in a room.

Two-way, three-way and in-ear designs

A three-way design splits the band twice, often at a few hundred hertz and around 3 kHz, and gives a dedicated midrange driver the voice band between them. The Borea BR09 has a dedicated midrange, and its review explains that because the midrange driver plays only the midrange, it can use a short-travel suspension. The price is a second crossover region, a taller cabinet and a costlier network.

Driver count alone says little. The Diora Acoustics Chors 5 is a two-way floor-stander whose two woofers play the midrange and bass together, and its review argues that this helps the bass extension, since neither driver is cut off at the low end the way a three-way midrange would be.

Multi-driver in-ear monitors use crossovers too. The Kiwi Ears Orchestra Lite splits its eight balanced armatures across a three-way crossover, two for the treble, four for the mids and two for the lows, and its review notes that the drivers and crossover are why the nozzle had to be so thick. A crossover also makes an earphone's impedance change with frequency, which is why a source's output impedance can shift the tuning of a multi-driver IEM; the impedance and balanced armature pages cover both. For speakers, the best speakers guide ranks every two-way and three-way design we've reviewed.