What is a Speaker Crossover And How It Works

What is a Speaker Crossover And How It Works

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A crossover is an electronic unit or digital processor which directs different frequencies of an audio signal to different drivers in a loudspeaker system. 

Music has both high frequencies and low frequencies. A woman’s voice is usually at a higher frequency, for example than a man’s voice. A bass guitar or a big drum can reach lower frequencies than any human voice, and a flute or piccolo plays at higher frequencies.

And speaker systems usually have different drivers to best play these different frequencies – larger woofers for playing the low frequencies in music, midrange drivers for playing much of the middle registers, and small tweeters for playing the high frequencies.

A crossover splits the music into two or three different signals (sometimes more, depending on how many different drivers a speaker system has), and sends the low frequency, or bass, signal to the woofer, the high frequency, or treble, signal to the tweeter and so on.

What is a Crossover and Why Is It Important?

A crossover is a very important part of any stereo or home theater system, and allows the music, dialog  or other sounds to be reproduced more clearly and with much greater fidelity.

Again, to state it as simply as possible, the crossover splits audio information (music, movie soundtrack, etc) into different signals, based on frequency, and sends those signals to the correct speaker driver – to the tweeter for high frequencies and the woofer for low frequencies.

There are different types of crossovers in use, although despite technological developments the most basic and oldest crossover design remains today by far the most popular and widely used – the good ol’ passive, in-speaker electronic crossover. Another extremely popular crossover, which many people have without even realizing it, is the crossover which sends only deep bass music to their home theater’s subwoofer. Not just home theater systems, either – many audiophile or good car stereo systems might also have a subwoofer and crossover.

Let’s look in more detail at the different types of audio crossovers, how they differ and what they are used for. Crossovers can be divided into the following categories:

Active versus Passive Crossovers

An active crossover is often a separate component, while passive crossovers generally are built right into the speaker’s box. Active crossovers go between a preamplifier and two or more amplifiers – before the amplifier – and passive crossovers instead receive the amplified signal as it comes into the speaker – after the amplifier.

Active Crossovers can have strong advantages over passive crossovers, including better sound quality, greater flexibility and more adjustability. They can be expensive, though, require AC power, and have to be set up and configured properly.

Again, passive crossovers are far more popular. This is largely due to the fact that they are already built into almost every speaker system. As such, you don’t have to purchase anything separate, or even think about the crossover – just plug the speakers in and away you go.

For high end stereo or home theater systems, though, active crossovers allow the audiophile or videophile to use separate amplifiers for a subwoofer (the most common use), to fine tune the crossover frequencies and levels for differences in rooms, speakers and electronics, and to generally enjoy higher quality reproduction of music, movie soundtracks or any other playback sounds.

Active crossovers are also more common in higher end car audio systems, where there are often different amplifiers and different speakers for high and low frequencies.

Electronic versus Digital Crossovers

For decades now, the most common type of crossover has used electronic components – mainly capacitors and inductors – to divide the different audio frequencies into different signals, and today this electronic, or analog, crossover is still by far the most common approach.

It is relatively easy, though, to program digital code which will divide a digital music signal into different frequencies, and this type of DSP (digital signal processing) crossover is already in use in some high end audiophile systems which use primarily, or exclusively, digital music sources. Digital crossovers can also be found in home theater receivers, for instance in providing a separate output to connect to a subwoofer.

Digital crossovers, however, can only work in the digital realm, which means that if you are using a turntable, or any other analog music source like open reel or cassette tapes, you would need to convert that music to a digital signal before the crossover would work – complicated, and this means you’d lose that “analog sound” advantage.

2-Way versus 3-Way Crossovers

This distinction is pretty easy to understand. If you have a speaker with only two individual speaker drivers in the box – a tweeter and a woofer – this means that the speaker will have a two-way crossover, which separates the incoming music into two signals: high frequencies for the tweeter, and low frequencies for the woofer.

If your speaker has three speaker drivers – a tweeter, a midrange and a woofer, this speaker will have a three-way crossover, and will create a third signal with only the middle frequencies, and feed that signal to the midrange drive.

Indeed, though 2-way and 3-way crossovers are the most common, speakers can have more than three drivers – a six way system, for example, may have a sub-woofer, a woofer, a mid-bass driver, a midrange, a tweeter and a super-tweeter, and would then need a six-way crossover – and some speaker systems have even more drivers than that!

Read more here: 2-Way vs. 3-Way Speakers: What’s the difference?

Size Matters – Big versus Small

Let’s say you have just purchased a subwoofer, active crossover and amplifier for your home or car stereo or your home theater system. When you are setting up your new crossover, and considering how big and powerful your new subwoofer and amplifier are, the temptation is often strong to adjust the crossover for a very big range of frequencies – up to 500 or 1,000 Hz, for instance, which is well into the midrange frequencies. After all, the subwoofer is so big and nice, and there is so much power, that you might as well use it for as much of the music as possible, right?

But this would be a mistake, and will really hurt the performance of the subwoofer and the overall sound of the system. Subwoofers are designed only for the deepest frequencies, and no matter how good or expensive a subwoofer is, it will never sound good for higher frequencies. The following guide might help, showing recommended crossover frequencies for different speaker/subwoofer combinations:

  • Small bookshelf or satellite speakers (4-5 inch woofer): 150-200 Hz
  • Medium sized bookshelf speakers (6-8 inch woofer): 75-100 Hz
  • Large floor-standing speakers (10-12 inch woofer): 60-80 Hz
  • Very Large floor-standers (15+ inch woofer): 40-60 Hz

The manufacturer of your subwoofer should also offer specific recommendations for crossover frequencies best for that unit and for various combinations.

How Exactly does a Crossover Work?

While it is pretty easy to understand the basics of what a crossover does, the technical aspects of its operation are a bit more complex, but still very interesting. Let’s look in more detail at a basic two-way passive crossover.

As we said before, a crossover will send high frequencies to the tweeter, a range of frequencies which that tweeter can most effectively and safely reproduce, and it will do the same with the woofer, sending only those deeper frequencies the woofer can handle with good sound and no damage.

To separate the high frequencies from the overall musical signal, the crossover will use what is called a high-pass filter, which passes only high frequencies and holds back everything else. 

And to separate the low frequencies from the overall musical signal, the crossover will use what is called a low-pass filter, which allows only low frequencies through and holds back everything higher.

There are two main specifications involved – the first is the frequency below which the pass filter will impede, or cut off, the musical signal. The second is the slope, or the speed at which the musical signal rolls off below the cut-off frequency.

If the slope is too sharp, and absolutely nothing below (high-pass) or above (low-pass) the crossover frequency is allowed though, the transition between the tweeter and woofer will not be smooth, and the speaker won’t sound good. If the slope is too shallow, and too much is allowed through beyond the crossover frequency, the tweeter will work too hard trying to reproduce low frequencies, or the woofer trying to reproduce high frequencies, and again the speaker’s sound will suffer.

A crossover might also control or adjust other aspects of the audio signal as well, such as phase, time and overall level (volume), in order to make the speaker sound smoother, to lower distortion, or to make the stereo speakers or surround speakers have better imaging.

What Makes a Crossover?

Even though the job a crossover does is extremely important, even crucial, to the sound of a speaker system, and it seems to do many different things – pass filtering, frequency slope roll-off, phase and time correction, level control and more – the components of a crossover are quite simple. A basic passive or active electronic crossover may include any of these items:

  • Capacitor – simply put, a capacitor resists the flow of electricity, like in the musical signal, and the resistance decreases as the frequency gets higher. This makes a capacitor the main component of a high-pass filter, as it will let more high frequency signal through to the tweeter.
  • Inductor – basically the same thing as a capacitor, but in reverse, the resistance increasing as the frequency gets higher. This makes a inductor the main component of a low-pass filter, as it will let more low frequency signal through to the woofer.
  • Resistor – a resistor resists electricity across the entire bandwidth of an electrical signal, regardless of frequency, and so resistors are used to equalize the amount of musical signal going to the two (or more) drivers, the woofer and tweeter, making the overall sound more balanced.

Types of Car Audio Crossovers and How they Work

As in home stereo and home theater systems, the crossovers used in car stereo systems can easily be divided into two basic types passive and active. The difference is that with higher end car stereos –almost anything above and beyond the basic factory installs – active crossover systems are far more common than they are in home stereo systems.

If you have car stereo speakers with both larger woofer drivers and smaller tweeter and/or midrange drivers in the same speaker, that speaker will have a simple passive crossover built in which directs the appropriate frequencies to the correct drivers.

If you have separate speakers, like woofers in the back deck and tweeters up front, the system might well have some sort of active crossover, which directs a pre-amplified (line level) musical signal to different amplifiers, depending on frequency, and those will send the amplified musical signal to the appropriate speaker.

And if you are trying to decide what type of car stereo system to get, and plan to install it yourself, in general active crossovers with multiple amplifiers have better sound – sometimes a lot better – but are more expensive and more difficult and time-consuming to install. Systems with a single amplifier and a passive crossover are much easier to install, and often sound very good, but perhaps not on the level of a full-blown multi-amp system.

How to Choose the best Crossover for You?

Although this is not a buyer’s guide, I’d still love to mention a couple of extremely good crossovers of various types, units which offer superb sound, flexibility in adjustment, excellent build quality and reliability and great value for the money.

For a car audio system, one of the best active crossover systems we’ve found, with lots of control and flexibility and amazing sound quality for the money is the BOSS Audio Systems BX55 Three-Way active crossover. It has level and frequency controls for front and rear speakers and even more controls for the subwoofer, making the BOSS BX55 2 a perfect choice for any combination of speakers and amplifiers, any car and any type of music.

If you want a step up in sound quality, and a unit which will be the equal of even the best amps and speakers, you can’t do much better than the Clarion MCD360 Three-Way active crossover, with its legendary sound quality and reliability and a full array of controls and adjustments.

For passive crossovers for your car stereo, keep in mind that any multi-driver speaker will have a crossover built in. Still, you may want to install separate tweeters, midrange drivers and woofers in various places in your car and run them all from your head unit (the main stereo/amplifier built into the dash). For this, a wonderful choice is the Audiopipe CRX-203 Two-Way or the Audiopipe CRX-303 Three-Way passive crossover, which are super-easy to install and have really high power handling capacity – and they sound great! 

You might also want to check out the wonderful Audiopipe subwoofers, which have really clear, powerful deep bass and ultra low distortion – a truly high-end product.

For home audio systems or commercial set-ups, the  Behringer Super-X Pro CX3400 Two-Way/Three-Way professional crossover is a really superb unit, and can be used with almost any amplifiers (as long as they have XLR connectors) and speakers, and is a great choice for both home and professional use. 

And for an excellent bargain alternative, still with professional quality construction and great sound, the Technical Pro XO3 Three-Way crossover also offers XLR connectors and a full range of controls, as well as beautiful sound quality, clear, dynamic and detailed – another great choice for home or commercial sound systems.

Why Build a Crossover?

You may be wondering if you should build your own crossover from components, or simply purchase one of the many available online, which are often very inexpensive.

If you are designing your own speakers, and need to match the crossover frequencies to the specific speaker drivers and cabinet you are using, it might make more sense to build your own passive crossover, and it is a fairly straight-forward project. 

If, however, you are looking for an active crossover for your home stereo or home theater system, or for your car stereo system, it almost certainly makes much more sense to purchase one of the many excellent active crossovers already available, like those listed above in our recommendations for best active crossovers. They can be adjusted in many different ways to best suit your speakers and amplifiers, your room’s or car’s acoustics, the size and types of amplifiers you are using and other factors – including your own listening preference.

The same is true of passive crossovers for your car audio system, and the units listed above work well with pretty much any car, amplifiers and speakers, and are affordable and a snap to install. We love a good DIY project as much as anybody, but when great units are this inexpensive, this good and this easy, it’s a no-brainer!

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