In a passive crossover, the inductor is the part Rod Elliott of Elliott Sound Products (ESP) calls, in general, the world’s worst passive component. Core choice is the trade that shows up first: size and DC resistance on one side, linearity on the other.
Short answer
For a low-power “utility” speaker, ESP says you’ll probably get away with a magnetic-cored inductor, but an air-cored coil is always better (ESP, “6dB Passive Crossovers”). Air-core coils don’t saturate. Cored coils (iron, ferrite) need fewer turns for the same inductance, so they can be smaller with lower DC resistance (DCR) — at the cost of distortion when the core saturates.
Why cores distort
ESP (“6dB Passive Crossovers”): unless the core is much larger than theoretically required, it will suffer from partial saturation, and that introduces distortion. Saturation depends on current and frequency, and is worst with high currents at low frequencies. That is why a woofer series inductor is the hard case.
What DCR does
ESP (6 dB article): coil DC resistance sits in series with the woofer. It reduces the damping the amplifier can apply, and it dissipates power as heat — watts the driver never gets. Thin wire makes a smaller coil with higher DCR; thicker wire lowers DCR but grows the coil. ESP’s example: 0.66 Ω at 5 A RMS dissipates 16.5 W, and hotter copper rises further in resistance.
Placement
ESP (passive crossover design): mount inductors at right angles to each other to cut mutual coupling, and keep them away from steel. ESP measured a 460 µH air-core coil that rose to 480 µH at 10 mm from a thin (0.8 mm) steel plate, and suggests 50 mm as the absolute minimum clearance in most cases. Coil orientation detail also lives on crossover layout mistakes.
Where it sits in the network
Values and slope still come from the published network and first-order vs second-order crossovers. Active vs passive is on active or passive crossover. Capacitor dielectric choice is on crossover capacitors: film vs electrolytic. Back to the Parts & kits hub.