The Wright Model: Accurate Voice Coil Inductance Modelling
If you have ever compared a measured impedance curve to a simulated one and noticed the high-frequency end does not match, voice coil inductance modelling is almost certainly the reason.
The problem with ideal inductance
The simplest driver model treats the voice coil inductance (Le) as an ideal inductor. An ideal inductor's impedance rises in direct proportion to frequency — double the frequency, double the impedance.
Real voice coils do not behave this way. Measured impedance curves show a high-frequency rise that is shallower than an ideal inductor predicts. The impedance rises roughly proportional to √f rather than f.
This discrepancy matters for several reasons:
Crossover design. If you are designing a passive crossover that includes the subwoofer's impedance in the network, an incorrect high-frequency impedance model produces wrong crossover slopes and points.
Impedance minimum calculations. Amplifier compatibility depends on the impedance minimum. An ideal inductor model can overestimate impedance at mid-frequencies, missing a dip that falls below the amplifier's minimum load.
Power delivery. The impedance curve determines how much current (and therefore power) the amplifier delivers at each frequency. Getting it wrong means the simulated SPL does not match reality.
Why real voice coils are not ideal inductors
A voice coil is wound around a former that sits inside a magnetic gap. The pole piece (the iron core of the magnet) is right next to the coil.
When alternating current flows through the coil, it induces eddy currents in the pole piece. These eddy currents create their own magnetic field that opposes the coil's field, effectively reducing the inductance. Because eddy currents increase with frequency, the effective inductance decreases with frequency - hence the shallower-than-ideal impedance rise.
Some drivers include a shorting ring (a copper or aluminium ring) around the pole piece specifically to enhance this effect. The shorting ring provides a low-resistance path for eddy currents, reducing the effective inductance at high frequencies and flattening the impedance curve. This reduces distortion and improves high-frequency response.
The Wright semi-inductance model
The Wright model replaces the ideal inductor with a semi-inductance element. Where an ideal inductor's impedance rises in direct proportion to frequency, the Wright model uses a relationship that rises more gradually — proportional to the square root of frequency rather than frequency itself.
This single conceptual change captures the eddy current effect with remarkable accuracy. The resulting impedance curve matches measured data far better than the ideal model across the full frequency range. RokketBox derives the necessary parameters automatically from the driver's published Le specification.
What this means in practice
The difference between ideal and Wright inductance models is most visible above 200 Hz - above the subwoofer's primary operating range. But it matters for:
Impedance-compensated systems. Any system that uses the impedance curve for calculations (passive crossovers, amplifier loading analysis) benefits from the correct model.
4th-order bandpass enclosures. The bandpass response is sensitive to the impedance curve shape because the coupled chambers interact through the driver's impedance. An incorrect inductance model shifts the bandpass response.
Impedance matching. When running multiple drivers on a single amplifier channel, the combined impedance depends on each driver's full curve. Accurate modelling prevents surprises.
How RokketBox implements it
RokketBox uses the Wright semi-inductance model by default for all driver simulations. When you look at the impedance plot in the simulator, the high-frequency rise reflects the semi-inductance behaviour, not an ideal inductor.
If a driver's T/S parameters include Le (as most do), RokketBox converts it to the Wright semi-inductance coefficient automatically. The result is an impedance curve that matches what you would measure with a real impedance analyser - not the textbook idealisation that most online calculators produce.