Audio Crossover Networks
A crossover network divides an audio signal into separate frequency bands that are sent to appropriate speaker drivers: woofers for low frequencies, midrange drivers for middle frequencies, and tweeters for high frequencies. Passive crossovers use inductors, capacitors, and resistors to create frequency-dependent filters without requiring external power.
The crossover frequency is the point where the output from both the low-pass and high-pass sections is equal, typically down 3 dB from the passband level. Proper crossover design is critical for smooth frequency response, proper phase alignment between drivers, and protecting tweeters from low-frequency power that could cause damage. The component values depend on the desired crossover frequency, speaker impedance, and filter order.
Crossover Formulas
Common Crossover Points
| Application | Frequency | Drivers Split |
|---|---|---|
| Subwoofer/Woofer | 80-120 Hz | Sub to main speakers |
| Woofer/Midrange | 300-800 Hz | Woofer to mid driver |
| Midrange/Tweeter | 2-5 kHz | Mid to tweeter |
| Full-range 2-way | 2-3 kHz | Woofer to tweeter |
Filter Types
- 1st Order (6 dB/octave): Simplest, one component per driver, excellent phase response, gentle slope.
- 2nd Order (12 dB/octave): Better out-of-band rejection, requires two components per driver, 180 degree phase shift.
- 3rd Order (18 dB/octave): Good balance of slope and phase behavior, three components per driver.
- 4th Order (24 dB/octave): Linkwitz-Riley alignment provides flat summed response, four components per driver.
Frequently Asked Questions
What crossover frequency should I use for a 2-way speaker?
A typical 2-way speaker crosses over between 2000-3500 Hz. The exact frequency depends on the woofer's ability to reproduce higher frequencies and the tweeter's ability to handle lower frequencies. Crossing over too low risks damaging the tweeter; crossing over too high may cause the woofer to beam (become directional) at higher frequencies.
Does speaker impedance stay constant with frequency?
No. Speaker impedance varies significantly with frequency, with peaks at resonance and rises at high frequencies due to voice coil inductance. The nominal impedance (4, 6, or 8 ohms) is an approximation. For more accurate crossover design, use impedance measurements or impedance compensation networks (Zobel networks) to flatten the impedance curve.
Are passive crossovers better than active crossovers?
Both have advantages. Passive crossovers are simpler (no external power needed), can be placed inside the speaker enclosure, and have no noise floor. Active crossovers offer more precise control, adjustable crossover points, steeper slopes, and eliminate power loss in passive components. Professional and high-end systems increasingly use active crossovers (DSP-based) for their flexibility and precision.