Car Audio Crossovers FAQ

Most of the time, the processing is done by the hardware and tuning software inside your system's receiver and amplifier. You don't really see each aspect of system-tuning as the work of separate and distinct processors like crossovers.

Here are some of the most common questions people have about how crossover systems work.

Crossover questions

Those are the questions. Now let's get to the answers.

Q: What are crossovers and what are they for?

A: A crossover is an electronic device that takes a single full-range input signal and separates it into high-, mid-, and low-range frequencies — so tweeters, midrange speakers, and subwoofers will each get fed only the range of frequencies they were designed to play. Sounds with frequencies outside each designated range are attenuated, turned down and blocked. You can think of a crossover network as an audio traffic controller, directing highs to your tweeters, midrange to your woofers, and low bass to your sub.

Every speaker system needs a crossover of some sort so the speakers can perform at their best ability. Component speaker systems come with either separate outboard passive crossover boxes or with the passive crossovers installed in-line or on the drivers. Every full-range, coaxial speaker – with its tweeter mounted in front of the woofer cone – already has a tiny passive crossover network built into it somewhere.

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The guts of an Audiofrog GS410C 2-way passive crossover

Q: What're "active" and "passive" crossover systems?

A: There are two kinds of crossovers: active and passive. The difference is in where they are in the path of the signal flow (i.e., before the amplifier or after it) and how they work.

Active crossover system

An active, or electronic crossover system is one in which every driver (tweeter, woofer, subwoofer) is run by a separate amp channel. The crossover needs power connections and goes between the receiver and the amp. It receivers low-level preamp signals and electronically divides the signal to send each band of frequencies in the right direction.

Active crossovers are usually adjustable (you can select the crossover points) and often have other features like bass boost circuits for subwoofers. Another bonus when using an electronic crossover is that you can independently control the relative volumes of all your different drivers.

Passive crossover system

Passive crossovers go between the amplifier and the drivers (tweeters, speakers, and subs). A passive crossover circuit is built with coils, capacitors, and resistors beefy enough to handle the high output power of the amplifier. This is the type of crossover system you'll find in component speaker systems.

Some passive crossovers include a "tweeter level" switch, which gives you some control over how loud the tweeter plays relative to the woofer (see further below). A disadvantage of using passive crossovers is that they filter out frequencies already amplified, creating extra heat and lowering speaker efficiency.

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These Crutchfield Bass Blockers eliminate signals below 600 Hz.

Q: What're "high-pass and low-pass filters"? How should I set or adjust them?

A: In car audio, electronic crossover filters — high-pass, low-pass, bandpass, and subsonic filters — divide full-range input signals into discrete frequency-limited ranges or "bands" of sound so each of the different kinds of speaker can play just what they were designed to handle and output cleanly without distorting — tweeters playing high frequencies only, subwoofers low bass, etc.

• A high-pass filter — sometimes referred to as a "low-cut" filter — removes the notes below the filter's frequency setting so tweeters and midrange drivers won't try to play bass notes that they weren't designed to play without distorting. Bass blockers are small inline high-pass filters used to protect tweeters.
• A low-pass, or "high-cut," filter removes the notes above the filter's frequency setting so mid-bass woofers and subwoofers won't try to play high notes that they weren't designed to play without distorting or muddying the pure bass tones.
• A bandpass filter employs both high-pass and low-pass filters so midrange speakers can play cleanly with the very high and very low notes kept away.
• A subsonic filter, sometimes called an infrasonic filter, is used to remove the extremely low bass notes (often below the range of human hearing) that the amplifier would otherwise waste energy trying to reproduce. Subsonic filters are also used with ported subwoofer enclosures to reduce the unwanted loud booming notes around the box's resonant frequency.

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How an active stereo 3-way crossover fits into a system

Q: Where and how do you install a crossover?

A: A speaker system's passive crossovers are often mounted between the woofer and the tweeter — usually nearer the woofer where there'll be more room — inside your vehicle's door or dash panel. Some installers, however, mount their system's passive crossovers next to the amplifier. Both methods work equally well — whichever seems more convenient to you will be your deciding factor.

Bass blocker crossovers work best installed right next to or even on the high-frequency drivers and tweeters they're meant to protect.

Active crossovers go between the stereo and the amplifier, so stand-alone crossovers like line output converters with low-pass outputs for subs go in the dash behind the receiver, if there's room, or next to the amplifier, where power and ground connections are easier to access.

For more information see our Crossover/equalizer installation guide.

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Q: How do I adjust my crossover when adding a subwoofer

A: A good way to set up your adjustable high- and low-pass crossover filters is to fine-tune by ear.

1. Start with an approximation, setting both filters at 100 Hz.
2. Play a favorite, well-known song or test track. With the sub turned off, adjust the high-pass control so no low-frequency bass sounds play through your midrange speakers and tweeters.
3. Then, with the "full-range" speakers turned down or off and the subwoofer turned back on, listen to the music coming out of your sub alone and slowly adjust the low-pass filter until all the high- and mid-frequency notes disappear. The low-pass filter not only eliminates the notes you don't want your subwoofer to play, it also acts like a tone control to capture the details of the deep bass. Filter out the cymbals, strings, vocals, and guitars — leave the bass guitar, synths, low tom-toms, and kick drums.
4. With all the speakers and subs playing, you can tweak the controls the small amounts that may be necessary to smooth out any high and low bumps in the audible response around the crossover frequencies.

Crossover filters control the amounts of signal going to different elements in a sound system, they should not be used as tone controls. That's the job of an equalizer or sound processor.

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Q: What's the selector switch on my passive crossover for?

A: Component speaker systems have passive crossovers. Some have tweeter level switches so that you can adjust the highs relative to the mids. Ideally, a crossover will put out the same level of signal, or volume, to the tweeter as it does to the woofer. But many people find that this sounds unbalanced and too shrill or bright. Because of this, many speaker manufacturers put tweeter attenuators in their crossovers.

The settings can include:

• 0 dB, when the tweeter level is the same as the woofer
• -3 dB, for a little attenuation
• sometimes -6 dB, for a lower tweeter level
• and sometimes even +3 dB, for those who like the highs brighter

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PAC LPA-E.4 LocPro Advanced line output converter with low-pass crossover and bass boost EQ.

Q: How do I choose the right crossover?

A: For protecting small speakers and tweeters from distorting and damaging bass notes, you can use a Bass Blocker, a type of high-pass crossover filter, made to connect inline with a small speaker.

For creating a signal suitable for a subwoofer system to use, you can get a Line Output Converter (LOC) with a built-in low-pass filtered subwoofer output. LOCs provide practical solutions for adding amplifiers to factory systems and usually include crossovers in their circuitry for protection and signal distribution purposes.

Read our article about line output converters for more information.

Crossovers built into your amplifier and digital signal processor. For the ultimate in crossover features, get a digital signal processor (DSP). The software will allow you to set up and control, sometimes automatically, virtually every aspect of frequency-band distribution and balancing good sound.

Check out our DSP buying guide and What does a DSP do? article for more details.

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Q: Have more questions?

A: Call Crutchfield. For sorting out which products with what features to get, give us a phone call and talk to one of our Advisors. We can help you choose the right gear and give you the right advice on how to install it in your vehicle.

For help installing and setting up your new system, you can call Crutchfield Tech Support for free advice on how to get everything running smoothly and sounding great.