Crossovers: Signal Traffic Cops In A Car Audio System

Not every speaker can play every frequency: Tweeters can’t woof and subwoofers can’t tweet. The signal that comes out of a car audio head unit is full range, meaning that it contains the full range of frequencies in a given piece of music or program material, from the lowest lows to the highest highs. Because a high-end car audio system usually consists of specialized speakers — subwoofers, midranges, and tweeters — that reproduce certain frequencies, there has to be a way for those frequencies to get separated and sent out to the appropriate speakers. That’s the job of crossovers, which act like signal traffic cops in a car audio system.

Crossovers come in two types, active and passive.

Get active

Active or electronic crossovers are always placed before the amplifiers in the car audio signal chain, and divide the audio signal when it’s still at line level or un-amplified (see Figure 17-7). Many amplifiers have an active crossover built in so you don’t need to add a separate component. They’re called active because electronic crossovers require a power source to work.

Figure 17-7:

An Audio- Control electronic crossover.

 

 

Passive resistance

Passive crossovers come after the amplifiers (but before the speakers) and filter an amplified signal. Passive crossovers are typically included with speak¬ers that offer more than one driver, such as component sets (see Figure 17-8).

Although more expensive coaxials come with outboard or separate passive crossovers, more typically, a small and simple filter called a capacitor blocks the low frequencies going to the tweeters and the midrange gets a full-range signal. The passive crossover networks that come with component speakers, on the other hand, include components called inductors that block high frequencies from the midrange. Many passive crossover networks allow adjustment of the crossover point and slope, which I cover in the following section.

Points, slopes, and orders

The crossover point refers to the frequency at which a signal begins to be attenuated or reduced by the crossover. For example, 75 Hz is a common crossover point for subwoofers, meaning that the signal begins to be reduced at above 75 Hz.

The crossover slope refers to the rate at which the signal is reduced or rolled off beyond the crossover point. This always occurs in multiples of 6 decibels (dB), and the higher the decibels, the steeper the slope (see Figure 17-9). Slopes are also referred to as orders: A first-order crossover has a 6-dB-per- octave slope, a second-order a 12-dB-per-octave slope, and so on.

Figure 17-8:

An Infinity component speaker set with passive crossovers.

Courtesy of Audio Control

Many car audio systems use an electronic crossover to divide the full-range signal from the head unit to individual amps. In other systems, a full-range signal is sent to each amp that has a built-in crossover. Usually a component speaker set’s passive crossovers further divide the signal between the midranges and tweeters. Very rarely are active crossovers used to entirely filter the signals in a system.

It’s almost always preferable to use the passive crossovers that are included with a set of speakers because they were designed specifically for the speaker’s response characteristics.

Crossover types

Crossovers come in three basic configurations:

Low-pass crossovers (also called subwoofer crossovers) filter out all but low frequencies for subwoofers.

High-pass crossovers filter out all but high frequencies for tweeters.

Bandpass crossovers filter out all but a predetermined band of frequencies, usually in the midrange area.

Electronic crossovers can include all three types of crossovers, whereas passive crossovers usually just include a low-pass or high-pass section.