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When you hear people talk about car audio, you often hear about head units, amplifiers, speakers, subwoofers, and even car audio batteries and stiffening capacitors. One thing you rarely hear mentioned is the crossover; more often than not, this is a weak link in many car stereo systems. There is no such thing as a car that makes 1000 HP and achieves 100 MPG. Neither is there a speaker that plays 20Hz and 20,000Hz. And yes, when it comes to AUDIO, especially the two-channel audiophile crow that insists that this is possible, but once you put a measuring mic to it, the claims fade away like Joe Biden’s Twitter followers.

The range of the human hearing (20hz – 20khz) is often broken down into three segments when talking about sound reproduction. Those are bass, midrange, and treble, and although there is an upper and lower sub-segment for each, we will stick to those for simplicity’s sake.

If your goal is to build a high-quality car stereo system, you need to consider having a dedicated speaker to play each segment. That is the fundamental principle of an audiophile sound system. You will need a speaker for the bass capable of playing from 20hz to 100hz, this type of speaker is often called a subwoofer. You will also need a speaker that plays from 100hz to 3000hz. That is called a midrange speaker. Last, you will need a speaker that plays from 3000hz to at least 20,000hz. I say at least because it is even better if it plays beyond that. One of the reasons we use choose speakers for each of these frequency ranges is that each can play a given frequency range with excellent efficiency. In other words, a tweeter is not good at playing bass. Neither is a subwoofer at playing treble.

The Role of the Car Audio Crossover

Here is an issue, and that is that the output of the amplifier, for the most part, is full-range (20hz – 20khz). In other words, bass, mid, and high frequencies are all present. Here is where the Car Audio Crossover comes in. The crossover’s role is to split these frequency segments so that each speaker gets the range they are most efficient at playing. That is not as easy as that since there are many different typologies and types. However, regardless of that, there are two things to keep in mind. Those are slope and frequency.

Crossover Slope: In audio, filters are measured in what is known as order. An order equal a 6bd attenuation from the crossover frequency. So if you have a first order or 6db filter at 100hz, the signal will be 6dbs down in amplitude at 200hz. A first-order filter is often not enough to get rid of damaging frequencies, which is why a second-order filter is the most common filter in car audio. Third-order 18db filters are not rare but seen only in tweeters for the most part. Fourth-order 24db crossover is seldom used in car stereo because of phasing issues present in a confined space such as a vehicle.

Crossover Frequency: As previously hinted, the crossover frequency is the frequency where the filter starts to attenuate. Unlike in-home audio, it is not rare to gap the crossover frequencies to attenuate resonance from hard surfaces found in the vehicle. What does this mean? Let’s say that your tweeter and midrange should be crossed at 3500hz. But after taking an RTA measurement, you have a hump center at 4000hz. This is happening because the vehicle is summing the output of the tweeter and midrange. A rookie will try to use eq. A pro would leave the tweeter at 3500hz and take the midrange down to like 3200~3300hz, or until the hump is gone.

Types of Car Audio Crossover

After determining the slope and crossover frequency needed, you must choose a crossover type. And even though audio crossovers come in all shapes, sizes, and prices, they can be grouped into two main categories that being active and passive. The passive crossover requires no power and consist of a combination of inductors, capacitors, and resistors to shape the frequency range a speaker sees from the amplifiers. On the other hand, an active crossover needs to be powered, making it more susceptible to power line noise and distortion. Active crossovers also come in a wide range of typologies. The most common are fixed, programmable, variable, and digital.

Passive Crossover: As the name describe, a passive crossover requires no power. It utilizes coil (inductors), capacitors, and resistors to cut or pass frequencies. Although these are my favorite type of crossover, they are the most hated and misunderstood by the industry. That is because few truly master the design aspect. I like passive crossover because they go beyond filtering frequencies. They are pretty helpful in addressing the electromechanical deficiencies of the driver. Another worth mentioning, passive crossovers can get expensive when you use quality components.

DPS (Digital Sound Processor): These are the latest and greatest, according to some. Yes, DSP crossovers have come a long way not only in affordability but power as well. Most car audio DSP processors nowadays can be controlled via a smartphone app and can take a live sample from the listening environment and make accurate and predictable adjustments. However, they fail to address any electromechanical deficiencies the speakers may show. I think they tend to sound thin, but if you are listening to MP3s, that would not matter anyway.

Variable Electronic Crossover: These are the crossover most people will recognize as an electronic crossovers. Although electronic, it does not utilize a microprocessor. Therefore, for the most part, all signal shaping and filtering are done in the analog domain. These crossovers use pots and sometimes a switch to multiply the frequency. They are very common and affordable, although, as with most things in life, you get what you pay for, so don’t go overly cheap!

Programmable Electronic Crossover: If you are eyeing a high-end electronic crossover, you will likely be looking at a programmable electronic crossover. Rockford Fosgate and AudioControl make the most common pieces in this category. Both companies also employ this technology on their amps. These crossovers use chips or resistors soldered to a small pc board. Programmable electronic crossovers offer better signal-to-noise ratio and broader frequency range and flexibility.

Fixed Electronic Crossover: These electronic crossovers have a fixed frequency and, more often than not, use switches to select highpass, lowpass, bandpass, or bypass. It is rare to find it as a stand-alone unit, but almost a standard option in many amplifiers regardless of cost. Choosing an amp with the right crossover is something to consider.