What Is A Crossover Frequency? What Does A Crossover Do? A Helpful Guide

Wondering what crossovers do and what a crossover frequency is? Trust me, I know – it can be a bit confusing at first.

Crossovers are incredibly important for a great-sounding stereo system whether in your home, car or nearly anywhere that speakers and an amplifier are used.

In this detailed post, I’ll explain what they are, how they work, and much, much more in a way that anyone can understand.  

Infographic – Audio crossover facts

What does a crossover do infographic diagram

What is a crossover frequency? What does a crossover do?

Crossover frequency and crossover basics summarized
  • Crossovers are used to separate an incoming musical signal into 1 or more outputs. They offer a way to “cut off” certain sound ranges to send the best range to each type of speaker (For example, tweeters and woofers in a 2-way speaker system)
  • A crossover works using the principle of electronic filters to filter out (block) a range of musical sound frequencies as desired.
  • A crossover frequency is the sound frequency that starts the cutoff point for crossover filters. It’s the frequency point at which signals are reduced by 3 decibels (represented as -3dB)
  • A crossover’s outputs are the signal ranges allowed to pass such as high-pass (lower frequencies are blocked) and low-pass (higher frequencies are blocked)
  • There are 2 types of crossovers: active (electronic) and passive (speaker) types. Both types are very commonly found in home, car stereo, and professional audio systems

When we think about musical signals we don’t always realize the important things going on behind the scenes. In fact, you’ll almost never find a good-sounding speaker system that isn’t using 1 or more types of crossovers.

That’s how important they are!

A crossover (audio crossover) is an electrical or electronic assembly that separates a musical sound source and provides outputs best suited for certain types of speakers.

There are 2 types of crossovers:

  • Active (electronic) crossovers
  • Passive (speaker) crossovers

Crossover frequencies explained

Shown: One of the most common crossovers used directly on speakers and the crossover frequency as a real-world example. As tweeters can’t produce bass sounds, they distort and can even be damaged by heavy bass. Using a crossover, therefore, makes it possible to block (filter out) unwanted sounds below the crossover frequency. (Shown is a typical frequency used at 3,500 Hz [3.5 KiloHertz]).

The crossover frequency is the sound frequency point at which sounds after that will be greatly reduced, effectively blocking them.

We use it as a reference point at which the output to a speaker (or the input to an amplifier, when using active crossovers) is reduced by 3 decibels (-3dB). Normally the crossover frequency is used as a starting point in mathematical computations for crossover design.

In the world of electronics, it’s also sometimes called the corner frequency or cutoff frequency.

What’s the simple answer?

All sound frequencies after the crossover frequency are cut more and more past it, with an increasingly steep reduction – to the point where they’re almost completely blocked.

In other words, a crossover filters out a range of sound you’d like to prevent reaching speakers, starting at the crossover frequency.

What does sound “frequency” mean?

Diagram explaining sound frequency definitionIn this simple diagram, you can see what I mean by “frequency.” After all, the word simply means “how many times something happens.” Likewise, sound frequencies are assigned a number by the number of times they occur per second.

When we talk about “frequency” we’re referring to a number range for the human ear. For math & engineering purposes most of the time we use the range of 20 to 20,000 Hertz (20 to 20 KiloHertz, or 20 thousand Hertz).

In reality, the human ear can only hear down to around 30 Hz and near 16 KiloHertz, although it depends on your ears.

Hertz is a label used to represent frequency in terms of cycles per second. It’s because all sound waves (and electronic audio signals too) are alternating waves that happen many times per second.

“Hz”, “KiloHertz”, “kHz” are shorter ways of writing it (Kilo = the thousands marker, as you might recall from math class).

As an example, here are some of the most common sound frequencies that crossovers help with:

  • Bass: 20-100Hz or so
  • Midrange: (vocals, instruments, and more) ~100Hz to around 3Khz
  • Treble: (high-frequency sounds) Around 3KHz to 20Khz

How does a crossover work? What is a speaker crossover?

As I mentioned earlier, there are 2 kinds of crossovers. That’s true even if they’re built into an amplifier or speaker cabinet itself. The same basic designs are used just in a different package.

1. Active (electronic) crossovers

Illustrated diagram of an electronic (active) crossover example

A typical example of a separate electronic (“active”) crossover. In this example of a separate crossover used with car stereo amplifiers (nearly identical to those used in home stereos, too) you can see the RCA jack audio inputs and the crossover’s adjustable/switchable outputs. One set of output jacks provides a high-pass signal to connect to an amp for driving tweeters or full-range speakers while blocking bass. The 2nd output is for providing a bass-only signal to the amp for woofers.

Illustrated view of a car amplifier built-in crossover and componentsA typical car amplifier’s built-in electronic crossover circuitry illustrated. Sometimes called the “front end”, an amplifier’s internal crossover section is made up of a few basic electronic parts: Variable resistors, operational amplifier chips, capacitors, and fixed value resistors. They’re designed just like separate crossovers to give adjustable features & variable crossover frequency settings.

Electronic crossovers are also sometimes called “active” crossovers as unlike speaker crossovers, they need a power supply connection to work. Also, unlike speaker crossovers, they’re used before an amplifier.

While speaker crossovers connect directly to the higher-power output terminals of an amp and then to speakers, electronic crossovers work only with small signals. They’re connected to the outputs of a stereo in most cases.

How electronic crossovers work

Image showing a typical op amp IC and low pass crossover circuit example

Left: A typical operational amplifier (Op amp) integrated circuit (IC), the Texas Instruments TL072. Right: An example of a low-pass crossover circuit using an op-amp to filter out high-frequency sounds.

Electronic crossovers may sound very complicated (and they are, at least in some ways) but they’re actually based on pretty basic principles.

They work using a variety of electronic filter circuits based around a very common electronic component: the operational amplifier (“op amp”). Op amps are tiny multi-purpose amplifiers that are very useful for amplifying or changing an input signal in many ways.

They, together with resistors and capacitors, can be used to control how a music signal is output and will block certain ranges of frequencies.

Alpine car amp electronic crossover schematic exmaple

A schematic of a typical electronic crossover. In this case, the left stereo channel of an Alpine car amplifier’s built-in crossover circuitry is shown.

Electronic crossover functional diagram showing the basic blocks of operation

When put together in a way in which you can select your preferred filter (high or low pass, for example) and adjust the crossover frequency they form a complete crossover unit.

Basically, they offer several adjustable filters so you can prevent a range of musical frequencies from going to the wrong speakers. The crossover frequency is usually adjustable using switches or dials to allow you to change it as you like.

Once an input signal is applied, you’ll get the following outputs (depending on the type, as there are many options available):

  • High-pass outputs to block bass from tweeters or to block low-end bass from main speakers. This allows more volume without distortion as small speakers can’t handle heavy bass well.
  • Low-pass outputs for bass: When used, this blocks the vocals and other higher frequency sounds that woofers and subwoofers can’t reproduce well. The result is good, clear, heavy-hitting bass.

2. Speaker (passive) crossovers

Home and car stereo speaker crossover examples illustrated and labeled

Top: A typical car stereo speaker crossover, with the main parts labeled. Bottom: A typical home stereo speaker crossover, which is extremely similar. (These are normally installed inside the speaker cabinet) Both use capacitors and inductors to form crossover filters and control the sound sent to tweeters, midrange speakers, or woofers for best audio sound quality.

Speaker crossovers are sometimes called “passive” crossovers as they don’t need an external power supply connection. They work using passive components: capacitors and inductors.

A speaker crossover is an electrical circuit that uses inductors and capacitors to filter a speaker signal and split it among 1 or more outputs. The outputs depend upon the frequency response of the speakers used.

Unlike electronic crossovers, normally they’re connected to the outputs of an amplifier and then to the speakers you’d like to use.

2 way speaker system and crossover diagram

One of the most common speaker crossover types in use today: A 2nd-order 2-way speaker crossover with tweeter and midrange/woofer outputs. Inductors are represented with an “L” symbol and capacitors with a “C” symbol. A “2nd order” crossover just means that the second stage of parts is used to make the crossover filter out the unwanted frequencies even more effectively.

Capacitors and inductors have some interesting properties depending upon the frequency of a signal applied to them:

  • Capacitors have more “resistance” (called impedance, in this case) to a low-frequency signal than a higher one. The lower the frequency, the less signal that is allowed to pass.
  • Inductors are coils of wire that have more resistance to a high-frequency signal than a lower one. Therefore they filter out higher sound frequencies.

This works because when a capacitor or inductor has a signal applied to it that’s past the crossover frequency (depending on how it’s used), the resistance increases, which reduces the speaker voltage.

This means the speaker will receive less and less of the speaker signal that we want to block.

In all cases, the part value is chosen according to the speaker “Ohms” (impedance rating) it’s planned to be used with. That’s super important!

Note: Speaker crossovers can only be used with the speaker impedance they’re designed for, or they won’t sound the same.

For example, using an 8 Ohm home speaker crossover with a 4 Ohm car speaker won’t work correctly. That’s because the part values were chosen for one impedance only. When you change that, it dramatically changes the crossover frequency!

Image of a tweeter used with inline bass blocker capacitor speaker crossover example diagramWhen used in series with a tweeter, a crossover blocks damaging and distorting bass that tweeters can’t handle. Capacitors like in this example can be used as a simple speaker crossover for tweeters.

Speaker crossovers are designed in many ways but all have the same basic design structure – only the details change.

They’re also often labeled with names like “1st order”, -6dB/octave, “2nd order”, -12dB/octave, and so on. I’ll explain a bit more about that as we go.

For now, you only really need to know that 2nd order and 3rd order crossovers are the same thing but with more crossover stages, or “orders”, added to make the filtering ability even more effective.

What are decibels and why do we use them for audio?

Diagram showing the formula for crossover voltage in decibels with example math problem solved

Crossovers (and a lot of other audio electronics & equipment) are measured using Decibels. Decibels (“dB”) are a convenient mathematical way of dealing with numbers that occur as powers of 10, unlike linear numbers, which occur in a straight line. Shown is an example of figuring out the reduction, in dB, of a crossover output.

In the real world, lots of measurements deal with things that don’t increase or decrease in a straight line (“linear”) but instead on a curve (“non-linear”, or logarithms).

I won’t bore you with heavy math here, but we use Decibels in the world of audio as a mathematical way of dealing with musical electrical signals. That’s because a lot of it happens not in a straight line but in curves.

That is, much of the audio world works with powers of 10 (logarithms, which you might remember from your algebra class). Hence the need for a way to deal with those – that’s where the dB representation comes in handy.

And it’s not just crossovers that work with decibels but even your own ears are “logarithmic”! That is, the volume your ears perceive is measured in dB, too.

What is a crossover “slope”?

Crossover slope diagram and examples illustrated

Diagram showing the crossover slope, or cutoff steepness, for the most common crossover types. Crossovers have “orders” – that is, 2nd, 3rd, or more stages that increase their ability to filter out the unwanted sounds frequencies sent to a speaker.

A crossover slope is the steepness of a crossover’s filtering ability. In other words, it’s how much a crossover’s blocking (filtering) ability is past the crossover frequency point.

Slopes, just like the crossover frequency, are determined according to a level in decibels (dB). The negative symbol is used to show they represent an attenuation, or reduction, of the signal. (Which of course is how crossovers work!)

As you might have guessed, the larger the steepness (greater the slope), the more effective the crossover is at filtering out bass sent to a tweeter, as an example. Likewise for other speakers connected to it.

In the audio world, we commonly refer to frequencies in octavesAn octave is a doubling or halving of a frequency number.

For example, when we refer to a crossover having a cutoff of -6dB per octave, we mean it will continue to cut the input signal more by a factor of 6dB for every doubling of the previous frequency.

Like this: (Low pass crossover frequency) -6dB @ 1KHz, -12dB @ 2KHz, -18dB @ 4KHz, -24dB @ 8KHz, –32dB @ 16KHz, up to 20KHz.

What are the most common and best crossover slopes?

12dB per octave speaker crossover example image

There’s a lot more to say here, but crossovers are designed to be a good compromised between complexity, price, and sound quality. While you might think “the higher order, the better” would always apply, things get much more complicated once you get past 3rd or 4th order crossovers.

Generally speaking, a -12dB crossover slope is one of the best compromises and works well for most speaker systems used today.

One reason is that it’s simple. has fewer design complications, but still gives a good cutoff ability that works great both for single speakers or 2-way speakers.

In general, the most commonly used are:

  • -6dB
  • -12dB (the most popular, by far)
  • -18dB

Electronic and 2-way speaker crossovers are nearly always -12dB models.

What is a two way speaker?

Image showing diagram with home and car stereo 2-way speaker examples

Examples of very common 2-way speakers you’ll find in either car or home stereos (in addition to other types as well). They have nearly the same things in common except that home stereo speakers are usually placed in a speaker box (speaker cabinet/enclosure) while car speakers may be installed separately in many cases. Both use a 2-way crossover to produce a very nice sound.

What are 2 way speakers?
  • 2-way speakers are a speaker system in which 2 speakers work together to produce the full range of sound. Audio from a stereo amplifier is divided between the speakers by a 2-way speaker crossover.
  • 2-way speakers are the most common type in the world, and many offer low-cost with great sound
  • These types of speakers use a tweeter for high frequencies and a woofer speaker for the midrange and bass portions of the music.
  • While the type of crossover varies from model to model, one of the most common and best-performing is the 2nd order crossover with a slope of -12dB per octave.

2-way speakers use 2 speakers on each channel and a crossover to divide the audio frequencies reproduced between the two. Each speaker receives a signal range it’s best suited for.

Illustrated diagram showing examples of 2 way home and car stereo speakers with 2 way crossovers

For example in 2-way speaker design:

  • Tweeters receive only high frequencies – typically around 3.5KHz and above
  • Woofers only receive lower frequencies – typically around 3.5KHz and below

The crossover frequency used varies by design needs, to there’s no “one” crossover frequency that works in all cases. Additionally, crossovers must be matched to the right impedance (Ohms rating) for the speakers they’re designed to work with.

The 2-way crossover evenly splits the incoming sound and sends it to the correct speaker such as the tweeter and a woofer.

Image of coaxial 2 way car speaker example

Coaxial speakers are 2-way speakers, too. In fact, in the example shown here, you can see crossovers on the rear of the speaker. Just like separate crossovers, lower frequencies are directed to the large woofer and highs are sent to the tweeter.

The result is that the sound produced is a full range of sound, but without distortion or poor performance you’d get when trying to play the same range in only 1 speaker. In other words, a 2-way speaker design can produce a clean, detailed sound.

In many systems, you won’t necessarily need expensive components or speakers to get great sound. Even low-cost 2-way speakers can sound very nice!

What is a good crossover frequency? What crossover frequency should I use?

The truth is, there’s no good set of crossover frequencies that work for every speaker. It depends on a lot of things.

However, here are some of the most common frequencies that work well in many cases. This is based on my experience with speaker design and many stereo installations.

Recommended crossover frequency table

Speaker/System TypeCrossover Freq. & TypeNotes
Subwoofers 70-80 Hz (low pass)

Good low-pass frequency range for subwoofer bass & blocking midrange sounds. Best for pure, clear bass sound that "hits."

Car main (full range) speakers 56-60Hz (high pass)

Blocks low-end bass that causes distortion or speakers to "bottom out." Great compromise between full-range sound and midrange bass capability.

Tweeters or 2-way speakers 3-3.5KHz (high pass, or high/low-pass)

Most 2-way or 1-way (tweeter) crossovers use a frequency near this as most tweeters can't handle sounds below this range. Same for woofers above this range.

Midrange/woofer 1K-3.5KHz (low pass)

Woofers and many midrange speakers do not perform well above this general range. They're poor for treble and a tweeter should be added.

3-way system 500Hz & 3.5KHz (Woofer/tweeter crossover points)

Similar to 2-way systems the upper freq. would be the same. Midrange drivers in a 3-way system often do not perform well below 500Hz or 250Hz in many cases.

Recommended reading

Product image of the Loudspeaker Design Cookbook by Vance Dickason

Want to learn a lot more and truly understand speakers, crossovers, and how to design a great sounding system of your own? Here’s the very book I learned a lot from myself.

You can pick up your own copy of the famous Loudspeaker Design Cookbook at Amazon today. It’s an excellent source of information including formulas you too can use to build your own car or home loudspeaker projects.

Interested in learning more about what tweeters do and the different kinds? Check out my wonderful guide to tweeters here.

For some great articles about crossovers, speakers, and lots of DIY projects, check out the Elliot Sound Products page.

Did I leave anything out? Drop a note!

I hope you’ve found my post helpful, clear, informative, and most of all what you were looking for. If you’ve still got questions, suggestions, or just want to say hi, leave a comment below or send me a message from my contact page.

Your comments are welcome!

  1. Thanks for a helpful and thorough explanation! :)

    What I’m trying to grasp is how you go about changing the crossover frequency. Let’s say from 2500 to 3500Hz. Do you need to change the cap and the inductor to ones with different values? And use some kind of calculation for that?

    Reply
    • Hey there, I’m glad you liked it! My belief is that it’s important to try an explain things in a way that doesn’t make your head hurt, ha ha.

      To change the crossover frequency, you either have to (1) [yes] change the capacitor(s) & inductor(s) depending on the design, or (2) change the Ohms load connected to it. The issue with #2 is that when adding resistors in series with a speaker you lose volume (decibels) as some power is lost across the resistor. Sometimes it’s barely noticeable, other times it’s a problem.

      For #1, yes, you need to calculate the components depending on the type of crossover (Linkwitz-Riley, Butterworth, etc.) and order it is (1st order [-6dB,] 2nd order [-12dB]. If you’re swapping all of the components then it doesn’t matter as you’ll end up with the right values. Linkwitz-Riley is one of the most commonly used and is good.

      Here’s a good speaker crossover calculator you can see that will help: https://www.parts-express.com/crossover-calculators They have diagrams and you can play around with it.

      When you start looking for parts, don’t be surprised if you can’t find the exact values. That’s ok. You just need to get them fairly close and you can add parts to get the values you need if necessary. Capacitors add in parallel and inductors add in series).

      (Ex.: can’t find a 3.75uF capacitor? You can use a 2.2uF + 1.5uF in parallel = 3.7uF which is close enough).

      Reply
      • Yeah, that’s good belief, you succeeded! =) Thanks for the detailed reply, and a great link! Very useful to play around with the calculator.

        I’m not sure if my old two-way Pioneers have a 6, 12 or 18 dB crossover, but since there’s only one cap and one inductor (and an L-pad for the tweeter), it should be a first order, right?

        In any case, I’m not getting the right numbers with what I’m trying. The cap is 3.5 uF (63 V) and the inductor is 2.5 mH. The crossover should be (and I’m pretty sure all parts are original) at 2200 Hz. And both woofers and tweeters are 8 ohm. Should the L-Pad be included in the calculation somehow? And does the voltage of the cap matter, or should you just try to find one within fairly close range?

        I should mention that what I’m thinking about trying, is putting in some new tweeters and set the crossover to around 3000-3500. :)

        Reply
        • Hi there, as you can see from the diagrams in the link I sent, yours is likely a 2nd order (-12dB/octave) crossover. I’m not sure about the capacitor & inductor values you mentioned as even though it’s supposed to be 2.2KHz crossover frequency, the values depend on the kind of network used and what they designed if for.

          The voltage is just a rating to be sure the components (capacitor mainly) can handle the voltage put out by the amplifier. 63V should be fine for a home receiver or amp. (Higher is fine too, but not mandatory) That isn’t a factor for the behavior of the crossover.

          L-pads are normally connected so the crossover sees an 8 ohm load regardless of the L-pad setting/tweeter volume, so you can usually treat it like any 8 ohm speaker load.

          So the easiest thing to do is just use a 2nd order design like in the calculator page, choose the freq. you want, and select Linkwitz-Riley. Then you can tinker with changing the frequency a little bit as it may help give you values that are easier to find.

          It’s critical however to be 100% sure the speakers are 8 ohms (or whatever they supposedly are) for that. You can measure them with a test meter set to Ohms and they’ll read somewhere around ~6 to 7 etc Ohms if they’re 8 ohm speakers.

          You can also just pick up some ready-made crossovers to save the time, money, & hassle if you like.

          Reply
  2. Hmm, yep, a ready-made xo should be the easiest way. And the Daytons seem to be good value for money. Not as cheap as just changing a cap, though. :)

    All I can see on my xo is a 3.5uF cap, a 2.5mH inducer and a big level control for the tweeter. Are you sure that’s a second order?

    Reply
  3. Great page. Thanks Marty. I have a clearer understanding of how my sound system works and what the numbers on the labels of my speakers mean!

    Appreciate your good work.

    Reply

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