Crossovers are important for a great sounding audio system whether in your home, car, or even boat. But what exactly is a crossover frequency?
In this detailed post, I’ll explain what they are, how they work, and much, much more in a way that anyone can understand. Let’s get started!
- Infographic – Audio crossover facts
- What is a crossover frequency? What does a crossover do?
- How does a crossover work? What is a speaker crossover?
- What are decibels and why do we use them for audio?
- What is a crossover slope?
- What is a two way speaker?
- What is a good crossover frequency?
- Why a crossover is important for good subwoofer bass
- How to use a crossover for rear fill
- Recommended reading
Infographic – Audio crossover facts
What is a crossover frequency? What does a crossover do?
They’re a critical part of good sound fidelity. In fact, you’ll almost never find a good-sounding speaker system that isn’t using 1 or more types of crossovers.
There are 2 types of crossovers you’ll use with your home or car audio system:
- An active crossover (electronic)
- A passive crossover (speaker crossover)
Crossover frequencies explained
As tweeters can’t produce low frequency sound, they distort and can even be damaged by heavy bass. To prevent this, we can block unwanted sounds. (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).
Sometimes called the corner frequency or cutoff frequency, it’s used as a starting point in math calculations for audio design.
What does sound “frequency” mean?
A musical signal’s frequency is described by the number of times per second an alternative wave is completed.
When we talk about “frequency” we’re referring to a range of sound the human ear can hear. By convention, a range of 20 to 20,000 Hertz (20 to 20 KiloHertz, or 20 thousand Hertz) is used to represent this.
In reality, the human ear can only hear down to around 30 Hz and near 16 KiloHertz, although it depends on the hearing ability of the person.
Hertz is used to represent units of frequency in cycles per second. That’s because sound waves (and electronic audio signals too) are made of alternating waves that happen many times per second.
“Hz”, “KiloHertz”, “kHz” are shorter ways of writing it (Kilo = one thousand).
Here are some of the most common sound ranges 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?
1. Active crossover (electronic crossover) basics
A typical example of a separate electronic active crossover. One set of output jacks provides a high pass filter output to the amp for driving tweeters or the main speaker pair. The 2nd pair is often a subwoofer crossover frequency output.
A car amplifier’s internal crossover section is made of basic electronic components: Variable resistors, operational amplifier chips, capacitors, and fixed value resistors. They’re essentially electronic crossovers built into the amp, saving the need to use one separately.
Electronic crossovers are also sometimes called “active” crossovers as unlike speaker crossovers, they need a power source to work. While passive crossovers connect directly to the higher-power output terminals of an amp, electronic crossovers work only with low voltage signals.
They’re connected to the line level RCA jacks or a car stereo, car amp, or home AV receiver for example.
How electronic crossovers work
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 useful for amplifying or changing an input signal in many ways.
They, together with resistors and capacitors, can be used to control the musical signal.
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.
They can be designed to change modes with a switch. They often use adjustable resistors dials to allow changing the cutoff frequency.
Once an input signal is applied, you’ll get the following outputs depending on the type:
- High pass filter: Blocks subwoofer bass frequencies from tweeters or from the main speakers. This allows more volume without distortion as small speakers can’t handle heavy bass well.
- Low pass filter: This blocks vocals and other high frequency sound that a woofer or subwoofer can’t reproduce well. Also called the LFE channel for a home receiver subwoofer output.
- Band pass filter: A bandpass filter is a combination of the other too – it limits the output to only midrange frequencies (for use with a midrange driver set).
2. Speaker (passive) crossovers
Top: 2-way component crossovers for car component speakers. Bottom: A home stereo speaker crossover like you might find in a bookshelf speaker set. Both use capacitors and inductors to filter and control the sound sent to a tweeter, midrange speaker, or woofer.
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 with speaker wire to your speaker driver set, one for each speaker channel.
A 2nd-order 2-way speaker crossover is shown here. Inductors use an “L” symbol and capacitors use a “C”. A “2nd order” type has a second stage of parts to more effectively filter out sounds than a first order design.
Capacitors and inductors have some interesting properties depending upon the frequency of a signal applied to them:
- Capacitors have more “resistance” (called impedance) 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.
When a capacitor or inductor has a signal applied to it that’s past the crossover frequency the resistance increases, reducing the speaker voltage output since it’s lost across the capacitor or inductor.
This means the speaker will receive less and less of the speaker signal that we want to block the further the sound frequency is past the crossover point. Part values are chosen according to the speaker impedance (Ohms) they’re to be used with since this affects how they behave.
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. 2nd order, 3rd order, etc designs use the same principles but with more stages, or “orders”, added to make them even more effective at block sound.
What are decibels and why do we use them for audio?
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 to measure musical electrical signals. That’s because a lot of it happens not in a straight line but in curves.
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. This means the way our ears perceive volume works on a logarithmic scale.
What is a crossover slope?
Diagram showing the slope, or cutoff steepness, for the most common types. Crossovers have “orders” – that is, 2nd, 3rd, or more stages that increase their ability to filter out the unwanted sounds frequencies.
The slope is the steepness of a crossover’s filtering ability. In other words, it’s how effective it is past the crossover frequency point.
Slopes, just like the crossover frequency, are categorized in terms of decibels (dB) per octave. The negative symbol is used to show they represent an attenuation, or reduction, of the signal.
What are the most common and best crossover slopes?
Crossovers are usually designed to be a good compromise between complexity, price, and sound quality. While you might think “the higher order, the better” would always be right, things get more complicated once you get past 2nd or 3rd order designs.
Generally speaking, a -12dB crossover slope is often the best choice and works well for most speaker systems. A subwoofer usually sounds very good with a 12dB or 18dB slope.
One reason is that it has a relatively affordable and uncomplicated design but still gives a good cutoff ability. This works great both for single speakers or 2-way speakers.
In general, the most commonly used slopes are:
Electronic and 2-way speaker crossovers are typically -12dB.
What is a two way speaker?
Common 2-way speakers you’ll find in car or home speaker systems. They’re very similar except that home stereo speakers are usually placed in a speaker box while car speakers may be installed separately.
Each speaker is provided only the range of sound it can produce well:
- Tweeters receive only high frequencies – typically around 3KHz and above
- Woofers only receive lower frequencies – typically around 3KHz and below
The crossover frequency used varies by design needs, so there’s not one that works in all cases. Additionally, they must be used with the correct impedance (Ohms rating) for the speakers they’re designed for.
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 a full range of sound 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 offers clean, detailed sound you wouldn’t get otherwise.
In many cases, you won’t necessarily need expensive components or speakers to get great sound. Even budget 2-way speakers can sound very nice!
What is a good crossover frequency?
The truth is, there’s no perfect set of crossover frequencies that works 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 what most home or car audio system needs are along with my own speaker design and installation experience.
What crossover frequency should I use?
Recommended crossover frequency table
|Speaker/System Type||Crossover Freq. & Type|
|Subwoofers||70-80 Hz (low pass) – Best for pure, clear bass sound that “hits.”|
|Car main speakers (full range)||56-60Hz (high pass) – Blocks low-end bass that causes distortion or speakers to “bottom out.”|
|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|
|Midrange/woofer||1K-3.5KHz (low pass) – Woofers and many midrange speakers do not perform well above this general range.|
|3-way system||500Hz & 3.5KHz (Woofer/tweeter crossover points) – Similar to 2-way systems the upper freq. would be the same.|
Why a crossover is important for good subwoofer bass
A passive subwoofer crossover can’t match the performance of an electronic one if you want great bass.
Ever wonder how some people have pure, hard-hitting bass in their car, or why your home theater subwoofer sounds so good? It’s because of today’s electronic crossovers. Unlike full-range speakers, a subwoofer is especially susceptible to poor sound if midrange frequencies are allowed to play through it. The pure bass sound you’ve heard requires a good crossover.
A true subwoofer has a fairly limited frequency response (about 20 to 100Hz typically) so it’s important to get the most out of them.
While it is possible to get fairly decent bass sound using a big, bulky, and sometimes hard-to-find inductor low-pass board, they’re often not nearly as good sounding like an electronic crossover with a better cutoff that can block out the vocals and treble.
Also, some include a bass boost feature to add some extra punch when you want more thump – that’s simply not possible with a passive one. They also can’t handle high power levels without a huge and expensive inductor coil.
How to use a crossover for rear fill
One problem with a stereo sound system using rear speakers is that the rear channels can “pull” the stereo image behind you and muddy up the sound. This is because higher-pitched sounds are used as cues for our brain to know where they’re coming from. That’s one reason you tend to turn the fader control more towards the front than the rear.
When our ears hear it coming from both the front and the back, it often doesn’t sound quite right. In surround sound systems playing surround sound, this isn’t a problem since it’s already mixed specifically to sound great. However, using your satellite speaker set with stereo sound will quickly show you the problem.
To get better rear fill in your own car or home system, a simple trick is to use a low pass filter set to block mids and highs. Try something like a 1KHz or 500Hz low pass and see what you think!
I highly recommend the famous Loudspeaker Design Cookbook. It’s an excellent source of information including formulas you can use to build your own pro-level car or home audio projects. I speak from experience!
For some great DIY projects and circuits, check out the Elliot Sound Products page.