A quick guide to understanding inductor coils

Inductors are an important component to any speaker, they restrict higher frequencies, allowing low frequencies to pass through unimpeded.

  • The higher the value of a inductor, measured in Henries, the more high frequencies are restricted
  • They come in a variety of sizes, gauges and form factors, each with their benefits and drawbacks.
  • Made using tightly-packed windings of copper wire or foil into a coil around an iron slug, bobbin, or simply held together by zip ties and possibly an extra coating of varnish or wax.

Common varieties include:

  • Iron-core
  • Air-core
  • Foil

Iron-Core Inductors

Iron-core inductors are the most common type of coil found in loudspeaker design.
They offer low resistance and they require less copper compared to other types of inductors, helping to keep their size & cost down.
The iron core, often a ferrite slug or series of laminated plates, help boost the strength of the magnetic field produced by the the coil, allowing for smaller gauge wire and fewer windings.
However, the iron cores tend to store energy from the electrical field, which can reduce clarity, especially in the midrange and treble. They are best suited for frequencies below 200-300Hz, which avoids the loss in clarity in the higher frequencies of a driver.
Solid steel slugs hold the most charge, and should be avoided whenever possible.
Ferrous composite slugs/cylinders are an alternative for smaller coils.
Laminated-steel plates, offer the best quality option as each laminated plated will hold a smaller charge than if it were a solid piece of steel, preventing smearing into the midrange.

Air-Core Inductors

Air-core inductors are the solution to all of the issues that iron-core inductors create. They are the second most common type of inductors found in speakers, typically used in the mid and tweeter circuits, where sizes are more reasonable.
By removing a ferrous core, you remove the stored charge of the iron, vastly improving clarity of the signal passing through them, especially in the midrange and into the treble.
Air core inductors are less reactive than iron, so they requires more windings than an iron core. They will often be larger, and have a higher resistance than an equivalent gauge iron core.
A larger gauge wire can be used to compensate for the higher resistance, further increasing costs and size.
They are our coil of choice for the vast majority of crossover applications.

Foil Inductors

These are the premium option among inductors, and are typically made from high-purity copper foil, which is separated by a very thin poly film or wax paper.
Compared to traditional air-core inductors, foil inductors are a fairly subtle upgrade, further improving clarity, separation and layering within the soundstage when compared to traditional wire-wound air cores.
They are best suited for bass through to the upper midrange, as they become less effective into the treble, especially above 3000-4000Hz.
We offer foil inductors as an optional upgrade for our NX-line of speakers.

 What About Litz Inductors?

Litz inductors are simply inductors made using multiple twisted strands of wire.
They can be used as part of iron-core or air-core inductors, like any typical wire-wound inductor. their design offers reduced skin effect than traditional wire or foil inductors in higher frequency applications, especially above 4000Hz, where a foil inductor typically falls short. Unless you’re putting an small inductor in-line with a tweeter, to roll off the top end, there aren’t many other cases where using a litz coil would truely be beneficial.
They are fairly uncommon in speakers, and we don’t use them in our kits, especially when a standard air core inductor offers similar performance without the added expense.

Inductor Placement & Orientation

Unlike capacitors and resistors, Inductors are very sensitive in regards to the placement of other near-by inductors, and should be kept perpendicular to one another. (See 6 & 7)
 When 2 coils are place next to one another in the same (or incorrect) orientation, (see 2-5) they begin to couple with one another, which transmits undesired frequencies to the other coils in circuit. (essentially cross-talk)
If 2 coils are stacked onto one another (see #4 & #8) they are directly coupled to one another, essentially creating a transformer. Every signal that one coil received will be induced/transferred into the other nearby coil, and should be avoided.

 Note:  The distances shown in these diagrams are suggestions, and not always practical, especially in a compact speaker.