INDUCTORS
A quick guide to understanding inductor coils
Inductors are a critical component in any loudspeaker crossover. They are used to limit higher frequencies while allowing lower frequencies to pass more freely.
The inductance value — measured in Henries — determines how strongly high frequencies are attenuated. Larger values generally result in greater restriction of upper-frequency energy.
Inductors are available in a wide range of sizes, wire gauges, and construction styles, each offering its own balance of performance, efficiency, and cost.
They are typically constructed by winding copper wire or foil into a coil. This coil may be formed around an iron core or bobbin, or it may be self-supported in an air-core design and secured with varnish, wax, or ties.
Common inductor types include:
- Iron-core
- Air-core
- Foil
Iron-core inductors are among the most commonly used coil types in loudspeaker crossover design. They offer relatively low resistance and require less copper than many alternative designs, which helps keep both their size and cost manageable.
In these inductors, the core — often a ferrite slug or a stack of laminated steel plates — increases the strength of the magnetic field generated by the coil. This allows designers to achieve the desired inductance value with fewer windings or a smaller wire gauge.
However, iron-core inductors can store energy in the magnetic field, which may introduce subtle losses in clarity, particularly at higher frequencies. For this reason, they are typically best suited for low-frequency applications, often below approximately 200–300 Hz, where their efficiency benefits can be realized without significantly affecting midrange or treble performance.
Solid steel slugs tend to store the greatest amount of energy and are generally avoided in higher-fidelity designs. Ferrous composite cores can provide a workable alternative in smaller inductors. Laminated steel cores are often considered the preferred iron-core option, as the laminated structure reduces stored energy and helps minimize potential smearing into the midrange.
Air-core inductors address many of the limitations associated with iron-core designs. They are the second most common type of inductor found in loudspeaker crossovers and are frequently used in midrange and high-frequency circuits, where the required inductance values allow for more practical physical sizes.
By eliminating a ferrous core, air-core inductors avoid the magnetic energy storage effects associated with iron. This can help preserve signal clarity, particularly in the midrange and treble, where low distortion and transparency are most critical.
Because air is less effective than iron at concentrating magnetic flux, air-core inductors require more windings to achieve the same inductance value. As a result, they are often larger and exhibit higher resistance than an equivalent iron-core design.
To reduce this resistance, designers may use heavier-gauge wire, which further increases both size and cost.
For these reasons, air-core inductors are our preferred choice for the majority of crossover applications where maximum sonic transparency is the goal.
Foil inductors represent a premium approach to inductor design. They are typically constructed using high-purity copper foil separated by a thin insulating layer such as poly film or waxed paper.
Compared with conventional wire-wound air-core inductors, foil designs can offer subtle improvements in clarity, instrument separation, and perceived layering within the soundstage.
Foil inductors are often well suited for use from the bass region through the upper midrange. At very high frequencies — typically above approximately 3–4 kHz — their physical characteristics can make them less practical or effective than smaller wire-wound alternatives.
For this reason, foil inductors are commonly used in specific crossover positions where their performance benefits can be most fully realized.
We offer foil inductors as an optional upgrade in selected models within our NX speaker line.
Litz inductors are constructed using multiple individually insulated strands of wire that are twisted or woven together. This type of conductor can be used in both air-core and iron-core inductors, much like conventional wire-wound designs.
The primary advantage of Litz construction is reduced skin-effect losses at higher frequencies. This can make Litz inductors beneficial in certain high-frequency applications, particularly above roughly 4 kHz, where conventional foil designs may become less practical.
In loudspeaker crossovers, however, there are relatively few situations where a Litz inductor provides a meaningful performance advantage. One example may be the use of a small inductor in series with a tweeter to shape the extreme top-end response.
For this reason, Litz inductors are fairly uncommon in most loudspeaker designs. In many crossover applications, a well-designed air-core inductor can offer comparable performance with greater simplicity and lower cost. As a result, we do not currently use Litz inductors in our upgrade kits.
Inductor Placement & Orientation
Unlike capacitors and resistors, inductors are highly sensitive to the placement and orientation of nearby coils. To minimize unwanted interaction, adjacent inductors should generally be positioned perpendicular to one another whenever possible (see diagrams 6 and 7).
When two coils are placed close together in the same orientation (see diagrams 2–5), magnetic coupling can occur. This interaction allows signal energy from one inductor to be unintentionally transferred to another, which may introduce unwanted frequencies into the crossover circuit — a form of electrical “crosstalk.”
Stacking inductors directly on top of one another (see diagrams 4 and 8) results in very strong coupling, effectively forming a transformer. In this situation, signal energy present in one coil can be directly induced into the adjacent coil. This condition should be avoided whenever practical.
Note:
The spacing illustrated in these diagrams represents general guidelines. In compact loudspeaker designs, ideal distances may not always be achievable, so careful orientation becomes even more important.
