Audio 101 – Crossovers

In the last few articles, I talked about frequency response, on/off axis response, and the various types of drivers you’ll come across when designing speaker systems. Today, I bring some of that together and talk about crossovers. Crossovers are a critical part of a system design, and being able to understand the different types of crossovers as well as the functions they serve will be pivotal to your ability to create a solid system.

What does a crossover do?

To put it plainly, a crossover is an audio filter. The role of this audio filter is to filter audio frequencies that that are coming in from a signal source to a speaker driver so that the speaker driver only plays a specific range of frequencies. For example, we discussed tweeters as a type of driver in our last article. A crossover will filter all frequencies below a certain point that are coming in from the signal source in order to protect that tweeter. If our tweeter is capable of playing town to only 3,000Hz, the crossover will filter out all frequencies below 3,000Hz, so the tweeter does not play material from 20 to ~3,000Hz. Similarly, if our midbass driver in a 2-way system is only capable of playing up to 3,000Hz before distortion begins to rise, the crossover will filter out all frequencies above 3,000Hz.

 

Passive Crossovers

The two most common types of crossovers are passive and active. A passive crossover uses a set of electrical components that filters analog audio signals. This is done through the use of capacitors, inductors, and resistors. For the scope of this article, we will not be discussing exactly how each of those components affects the frequency response. The important thing to know is that they filter an analog signal. Generally speaking, passive crossovers are part of the speaker set you purchase. For example, if you buy a set of retail home theater speakers, you will find a crossover network comprised of the aforementioned components inside. If you buy a “component” speaker set for car audio purposes, you will be given a midbass driver, a tweeter, and a crossover box which also contains those crossover parts. In a typical system utilizing passive crossovers, the signal will start at the source (your TV, your phone, your Xbox, your CD player, etc.), then to your amplifier, then to your passive crossover, and then to each individual speaker.

The advantage to passive crossovers is that, at a beginner and intermediate level, they require no work from the designer. You simply hook up the home theater speaker to your amplifier or hook up your car audio component speaker set as instructed in the manual it comes with, and you’re ready to roll. The disadvantage to passive crossovers is that they are very limited in their ability to filter a given set of components, and they inherently make the system less efficient. In addition, you are at the mercy of the designer that put together your home speaker or component set and how well they understood the specific tweeter and midbass driver it is accompanied with. The vast majority of these crossover networks are “textbook” crossovers that don’t take into consideration the specific abilities of the drivers being used, but they get the job done, and many of them utilize cheap electrical components that cause sound quality to suffer.

The following are retail passive crossovers bundled with car audio component speaker sets:

Pioneer Component Crossovers

On the left, you’ll see a high-end passive crossover that came bundled in a PPI component set:

Component Set Crossover PPI

The following is three sets of 2-way passive crossovers for a custom home theater speaker:

Custom Speaker Crossover 2-way

The following is a 3-way custom home theater passive crossover set for the Statement Monitors design:

Custom Speaker Crossover 2-way

 

Active Crossovers

An active crossover filters the signal digitally. In most cases, this active crossover will take a low-level (RCA) signal input, convert the signal into a digital signal, process the signal using an on-board computer, convert it back to analog, and output two separate filtered signals. Since a crossover typically separates an incoming signal into two signals (in the case of a 2-way system) that a tweeter and a driver can use, an active crossover will have a low-level (RCA) output for each individual driver. This is the most common type of active crossover, although there are some amplified 2-way active crossovers that will output high level signals.

Active crossovers offer significant customization advantages over passive crossovers. With an active crossover, you are free to choose your own crossover point (the point at which the audio signal is filtered), to suit the specific components you are using. This gives you much greater control over the specific speaker drivers you are using, allowing you to choose the ones that best fit your design goals and budget. In addition, the ability to choose your own crossover point also allows you to get the most out of those speaker drivers; allowing you to cross the tweeter as low as it can go to enhance your sound stage. With most active crossovers, not only do you have the ability to set the crossover point, but you also have the ability to set the crossover slope. We’ll discuss more on that in a minute.

In a typical system utilizing an active crossover, the signal will start at the source (your TV, your phone, your Xbox, your CD player, etc.), then to the active crossover, then to the amplifier, and then to each individual speaker.This type of crossover is inherently more efficient and does not depend on the quality of the passive crossover’s electrical components. My favorite example of a passive crossover is the miniDSP (www.minidsp.com):

MiniDSP

MiniDSP

 

As you can see from the above example, on one end of the miniDSP, there are two RCA input connections. On the other end, there are 4 RCA output connections. This is a 2-way crossover, which outputs a stereo input into two stereo outputs; two for the tweeters and two for the midwoofers. In addition to being a crossover, the miniDSP also offers vast equalizer capabilities; whether it’s parametric or graphic, and the ability to tune time delay/alignment for each individual driver.

The disadvantage to an active crossover is the added complexity over simply plugging in a pre-made passive crossover. It is, however, much easier and simpler than designing your own passive crossover. In the case of the miniDSP, you need to own a laptop and have a decent understanding of how to use it. The software is fairly intuitive. Once you have connected to the miniDSP and have it all set up, you need to know what crossover points to use at a bare minimum, which means you need to have a solid grasp of what the specifications of the drivers you’re using actually mean. Some of this can be tuned through trial and error, but the results aren’t as good.

While some may argue that an active crossover is more expensive, it really depends on what level of sound quality you are trying to achieve. A miniDSP board with a miniDC accessory (required for car audio) will cost you just under $150 shipped, with speaker driver adding another $100-$150. However, that $250-$300 will buy you a whole lot more sound quality than a passive component set will.

 

Crossover Slopes

Now that you have a basic understanding of what crossover do and what the two most common types of crossovers are, we’ll dig a little deeper and look at how they work and what a crossover filter looks like graphically. What you need to know is that crossovers all have slopes. When I referred to the filter that “cuts off” a certain frequency range, I was referring more to a gradual fall.

Crossover slopes are referred to in “orders.” Orders refer to the decibels per octave that the output begins to fall in a certain frequency range . Orders are all in multiples of 6, so a 1st order is 6dB/octave, a 2nd order is 12dB/octave, a 3rd order is 18dB/octave, and so forth, all the way up to an 8th order, which is 48dB/octave.

To put this into practice, if we have a crossover on a tweeter set at 2,000Hz with a 2nd order filter, that crossover filter is designed to reduce output of all frequencies below 2,000Hz by 12 decibels for every octave below 2,000Hz. This means it will be reduced by 6dB for the first octave, 12dB for the second octave, 18dB for the third octave, and so forth. Since this can be a bit confusing, let’s look at some graphical representation of crossovers.

Here’s what a 2000Hz, 1st order crossover filter looks like. For the purposes of this article, I’ll only be using Butterworth filters (more on that in another article). The below image has a filter applied only on the tweeter; not on the midbass driver.

1st order crossover 6db

Note that the crossover point is not the point at which output begins to fall, but the point at which output is falling. For this Butterworth filter, the crossover point is the point at which the filter causes a -3db drop in output. Aslo note that the tweeter will still be playing frequencies below the crossover point; but it will simply be playing them at increasingly reduced output. Let’s look at another crossover, but this time we’ll make it a 2st order filter:

2nd order Crossover 12db per octave

If we add a midbass drive, also with a 2,000Hz, 2nd order filter, here’s what our crossover would look like:

2nd order crossover 12db

As you can see, 2,000Hz is the point at which the two slopes cross over, hence the term crossover point.

 

Crossover Types

Now that I’ve shown you some examples of crossover slopes, you should have an idea of what they look like and how they function. To build on this, I’d like to discuss the two types of crossover slopes and their technical terms. To put it simply, a crossover filter is typically either a high-pass or a low-pass filter.

A high-pass filter allows all frequencies above a given crossover point to “pass” through. In other words, if we have a 2,000Hz high-pass filter, then all frequencies above ~2,000Hz will pass through the filter and will be played by the driver, and all frequencies below ~2,000Hz will be reduced in amplitude by the filter. Inversely, a low-pass filter allows all frequencies below a given crossover point to “pass’ through.

It is important to know that more than one filter can be applied to a given driver. For example, in a 3-way speaker system, the midrange will have two filters; a high-pass and a low-pass, in order to allow the tweeter and the woofer to both play their part. In a 2-way car audio system, it is very common to have a high-pass and a low-pass on the woofer. The low-pass filter is obvious in order to cross over to the tweeter, but the high-pass can also be used to cross over to the subwoofer. A high-pass filter on a car midbass driver will prevent that driver from playing bass frequencies that may, at high volumes, cause it to exceed its mechanical excursion capabilities and cause distortion.

Let’s look at a more complex example. The following image contains the following crossover filters:

  • 2,000Hz 4th order high pass on the tweeter
  • 2,000Hz 4th order low pass on the midbass driver
  • 80Hz 8th order high pass on the midbass driver

MiniDSP Crossover Network

 

Connecting it all together

Now that we’ve gone over the basics of crossovers, what does this mean for you? Well, it should give you an idea of what you’re working with. Most passive crossovers that come with car audio component speaker sets are either 1st order or 2nd order filters. The reason for this is cost. In order to create a 4th order passive filter, you need two use two inductors and two capacitors. With copper inductors typically coming in at $6-$20 apiece or more per filter (remember, you have two filters per crossover box), you can quickly see why it would become cost-prohibitive.

That said, you can also quickly see the value in an active crossover such as the miniDSP. While you are limited to the crossover point set by the combination of electrical components in a 2-way car audio component speaker set, an active crossover like the miniDSP will allow you to do with the click of a mouse what would be much too expensive and time consuming to do with a passive crossover.

At this point, you may be thinking “so active crossovers are always better, right?” Well, not quite. It really depends on your goals and your environment. If you have a background knowledge of some of these concepts in car audio and don’t mind spending the extra $100-$150 on a miniDSP based configuration, you will be better off that way than with a passive crossover-based component set. In home theater though, the environment is much different. Most people will have one single home theater receiver, so using an active crossover would prove to be much more costly and complex. For home theater purposes, a passive crossover is usually best and provides great results.

Should you have a desire to learn more, you can learn how to design your own passive crossovers for home theater speakers that will rival even the higher end boutique speakers out there. There is a wealth of knowledge over on the techtalk.parts-express.com forums where you can learn to design your own home theater speakers and passive crossovers. I personally use an active crossover (miniDSP) in my own car, passive crossovers (Polk Audio component set) in my wife’s car, and passive crossovers in my 3-way home theater speakers, so there are definitely places where each of them work well.

I hope that this article has helped you gain a better understanding of crossovers, how they work, and what your options are when designing an audio system for either home or mobile use. The goal here is to not only explain crossovers, but also to make you aware that, especially with car audio, you do have viable options to the standard passive crossover-based component speaker sets.  If you have any questions, feel free to leave a comment.

Don’t forget to “like” this article!

– XTR

Leave a Reply