Inside the RokketBox Simulation Engine
Most subwoofer calculators online give you a frequency response curve that looks smooth. It is not. They are computing a handful of points and drawing lines between them. RokketBox uses a fundamentally different modelling approach — and a much denser frequency sweep.
Circuit-domain simulation
Traditional simplified subwoofer models use closed-form transfer functions derived from the Thiele-Small framework. These are fast and provide reasonable approximations for simple cases: a driver in a sealed or vented box with ideal port behaviour.
RokketBox uses circuit-domain simulation instead. The entire acoustic system — driver, enclosure, port, radiation load — is modelled as an equivalent electrical network where each physical behaviour (mass, compliance, damping, radiation, losses) maps to a corresponding circuit element. The full network is solved at each frequency point, which gives us several advantages over simplified models:
Composability. When we want to account for a new physical effect — port turbulence, voice coil losses, radiation loading — we add it to the network rather than rederiving the maths from scratch. The model grows with the physics instead of fighting it.
Accuracy at extremes. Simplified models break down at frequency extremes, near resonances, and with non-ideal components. The circuit approach handles these correctly because it solves the full system of interactions, not a pre-simplified approximation.
Multiple enclosure types. Sealed, vented, and bandpass enclosures are all variations of the same network topology. We do not need a separate model for each type — we reconfigure the network and solve.
Why so many points?
Most calculators use a few dozen frequency points and draw lines between them. RokketBox uses a much denser logarithmic sweep across the full operating range. This density matters for three reasons:
Narrow features. Impedance peaks, port resonances, and the impedance saddle point in vented enclosures are narrow features that 50 points can miss entirely. The tuning frequency might fall between two computed points, making the impedance saddle look like a plateau instead of a sharp dip.
Group delay accuracy. Group delay is the derivative of phase with respect to frequency. Numerical differentiation requires closely spaced points to produce smooth, accurate results. Coarse spacing creates noisy, jagged group delay curves.
Port velocity peaks. Port velocity can spike sharply near the tuning frequency. Missing the peak with coarse spacing means underreporting the maximum velocity - which means you might not catch a turbulence problem until you build the box and hear it chuffing.
Bessel/Struve radiation loading
Most simplified models treat the driver's radiation load as a simple constant. In reality, the radiation impedance of a circular piston (the cone) is a complex, frequency-dependent function described by Bessel and Struve functions.
At low frequencies (where the cone is small compared to the wavelength), the radiation resistance is low and the driver is an inefficient radiator. As frequency rises, radiation resistance increases until the cone circumference approaches one wavelength, after which the driver becomes a more efficient radiator.
RokketBox computes the full Bessel/Struve radiation impedance and includes it in the circuit model. This gives accurate absolute SPL values (not just relative response shapes) and correct impedance curves at the frequency extremes.
Iterative port turbulence
Port velocity and port impedance are mutually dependent: the impedance depends on the velocity (turbulence adds resistance), and the velocity depends on the impedance (which determines how much air flows through the port). This circular dependency means you cannot solve it in a single pass.
RokketBox handles this with an iterative convergence approach at each frequency point. The solver starts with the linear solution, evaluates the resulting turbulence, feeds that back into the model, and repeats until the answer stabilises. This captures the non-linear compression that occurs at high excursion levels — the effect where driving the system harder produces diminishing returns because turbulence absorbs an increasing fraction of the energy.
BL compression
The motor force factor (BL product) is not constant across the driver's excursion range. At rest, BL is at its peak. As the cone moves outward or inward, the voice coil shifts partially out of the magnetic gap, and BL drops — often by 30–50% at the rated Xmax.
RokketBox models this progressive loss of motor force using a smooth falloff curve derived from the driver's excursion limits. The result is an SPL prediction that reflects real-world power compression and rising distortion at high excursion levels, rather than the optimistic linear assumption most calculators make.
Lossy voice coil inductance
Voice coil inductance (Le) is not a simple ideal component. Real voice coils exhibit frequency-dependent losses due to eddy currents in the pole piece and shorting rings. RokketBox uses an advanced inductance model that captures the correct impedance rise at high frequencies — a shallower rise than an ideal inductor predicts, matching what you would actually measure on an impedance analyser.
Most simplified models either ignore Le entirely or treat it as ideal, both of which produce incorrect impedance curves above a few hundred Hz.
What you see in the simulator
Every plot in RokketBox - SPL, impedance, group delay, excursion, port velocity, transfer function - is derived from this single circuit-domain solution. There is no separate model for each plot; they are all different views of the same underlying physics.
When you change a parameter (box volume, tuning, port area), the engine resolves the entire frequency sweep with the full circuit model. On modern hardware, this is fast enough for real-time interaction as you adjust the sliders.