What Does A Capacitor Or Inductor Do In A Speaker Crossover?

Confused a bit about what exactly capacitors or inductors do in a speaker crossover? You’ve come to the right place!

In this article, it’s all here along with clear & easy to understand diagrams I’ve made myself. Let me help you learn more and understand more about these extremely useful audio parts.

Inductors vs capacitors – how are they different?

inductors vs capacitors image

Inductors and capacitors are the basic components behind all kinds of wonderful audio electronics including speaker systems at home or in the car. Each is considered a passive component parts type as they don’t require a power supply to function.

What’s really cool is how they’re somewhat like polar opposites but can work together in a non-powered crossover (passive crossover) to greatly improve the sound quality & get the most from your speakers.

What does an inductor do?

how does an inductor work diagram

An inductor is a tightly wound coil of wire with a specific number of loops used to take advantage of a property called inductance. Inductance is the tendency of a conductor (specifically a coil of wire in this case) to oppose a change in the electric current flowing through it due to the magnetic fields it generates.

They’re used in electric motors, solenoids, spark plug coils, and, of course, speaker crossover designs. Inductors behave like the opposite of capacitors: an inductor opposes high frequency signals meaning it passes low frequency audio signals more easily.

An electrical behavior called reactance is what makes this possible. When the frequency changes, so does an inductor or capacitor’s opposition to the flow of electricity.

  • Inductive reactance: An inductor builds up a higher resistance (impedance) to current as the frequency increases due to the inductance it has.
  • Capacitive reactance: a capacitor builds up a higher resistance (impedance) to current as the frequency decreases due to its capacitance.

Units of measurement for inductance

For inductors, the unit of measurement is the Henry. By convention, inductors are usually sold in units of milliHenries (1/1,000th of a Henry, or .001 Henries). A typical speaker crossover might use an inductor with a value of 10 mH, for example.

How does a capacitor work?

how does a capacitor work diagram

Capacitors store an electrical charge using extremely thin and tightly wound electric conductors separated by an insulator. This can be an electrolyte, mica, or several other types of materials. While they do not allow a direct current (DC) signal to pass, they do allow alternating current (AC) voltage & signals to pass.

They have an interesting characteristic: capacitors allow only high frequencies to pass – they increase their impedance (frequency-based resistance) when lower frequencies are applied.

The point at which this takes place is carefully chosen to be the crossover frequency. There are two fundamental types of capacitors with one in particular used for audio.

Units of measurement for capacitance

For capacitors, the unit of measurement is the Farad. By convention, capacitors are often sold in units of microFarads (1/1,000,000th of a Farad, or .000 001 F), sometimes written with the Greek letter mu “µ” to represent “micro.” As an example, when buying capacitors for your own crossovers you’ll see capacitors listed in “µF” sometimes.

Smaller ones may use picoFarads (pF) or nanoFarads (nF) which are even smaller and are used in electronics.

Capacitors used in audio nearly always tend to be in the microFarad range. For example, a bass blocker to a tweeter may use a 47 µF capacitor.

Common capacitor types to know

1. Electrolytic capacitor

examples of electrolytic capacitors

Electrolytic capacitors are essentially the most common and the most affordable type, hence their popularity in all kinds of electronics and speaker applications. You’ll frequently find them in a passive crossover or directly connected to a tweeter as a high pass crossover.

They have a thin metal case and contain an electrolyte between the super-thin charged conductive plates inside.

A non polarized electrolytic capacitor allows passing an alternating current (AC) waveform like that used for a musical signal. They’re also called “bipolar” capacitors. DC types, on the other hand, can’t and should only be used for direct current like in a power supply.

Note: non polarized capacitors normally are marked as such and are the only types you should use for audio. Polarized capacitors can be damaged (and even explode!) if used for audio applications. At the very least you’ll have a distorted sound that you’ll hate.

2. Film capacitor

examples of film capacitors

A film capacitor uses a thin film material to separate its charged plates and is typically a bit more expensive. They’re also longer-life, have better performance (in some cases) for audio, and may have higher temperature limits.

Film capacitors are also offered in high voltage types which are great for vacuum tube audio designs. They’re also a good upgrade for cheaper electrolytic capacitors if you’re handy with a soldering iron.

3. Ceramic capacitors

ceramic capacitor examples

Ceramic capacitors aren’t normally used in crossovers because their capacitance values are usually quite small (in the picoFarad range, for example) while we often need microFarad range values for speakers.

They’re used for other purposes typically, such as a bypass capacitor in a power supply to control electrical signal noise or in an active crossover.

How does a speaker crossover work?

image showing the parts in speaker crossovers illustrated

A crossover uses a capacitor, inductor, or both to limit the frequency range of audio sent to one or more speakers. This is extremely useful for preventing bass frequencies from reaching a tweeter or harsh-sounding midrange and treble from reaching a subwoofer.

The crossover point is often recommended by the loudspeaker manufacturer or picked as a good compromise between the limits of the frequency response of each speaker used. The filter frequency (also called the corner frequency or Fc at times) is directly affected by the speaker impedance.

Crossover slopes explained

diagram of what is a crossover slope

When we talk about the “order” of a crossover network, we’re referring to the number of stages (sections). This affects how effectively the slope – the audio filtering ability – is.

A 1st order design uses a single inductor or capacitor while 2 make up a 2nd order, three a 3rd order, and so on. Each stage (order) has a -6dB per octave slope with -12dB/octave being one of the most commonly used both for speaker or amplifier crossovers.

1st order crossover with a capacitor (high pass filter)

how a capacitor high pass crossover works diagram

A high pass filter works by passing higher frequencies to a speaker and opposing lower frequencies. At lower frequencies the impedance of a capacitor has a very high Ohm value, greatly reducing output voltage to the speaker.

Likewise, the opposite is true at high frequencies. You’ll often find them installed on a car or home audio tweeter set to block distorting and potentially damaging bass from being played.

1st order crossover with an inductor (low pass filter)

how an inductor low pass crossover works diagram

A low pass filter works by blocking higher frequencies to a speaker and allowing lower frequencies to pass. At higher frequencies, the impedance of the inductor means it has a very high Ohms value, greatly reducing the output to a speaker.

This type is normally used with a woofer or mid range speaker to prevent treble or “highs” they can’t produce well from being played.

2nd order 2-way crossovers with capacitors and inductors

Examples of 2 way crossovers and diagram

How they work

A 2-way, 2nd order speaker crossover network is essentially a high pass and low pass crossover filter combined in parallel.

However, as they add a second stage (2nd order, -12dB/octave filter), they’re better performers than a simple 1st order design. They’re the most common type also, using a woofer & tweeter to create a 2-way speaker system for home audio speakers or a component speaker set for car audio.

Even inexpensive 2-way 2nd order component sets can sound excellent with speaker drivers of decent quality and adequate design.

Because of the overlap point at their cutoff frequency, things can get a bit more complicated when it comes to 2 way crossovers as I’ll explain further below.

How it works:

  • Capacitor C1 reduces voltage output to the tweeter below the cutoff point. Inductor L1 reduces high-frequency signals that have reached it even further by passing them to the ground/negative (-) amplifier return path.
  • Inductor L2 reduces voltage output to the woofer above the cutoff point, passing low frequency signals to the speaker. Capacitor C2 passes additional high-frequency signals that have reached it to the ground/negative (-) amplifier return path.

The end result is two crossover stages that are staggered in series, meaning they compound together for a crossover slope that’s 2x as effective as a single stage (-6dB/octave) design. This is a -12dB/octave slope.

Why are 1st order crossovers used if 2nd order ones are better?

While 1st order crossover networks are less common now, they’re still around. You’ll often find them in:

  • Budget 2-way speaker cabinet systems
  • Designs where the speaker’s natural rolloff (falling frequency response) can be used to reduce the components required to achieve the same effect.
  • Simple inline speaker crossovers like bass blockers for car stereo use and related applications where something quick & easy is ideal.

Generally speaking, however, 12dB/octave crossovers are the most popular as they’re a good compromise between cost, parts count, and complexity. In fact, car amplifiers and home AV receivers commonly use a 12dB/octave (12dB/octave) design even in their electronic active crossovers.

Crossover design types

crossover network overlap comparison diagram

There’s a range of possible crossover network designs a designer might choose from, but a few are preferred over others:

  • Butterworth: In its standard configuration, the Butterworth design sums to +3dB at the cutoff frequency overlap. This can be handy for various design needs like accounting for different speaker drivers’ offsets (spacing between speakers at their central acoustical point).
  • Linkwitz-Riley: This type sums to a flat (0dB) output at the crossover frequency overlap and is one of the most commonly used. It’s an ideal choice in most cases.
  • Bessel: A Bessel design is not considered “all pass” like a Linkwitz-Riley and does not sum flat at the frequency point.
  • Chebyshev: Not often used, this can be helpful where there’s a need for a boosted output at the crossover point as the Chebyshev provides a +6dB sum.

Why does this matter? Ordinarily, it doesn’t matter much for the average person. However, if you’re interested in making your own speaker crossover it’s helpful to understand the options possible.

Each crossover network type uses a slightly different mathematical formula set to calculate the parts values you’ll need. In all cases, a good speaker design book can help you do it yourself if you’re interested in getting the best performance or if you’re interested in do-it-yourself (DIY) speaker projects.

Product image of the Loudspeaker Design Cookbook by Vance Dickason

Want to learn more advanced speaker design skills or make your own crossovers? I highly recommend the Loudspeaker Design Cookbook by Vance Dickason. It’s full of excellent information!

Aftermarket home or car speaker crossovers generally use a Linkwitz-Riley design and sum to a 0dB level at their crossover point. One reason is that it’s assumed the speakers you’re using will have an adequate output near that point. If that’s not the case you can investigate more advanced designs yourself and compensate for a particular speaker’s weaknesses.

Crossover phase concerns

diagram showing crossover speaker phase comparison

Crossovers have another issue to contend with: each single crossover component adds a 90° phase (shift) to the signal sent to the speaker. Capacitors and inductors have a “phase shift” when a signal passes through them.

For simple single-stage crossovers, this isn’t really a concern as it’s not something you’re very likely to notice, although it’s a detail for more advanced speaker designs. However, for 2nd order designs, this means there’s a 180° difference between the two outputs, often resulting in an “odd” sound and also means the sound isn’t arriving at the listener’s ears at the same time.

To remedy this, 2nd order/even order crossovers normally have the tweeter output reversed. This reversal of one speaker output means both speakers are “in phase” and there’s no longer an issue with sound delay. If you’ve ever bought 2-way speaker crossovers you probably didn’t even know it was designed that way on purpose!

What is a crossover Zobel network?

how a Zobel network works diagram

A Zobel network is an impedance equalization network used to compensate for the rise in speaker impedance over the frequency range due to voice coil inductance. For example, many speakers with a voice coil commonly show a rise in their total impedance as the frequency increases. You can see this on impedance graphs plotted for speakers.

Because a crossover’s behavior is directly affected by impedance, this simple design can improve the speaker system’s performance by compensating and “flattening” the normal rise in the speaker Ohm load that the crossover sees.

zobel network circuit diagram

The network uses a simple RC (resistor-capacitor) network in parallel with a speaker driver to offset the impedance, resulting in the crossover seeing a nearly flat impedance over the frequency response range.

The resistor ensures the minimum total impedance is always met while the capacitor works to decrease the total crossover Ohm load as the frequency rises.

Your comments are welcome.
  1. I have just completed my second set of speakers and am on to my third try – except I am going to take a shot at making my own crossover. This was an excellent summary and was very helpful for me. I would appreciate a clarification on the 2nd order tweeter reversal action. I have taken a crack at a design using xsim software and it is working out just fine and got a decently flat response – with the tweeter NOT reversed. Now that I read this, so when I reversed the tweeter, I got a massive drop in my FR curve. Yet, you are indicating that this is how it should be wired? So, I am confused. Ultimately, isn’t the only thing that matters is that one gets a flat fr curve (hopefully with the fewest components/lowest cost). So, in my original case, flat fr = tweeter NOT reversed and that is good?
    Thanks for your help.

    • Hi Steve. When relying on a simulation as opposed to actual speakers in the real world there can be very big differences. Generic design software can’t replicate everything about real-world speakers accurately, although better software can be fairly good. In this case, it sounds like perhaps there’s something else going on.

      The initial impression I get (without being able to see what you’re doing myself) is that the simulated speaker response either seems off or something isn’t correct. Herhaps the tweeter polarity in the simulation is already assumed to be reversed, as this is typically the case for nearly all 2-way 2nd order crossover designs.

      If you were to measure the actual real-world frequency response with a real-time analyzer (RTA) and a decent microphone, you should see something entirely different than what you’re reporting. To clarify why the reversed polarity matters, it’s because when we say “out of phase” this means that the two speakers are producing sound waves that are not coinciding at the same time.

      In this case, 180° difference will cause cancellation close to the crossover frequency. The frequency response above that point will be ok, however. It won’t sound right in person because of what your ears & brain hear, but the frequency output will be as expected pretty much.

      If you’re wanting to design & build a customized 2nd order crossover, a resource like The Loudspeaker Design Cookbook is better as you’ll learn much more than what free software with simplified output can show you.

      If you’d like to use a more accurate crossover design program, a good one that’s not very expensive is X-Over Pro. If there’s a particular parameter(s) set or design goal you wanting to work on it might be helpful for me to know.

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