Car Audio Crossover Settings: The Complete Technical Guide

Published March 26, 2026 | Engineering reference for installers and enthusiasts

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What is a Crossover? And Why It's Critical

A crossover is an electronic filter that routes specific frequencies to specific speakers. It prevents your tweeter from playing bass (it will blow) and your subwoofer from playing vocals (it can't).

In car audio DSP, crossovers are active (digital, software-based) rather than passive (analog, built into speakers). This gives you far more control: you can adjust frequency, slope, and phase independently for each speaker.

Why Crossover Design Matters

  • Driver protection: A tweeter rated for 250W has a maximum frequency it can handle. A crossover prevents bass energy from overexciting the tweeter cone.
  • Output optimization: Every driver has a frequency range where it excels. A crossover ensures it only plays those frequencies.
  • Phase alignment: When two drivers play adjacent frequencies, their phase relationship at the crossover point determines whether they sum flat (good) or have a peak/dip (bad).
  • Subjective blending: A poorly chosen crossover frequency creates an audible "seam" where the sound changes character. A well-chosen one is transparent.

Recommended Crossover Frequencies by Driver Configuration

Standard 3-Way System (Tweeter + Midwoofer + Subwoofer)

Driver Type Typical Crossover Point Acceptable Range Rationale
Tweeter HPF (high-pass filter) 3–4 kHz 2.5–5 kHz Protects tweeter. Lower = more detail but more stress on tweeter. Higher = more bass but slower response.
Midwoofer LPF (low-pass filter) 3–4 kHz 2.5–5 kHz Should match tweeter HPF for flat summation. Prevents midwoofer from playing too high frequencies where it loses directivity.
Subwoofer LPF 80 Hz 60–120 Hz Industry standard. Varies by room and subwoofer quality. Lower for smaller rooms, higher for larger cabins.
Midbass HPF (if present) 80 Hz 60–120 Hz Matches sub crossover. Prevents midbass from producing subsonic content.

Tweeter Crossover Frequency Selection Logic

The tweeter high-pass filter frequency depends on three factors:

1. Tweeter Resonance (Fs)

  • Check your tweeter's spec sheet for Fs (resonant frequency)
  • Rule: HPF frequency should be AT LEAST 1.5× the Fs
  • Example: Tweeter with Fs = 1.8 kHz → HPF should be ≥ 2.7 kHz (aim for 3–4 kHz)

2. Tweeter Size

  • 1.0–1.5" tweeter: 3–4 kHz typical (fragile, needs protection)
  • 0.75–1.0" tweeter: 4–5 kHz acceptable (robust)
  • 0.5–0.75" tweeter: 5–6 kHz acceptable (very small, can play high)

3. Blending Preference

  • Lower frequency (2.5–3 kHz) = seamless blend, tweeter plays more mid-detail, but higher stress
  • Higher frequency (4–5 kHz) = cleaner separation, tweeter less stressed, but might hear a "seam" in upper midrange
Professional Default: Most professional installers use 3.5–4 kHz for tweeters and 80 Hz for subwoofers because these frequencies balance protection, blending, and industry standardization.

Subwoofer Crossover Frequency Selection Logic

The subwoofer low-pass filter (LPF) frequency depends on room size and subwoofer quality:

Scenario Recommended Sub LPF Midbass HPF Why
Compact car (sedan, coupe) 80–100 Hz 80–100 Hz Smaller cabin = less low-frequency absorption. Higher crossover keeps midbass from trying to produce 40 Hz.
Large SUV / truck 60–80 Hz 60–80 Hz Larger cavity = flatter bass response. Lower crossover allows sub to integrate better.
Small sealed sub (8" or less) 100–120 Hz 100–120 Hz Small sealed subs are best in midbass range. Higher crossover plays to their strength.
Large ported sub (12"+ ported) 50–80 Hz 50–80 Hz Ported subs extend deeper. Lower crossover utilizes the extension.

Understanding Slopes & Filter Types

Slope (measured in dB/octave) controls how steeply the crossover attenuates a driver once it crosses the crossover point.

Common Slopes

Slope (dB/octave) Order Attenuation 1 Octave Out Use Case Character
6 dB/oct 1st -6 dB Rare; extreme gentleness Very wide blend zone; minimal protection
12 dB/oct 2nd -12 dB OEM systems, budget DSPs Gentle blend; less driver protection
18 dB/oct 3rd -18 dB Specialty applications (motorcycle, marine) Balanced protection and blending
24 dB/oct 4th (LR4) -24 dB PROFESSIONAL STANDARD for car audio Excellent protection; transparent summation
36 dB/oct 6th -36 dB Fragile tweeters; competition systems Maximum protection; phase complications
48 dB/oct 8th (LR8) -48 dB Multi-way competition; extreme protection Brick-wall separation; requires expert phase management

Filter Types: Butterworth vs. Linkwitz-Riley

Linkwitz-Riley (LR) is the professional standard for car audio because of one critical property:

When a tweeter HPF and midwoofer LPF are both set to the same frequency using LR4 (24 dB/oct), they sum to exactly 0 dB (perfectly flat) at the crossover point.

Why this matters:

  • No peak or dip at the crossover frequency
  • Both drivers are back in-phase (360° phase rotation = 0° relative phase)
  • Creates seamless, transparent blending

Butterworth summing: High-pass + Low-pass at same frequency = +3 dB peak at the crossover. This is usually undesirable (causes a "hump" in the response).

Phase Inversion Rule: When using 24 dB/octave (LR4), the combined phase shift is exactly 360 degrees—the drivers are back in-phase. When using 12 dB/octave (LR2 or Butterworth 2nd), the combined phase is 180 degrees—the drivers are out of phase. In this case, you MUST invert one driver's polarity to achieve flat summation.

The Mathematics Behind Filter Selection

Phase Rotation by Slope

Every filter order adds 90 degrees of phase rotation per order:

Filter Type Phase Shift per Section Combined (HPF + LPF) Driver Relationship
Butterworth 1st (6 dB/oct) 45° 90° (if both 1st order) Neither in nor out of phase; complicated
Butterworth 2nd (12 dB/oct) 90° 180° (if both 2nd order) OUT OF PHASE — MUST invert one driver
LR2 (12 dB/oct) 90° per cascaded section 180° total OUT OF PHASE — MUST invert one driver
Butterworth 4th (24 dB/oct) 360° 720° = 0° (full rotation) IN PHASE — no inversion needed
LR4 (24 dB/oct) 360° 720° = 0° IN PHASE — flat summation, transparent

This is why LR4 (24 dB/octave) is the default: the drivers naturally come back in-phase without tricks.

Implementation & Practical Tuning Steps

Step 1: Set Crossover Frequencies

  1. Check tweeter Fs spec → HPF = 1.5× Fs or higher
  2. Use 3.5–4 kHz for typical tweeters (unless spec says otherwise)
  3. Use 80 Hz for sub LPF (adjust ±20 Hz for room size)
  4. Mirror sub LPF frequency to midbass HPF

Step 2: Select Slope & Filter Type

  1. Choose Linkwitz-Riley 4th order (24 dB/oct) as your default
  2. If tweeter is fragile (0.5–0.75"), use 36 dB/oct (LR6)
  3. If using LR2 (12 dB/oct), you MUST invert one driver's polarity

Step 3: Verify Polarity

  1. Use the null test procedure (below) to confirm drivers are in-phase
  2. If using LR4 (24 dB/oct), normal polarity for both
  3. If using LR2 or 12 dB/oct slopes, invert one

Step 4: Time Alignment

  1. Measure physical distances from driver's ear to each speaker
  2. Calculate delays: delay_ms = distance_inches ÷ 13.4
  3. Enter delays into DSP
  4. Run null test to verify alignment at crossover frequency

The Null Test Procedure: Gold Standard for Verification

A null test is the most accurate way to verify that two speakers are time-aligned and in-phase at a specific frequency.

Equipment Needed

  • DSP with polarity (phase) invert capability
  • Tone generator (in-car test signal or app)
  • SPL meter (optional; you can listen)

Procedure: Aligning Tweeter + Midwoofer

  1. Mute all speakers except tweeter and midwoofer
  2. Set playback to MONO (sum left and right channels)
  3. Generate a test tone at the crossover frequency (e.g., 3.5 kHz)
  4. Set the midwoofer LPF to the crossover frequency (3.5 kHz)
  5. Set the tweeter HPF to the same frequency (3.5 kHz)
  6. In the DSP, INVERT the phase of the tweeter (180° flip)
  7. Play the 3.5 kHz tone and adjust the tweeter DELAY upward in 0.5 ms increments
  8. Listen for the deepest null (quietest point) — a clean null drops 15–25 dB below the unaligned level. A shallow null (less than 10 dB) means a phase error remains that delay alone can't fix; use an all-pass filter to correct it
  9. When you find the deepest null, note the delay value (this is your tweeter delay relative to midwoofer)
  10. Restore the tweeter phase to normal (no inversion)
  11. Verify the SPL increases back to normal with the corrected delay (you've now achieved phase alignment)

Procedure: Aligning Sub + Midbass

Repeat the same steps but:

  • Crossover frequency: 80 Hz (typical sub crossover)
  • Mute tweeter and midrange
  • Enable sub and midbass
  • Generate 80 Hz tone
  • Invert sub phase, adjust sub delay, find deepest null
  • Restore sub phase and verify

Quick Null Test Reference

For each driver pair:

  1. Mute everything else
  2. Invert ONE driver's phase
  3. Generate tone at their crossover frequency
  4. Adjust the OTHER driver's delay until you get the quietest null
  5. Restore phase, verify with correct delay

Frequently Asked Questions

Should I use the same crossover frequency for tweeter HPF and midwoofer LPF?

Yes, for a 2-way system. Set both to 3.5–4 kHz. This ensures they blend smoothly at the handoff. Some installers use 4.5 kHz for tweeter HPF and 3.5 kHz for midwoofer LPF if they want a smoother midrange; this is acceptable but requires more careful time alignment.

What if my DSP only supports 12 dB/octave (2nd-order) slopes?

You can still get good results, but you MUST invert one driver's polarity. Set tweeter HPF and midwoofer LPF to the same frequency, invert the tweeter, and perform a null test to find the correct delay. The blend won't be quite as transparent as LR4, but it's acceptable.

Can I use different slopes on HPF vs LPF?

Generally, no. Matching slopes (both 24 dB/oct) ensure predictable summation. If you mix slopes (e.g., HPF 24 dB/oct + LPF 12 dB/oct), the summation becomes asymmetrical and unpredictable. Avoid it.

My tweeter has Fs = 2.2 kHz. Can I crossover at 3 kHz?

3 kHz is 1.36× the Fs, which is slightly low. Aim for 4–5 kHz instead (1.8–2.3× the Fs). At 3 kHz, you're too close to the tweeter's resonance and risk peaky response and poor damping. Higher is safer.

Why do I hear a "hole" in the midrange after setting crossovers?

This typically indicates a dip at the crossover point, caused by one of: (1) bad polarity (one driver inverted when it shouldn't be), (2) bad time alignment (drivers arriving at different times), or (3) too-steep slopes creating phase complications. Run a null test to verify alignment and polarity.

Is 80 Hz the right sub crossover for my car?

80 Hz is the industry standard and works for most cars. However: if your car is very small (compact car), try 100 Hz. If your car is very large (truck/SUV), try 60–70 Hz. If your sub is small (8" sealed), try 100 Hz. The null test will tell you the optimal frequency for your setup.

What's the difference between slope and steepness?

Slope and steepness are the same thing. A 24 dB/octave crossover means the signal is attenuated by 24 dB for every octave (doubling of frequency) you move away from the crossover point. An octave above 1 kHz crossover is 2 kHz; at 24 dB/oct, the output is -24 dB. Two octaves above (4 kHz) it's -48 dB.

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Related reading: Learn more about complete DSP tuning and motorcycle audio setup.