Air coils

Air coils are consistently the “simplest” and at the same time physically cleanest coil design: They do not have a ferro- or ferrimagnetic core, but only work with the magnetic field of the winding in air. This completely eliminates core-related effects such as saturation, hysteresis and remagnetization losses. The inductance remains very stable over the course of the current, and the filter behavior does not change with level or dynamics. This is precisely why air coils are considered a reference in demanding switches when it comes to impulse fidelity and freedom from distortion.

However, the sound practice of air coils is determined less by the inductance “L” than by what is also inevitable: the DC resistance (R DC). To achieve a specific inductance without a core, you need more windings and therefore more conductor length — this increases DC resistance unless you work with a correspondingly larger conductor cross section. In bass, R _DC has a direct effect on efficiency, damping and control: Too high series resistance reduces pressure and precision from the bass, while a low R _DC (due to large conductor cross-sections) makes the bass sound much more confident. This is why air coils are excellent in bass when they are designed for low resistance — then they are uncompromising in terms of electricity and sound, just bigger and more material-intensive.

In addition to resistance, two “invisible” aspects play a role in air coils: stray field and mechanics. Because the magnetic field is not guided in the core, the stray field is more pronounced. This makes installation in the crossover crucial: distance and 90-degree orientation with respect to other coils prevent unwanted magnetic coupling, which can otherwise shift filter values. Another mechanical factor is that alternating currents generate forces in the coil. A neatly fixed, dampened coil remains steady, reduces micro-movements and prevents energy from being converted into side effects instead of music.