A crossover is a component or group of components that limits the frequencies reaching a speaker. When separate drivers — subwoofers, woofers, midranges, and tweeters — are used in a system, each should only receive the frequency range it was designed to reproduce. Crossovers enforce those limits.
Crossover Points
The frequency range a filter allows full power through is called the passband. The frequency at which a filter begins reducing power is the cutoff point. The range between two adjoining cutoff points is the crossover area.
As frequencies move away from the passband, power reduction increases. When that reduction reaches 3dB, the crossover point has been reached. All crossovers and filters are rated at this point — also called the half-power point.
There are two main filter types. A high pass allows high frequencies while restricting low frequencies. A low pass passes low frequencies while restricting high frequencies. Combining both in series creates a band pass circuit that allows a specific band of frequencies through while restricting both extremes.
Slopes
Slope describes the rate at which a filter reduces power beyond the cutoff point. A 6dB per octave filter reduces power by 6dB for every octave from the cutoff. A 12dB per octave filter reduces twice as fast. An 18dB per octave filter three times as fast.
Example: a 6dB per octave low pass filter with a cutoff of 200Hz would reduce power by 9dB (6 + 3) at 400Hz and by another 6dB at 800Hz.
If all frequency bands are at the same acoustical level and speakers are well within their response range, slope matters less. When one section is significantly louder than an adjoining section, or a speaker is operating close to its maximum range, a steeper slope becomes important.
Diagram 3 shows what happens when a low frequency section is much louder than the adjoining section. The result is a broad peak in combined acoustical output at and above the crossover area. The crossover point, slope, or both can be adjusted to correct this.
If a crossover point is close to the edge of a speaker's response range, the speaker may distort near the crossover area — especially as power increases. A steeper slope can solve this by attenuating those frequencies more aggressively. As power is increased, the practical range of a speaker compresses, so crossover points and slopes that work at 50W may not be adequate at 100W.
6dB per octave passive filters are the most common in mobile audio and work well when crossover points are within the speakers' comfortable range. They also cost half as much as 12dB per octave passives and one third the cost of 18dB. For tweeters specifically, an 18dB per octave passive is worth considering — the steeper slope provides additional protection for a small, delicate driver.
Electronic Crossovers
An electronic (active) crossover operates at preamp level, limiting frequencies before the signal reaches the amplifier. It is a powered circuit that is independent of speaker impedance and creates no appreciable signal loss.
Active crossovers have filters built into the circuit board. Crossover points are changed by turning dials, switching frequency modules, or swapping fixed crossovers. Most active crossovers are 12dB per octave, though 18dB and higher are available.
A crossover with no frequency adjustment is fixed frequency. One that allows changes through a switch, knob, or module is adjustable or variable. Fixed units cost less; variable units offer flexibility with a single model.
Most electronic crossovers include individual output level controls for each channel, allowing gain matching for all amplifiers at one location. Some also allow the high pass and low pass filters to be set independently, enabling precise tuning of acoustic peaks or valleys near the crossover frequencies.
A key advantage of electronic crossovers is the ability to dedicate a separate amplifier to low frequencies. Amplifying bass requires more power than high frequencies. When an amplifier nears peak output, clipping can occur — and a clipping signal can destroy tweeters and small-voice-coil drivers. Separating low-frequency amplification lets the total system play louder and with lower distortion across all drivers.
Passive Crossovers
A passive crossover filter uses only coils (inductors) and capacitors. A 6dB per octave low pass is simply a coil in series on the speaker lead. A 12dB per octave adds a capacitor in parallel. An 18dB per octave adds a second coil in series. High pass circuits are built the same way with coils and capacitors swapped.
One important concept: crossovers affect impedance as well as frequency response. A 4Ω tweeter and a 4Ω woofer with a proper crossover present a 4Ω load — not 2Ω — even though the drivers are in parallel. The crossover network changes the impedance relationship.
Low Pass Circuits (Diagrams 4–6):
High Pass Circuits (Diagrams 7–9):
Band Pass Circuits (Diagrams 10–12):
Attenuation
High frequency drivers are commonly more efficient than low frequency drivers, creating a need to adjust levels for a uniform overall frequency response. L-pads are best suited for use in crossover circuits because they do not change the resistive load presented to the crossover.
Zobel Network
All voice coil drivers exhibit rising impedance caused by voice coil inductive reactance. For crossover circuits to operate correctly, this rising impedance must be equalized. The CR circuit shown in Diagram 14 accomplishes this. It can also be used on tweeter domes — not for network operation, but to eliminate harshness and ensure accurate application of L-type shelving networks.
Notch Filter
The notch filter dampens and eliminates the effects of driver resonance on crossover networks. When a driver has an undamped resonance peak located less than two octaves from a high pass crossover point, this circuit significantly improves driver performance. It is particularly useful for tweeter domes, midrange domes, and cone-type midrange drivers whose enclosure resonance is above 200Hz.
