Subwoofer Crossovers in Live Sound: Mains, Subs, and Alignment

TL;DR — A PA crossover sends lower frequencies toward subwoofers and higher frequencies toward the main loudspeakers. The crossover is a transition region, not a hard dividing line. Frequency, filter slope, polarity, delay, level, loudspeaker response, placement, and manufacturer processing all affect the result. Start with approved presets, then verify the deployed system. Put the system owner, processing boundary, outputs, presets, and acceptance process in the technical rider instead of prescribing one universal frequency.

What a PA crossover does

A crossover divides an audio signal into frequency bands for different loudspeaker sections. In a typical mains-and-subs system, a low-pass filter reduces higher-frequency content sent to the subwoofers, while a high-pass filter reduces lower-frequency content sent to the main loudspeakers.

The filters do not create a brick wall. Both systems normally contribute through a crossover region. Their acoustic outputs add according to level, phase, timing, polarity, placement, and the response of the cabinets and room. Two processor outputs with matching frequency labels can still combine poorly at the audience position.

A crossover may live in an active loudspeaker, amplifier, system processor, console matrix path, or a combination of devices. Identify the real processing boundary before changing anything.

Understand the settings

Do not copy a filter name and frequency from another rig. A setting that is correct for one matched product family, preset, or deployment may be wrong for another.

Why there is no universal crossover frequency

The useful transition depends on the main loudspeaker's low-frequency capability, the subwoofer's upper response, available amplifier and driver headroom, required level, cabinet placement, manufacturer presets, and the program material. The desired audience result matters more than a remembered number.

Use current manufacturer documentation for the exact cabinet, amplifier, processing, and configuration. When a matched preset exists, treat it as the starting system state. Do not bypass protection filters or alter protected parameters without the authority and competence to manage the consequences.

The crossover also cannot fix insufficient inventory. If the mains or subs cannot deliver the required coverage or output within their limits, moving the filter merely transfers stress between bands.

Align the acoustic result, not just the processor screen

Check polarity first

Confirm wiring, amplifier channels, presets, and polarity throughout the signal path. A polarity error can create a broad cancellation around the transition and may be mistaken for a frequency or level problem. The polarity versus phase guide explains why the terms are related but not interchangeable.

Account for time and phase

Mains and subs can have different acoustic centers, physical locations, processing latency, and phase responses. Delay may improve alignment at a chosen reference region, but a single delay value cannot make every seat identical. Moving a subwoofer, changing a preset, or changing the crossover filter can invalidate the previous alignment.

Balance level after the bands combine

Listen to and measure the mains alone, subs alone, and both together. Evaluate the overlap, not just isolated traces. Set the relative level for the production goal, noise limits, and available headroom. Boosting subs at the console and raising the system sub feed are different decisions with different ownership.

Separate console and system responsibilities

The system processor usually owns loudspeaker protection, crossover filters, cabinet equalization, band alignment, and zone timing. The console may provide left/right, mono, auxiliary, or matrix feeds to the system. A visiting engineer may be allowed to adjust a house-approved tonal contour or sub level while protected processing remains locked.

Agree on the boundary during the advance:

• who supplies and operates the PA system;
• which console outputs feed mains, subs, fills, and delays;
• whether subs receive a derived main feed or a separate mix destination;
• which processor parameters are locked;
• who can change system level or tonal balance;
• how the system is restored after a guest file or fault.

The aux versus matrix guide helps distinguish destination feeds. The PA system rider guide covers the larger handoff.

A practical setup and verification sequence

1. Confirm exact cabinet, amplifier, processor, cabling, and preset versions.
2. Inspect loudspeaker placement, orientation, wiring, and output assignments.
3. Load the approved manufacturer starting preset and protection settings.
4. Verify each band and enclosure group independently at a safe level.
5. Check polarity and signal routing before applying corrective EQ.
6. Measure timing and phase through the intended crossover region at useful reference positions.
7. Apply authorized delay, polarity, and level changes, then remeasure the combined response.
8. Listen with suitable program material across representative audience areas.
9. Confirm limiting, headroom, noise constraints, and fail-safe routing.
10. Save the final processor state and record who approved it.

Temperature, audience conditions, boundary effects, and listening position change the observed result. Verification must match the real deployment rather than a generic calculator.

Document the crossover and system handoff

Avoid writing “crossover at 80 Hz” as the entire requirement. That leaves filter type, slope, protection, delay, polarity, level, placement, preset, and ownership undefined.

Common crossover mistakes

• treating a processor frequency as proof of the acoustic crossover;
• copying settings between unmatched loudspeaker models;
• changing protection or preset filters to chase a room problem;
• correcting a polarity or timing fault with large EQ boosts;
• aligning at one point without checking the intended audience area;
• feeding subs separately without documenting routing and mute behavior;
• letting both the guest engineer and system technician change the same boundary;
• saving no final processor file or change record.

FAQ

What does a crossover do in a PA system?

It divides audio into frequency bands for different loudspeaker sections. In a mains-and-subs system, low-pass and high-pass filters create a transition region where the acoustic outputs must combine correctly.

What crossover frequency should I use for a subwoofer?

Use the current manufacturer preset or guidance for the exact mains, subs, amplification, processing, and deployment as the starting point. Verify the acoustic result; there is no universal frequency for every PA.

Should mains and subs overlap?

They normally overlap through a transition region because real filters attenuate progressively. The goal is controlled acoustic summation, not an imaginary gap or perfectly sharp handoff.

How do you align subwoofers with main speakers?

Confirm routing and polarity, then evaluate timing, phase, and level through the crossover region using appropriate measurement and listening positions. Apply only authorized processor changes and verify the combined result.

What crossover information belongs in a technical rider?

Document the exact system and presets, signal handoff, sub-feed method, output destinations, processing boundary, alignment owner, verification process, saved state, and recovery plan. Include a specific frequency only when the approved system design requires it.

Keep the system decision attached to the show

Build the technical rider in Techrider.live, record the PA handoff and responsible system role, and keep the approved configuration with the current show details. A clear ownership boundary prevents last-minute crossover changes from becoming an undocumented experiment.