12 Volt Encyclopedia · Volume VIII · Article 5
Car Audio Crossovers Explained: HPF, LPF, Band-Pass and Subsonic Filters
A crossover assigns frequency ranges; it is not a wall that instantly stops sound. Proper filtering protects drivers, reduces distortion and creates controlled overlap so adjacent speakers behave like one system.
High-pass filters
An HPF allows frequencies above its selected region to pass while progressively reducing lower frequencies. It protects tweeters and small speakers from excursion and power they cannot handle.
Low-pass filters
An LPF allows lower frequencies through and attenuates higher frequencies. Subwoofer LPFs reduce localization and prevent upper bass or midrange from reaching a driver not intended to reproduce it.
Band-pass operation
Combining an HPF and LPF creates a band-pass. A midbass might be high-passed near 70 Hz and low-passed near 300 Hz. A midrange might operate between several hundred hertz and several kilohertz.
Subsonic or infrasonic filters
A subsonic filter is a high-pass filter applied to a subwoofer channel. It limits very low frequencies that can waste excursion or unload a ported enclosure below tuning. The correct setting depends on enclosure alignment, driver and program material.
Starting points, not commandments
A subwoofer may begin near 70–90 Hz LPF; a door woofer may begin around 70–100 Hz HPF; a tweeter may require several kilohertz. These are examples only. Manufacturer limits, slopes, output goals and measurements govern final choices.
Electrical versus acoustic crossover
The DSP setting is the electrical filter. The driver’s natural response, enclosure and cabin add their own behavior. The measured acoustic slope may therefore be steeper, shallower or irregular compared with the menu setting.
Overlap and gaps
Too much overlap can create peaks or phase-dependent cancellation. Too little creates a response hole and makes each driver audible as a separate source. Level, polarity, delay and slope must be evaluated together.
Safe adjustment sequence
Begin with conservative protection, low level and minimal EQ. Verify every channel, measure individual drivers, align adjacent bands, then increase output gradually while watching excursion and distortion.
Installation and testing checklist
| Checkpoint | What to verify |
|---|---|
| Compatibility | Source output type, receiving input type and allowable voltage range match. |
| Routing | Signal wiring is protected, secured and separated from likely interference sources. |
| Termination | Connectors fit firmly without excessive force; no cable weight hangs from a jack. |
| Signal | Every channel is identified, clean and unclipped at the intended source level. |
| Final test | Polarity, channel assignment, noise floor and operation are verified with the vehicle running. |
Frequently asked questions
Is an 80 Hz crossover a brick wall?
No. Attenuation increases progressively beyond the selected point.
Should subwoofer LPF and door HPF use the same number?
They can, but acoustic response may require different frequencies or slopes.
Is subsonic filtering only for ported boxes?
It is especially useful there, but sealed systems may also benefit from excursion protection.
Can crossovers increase power handling?
They can reduce energy outside a driver’s safe range, improving practical headroom.
Should the amplifier and DSP filters both be active?
Usually one intentional filter set is easier to control unless combined slopes are specifically planned.