Crossover Basics for Sound Systems
Crossover basics come down to three settings per band: the frequency, the slope and the delay. Get them right and each driver plays only the band it was built for, the bands sum flat, and the subs are kept out of the frequencies that wreck them.
Passive vs active
- Passive: inductors, capacitors and resistors in the cab, after the amp. One amp channel per cab, but the components dissipate power, the filter interacts with the driver's changing impedance, and nothing is adjustable at the show.
- Active: filters before the amps, one amp channel per band. Slopes, delay, EQ and limiters are all adjustable, and each amp channel sees a single driver load. Most sound systems run active, from a dedicated crossover or a DSP.
Slopes
Each filter order adds 6 dB per octave: 12 dB/oct is second order, 24 dB/oct fourth order. Steeper slopes keep more out-of-band energy away from the driver.
| Type | Level of each side at the crossover point | Sum |
|---|---|---|
| Butterworth | −3 dB | +3 dB bump (in phase) |
| Linkwitz-Riley | −6 dB | Flat when the outputs are aligned |
Linkwitz-Riley 24 dB/oct (LR4) is the usual default in PA processing because the two sides sum flat and stay in phase with each other through the crossover region.
Typical crossover points
| Between | Typical range |
|---|---|
| Sub high-pass | A little below the box tuning (reflex) or horn cutoff |
| Sub to low-mid | 80 to 120 Hz |
| Low-mid to mid | 250 to 800 Hz, depending on the boxes |
| Mid to high (compression driver) | 1 to 2 kHz for 1" exit drivers; lower for larger formats |
Start from the cab designer's figures: a horn or scoop is designed around a band, and the crossover should match it. Keep each driver size on its own band. See 15 vs 18 inch subs.
Time alignment
Sound travels 343 m/s, so each metre of path difference is 2.9 ms. Subs on the floor and tops on a stack, or subs set back from the tops, arrive at the listener at different times, and the bands then sum with a phase error through the crossover region. Measure or calculate the distance difference, delay the nearer box, then fine-tune by measurement. Polarity checks come first: see phase and polarity.
Hardware
The XR204 is a four-way stereo active crossover: 2 × XLR in, 8 × XLR out, dynamic range above 106 dB and THD+N below 0.004%. For limiter and high-pass settings on the sub band, see limiter settings for subs.
Sources: filter responses are standard network theory (S. H. Linkwitz, "Active Crossover Networks for Noncoincident Drivers", JAES 1976). Speed of sound 343 m/s at 20 °C. Wavemark XR204 product data.
Questions
What is the best crossover slope for a sound system?
Linkwitz-Riley 24 dB per octave is the common default. Each side is 6 dB down at the crossover point, so the outputs sum flat when aligned, and the steep slope keeps out-of-band energy away from the drivers.
Should I use a passive or active crossover?
Active, for a sound system. Each band gets its own amp channel and adjustable slope, delay, EQ and limiting. Passive crossovers dissipate power and cannot be adjusted at the show.
Where should I cross subs over to the low-mids?
Commonly 80 to 120 Hz, but follow the cab designs. The sub band also needs a high-pass filter just below the box tuning or horn cutoff.
How much delay do I need to align subs and tops?
About 2.9 ms per metre of path difference, since sound travels 343 m/s. Delay the nearer box, then refine by measurement.