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Simulation Limitations

This simulator uses lumped-parameter modeling based on Thiele-Small parameters. It's accurate for low-frequency analysis within the linear operating range, but has specific limitations you should understand.

What This Tool Models Well

The simulator is accurate and reliable for:

  • Low-frequency response (typically below 200-300 Hz)
  • Sealed enclosure behavior across the operating range
  • Vented enclosure behavior including port resonance and tuning
  • Passive radiator systems with proper parameter entry
  • Impedance curves showing electrical load on the amplifier
  • Excursion prediction at moderate power levels
  • Comparing drivers in the same enclosure design
  • Alignment selection (QB3, BB4, etc.) and response shaping
  • Filter effects on response and excursion

For these applications, the simulator provides results that match real-world measurements well, assuming accurate T/S parameters and linear operation.

Fundamental Limitations

Lumped-Parameter Modeling

The simulator treats the driver and enclosure as a collection of discrete electrical, mechanical, and acoustic elements (resistances, masses, compliances). This works well at low frequencies where wavelengths are much larger than the driver and enclosure dimensions.

Breaks down when:

  • Wavelengths approach driver dimensions (typically above 200-300 Hz)
  • Enclosure dimensions become significant relative to wavelength
  • Complex enclosure geometries (horns, transmission lines) are involved

Linear Assumptions

The model assumes all parameters are constant and linear. In reality, T/S parameters change with excursion, frequency, temperature, and power level.

Not modeled:

  • Suspension nonlinearity: Cms changes with excursion (especially near Xmax)
  • Bl modulation: Force factor decreases as coil leaves gap
  • Thermal effects: Re increases with voice coil temperature, changing Qes
  • Parameter drift: Suspension stiffness changes with age and temperature
  • Inductance modulation: Le varies with coil position in some designs

At moderate power levels (well below Pe and Xmax), these effects are small. At very high power, they become significant and actual performance deviates from predictions.

What's Not Modeled

Enclosure Effects

Cabinet resonances and vibration:

  • Panel resonances that color the sound
  • Standing waves inside the enclosure
  • Diffraction from enclosure edges
  • Vibration transmission to mounting surfaces

Real enclosures aren't perfectly rigid. Thin panels can resonate and radiate sound. Internal standing waves can affect driver loading. These effects are design and construction-dependent.

Port and enclosure losses:

  • Port turbulence and nonlinear losses at high velocity
  • Acoustic resistance from fill material (partially modeled via Ql)
  • Leakage losses from imperfect sealing

The simulator uses simplified loss models (Ql for box losses, Qp for port losses). Actual losses depend on construction quality, fill material, and port design.

Driver Behavior

Cone breakup:

  • The cone stops acting as a rigid piston above a certain frequency
  • Breakup modes create peaks and nulls in response
  • Frequency depends on cone material, geometry, and size

For woofers, breakup typically occurs above 300-1000 Hz. This affects crossover design but not subwoofer applications.

Off-axis response:

  • The simulator shows on-axis response only
  • Directivity increases with frequency (beaming)
  • Off-axis nulls from cone geometry

At low frequencies, drivers radiate omnidirectionally. Above a few hundred Hz, directivity becomes significant. This matters for full-range speakers but less for subwoofers.

Distortion mechanisms:

  • Harmonic distortion from nonlinearities
  • Intermodulation distortion from multiple tones
  • Doppler distortion (minimal at low frequencies)

The simulator shows when you exceed Xmax or Pe, but doesn't predict distortion levels or spectra.

Room and Placement Effects

Boundary reinforcement:

  • The radiation space setting (half-space, quarter-space) approximates this
  • But real rooms have complex boundary interactions
  • Corner loading provides more gain than the simple model suggests

Room modes and standing waves:

  • Peaks and nulls at specific frequencies based on room dimensions
  • Frequency-dependent and position-dependent
  • Can be ±10 dB or more at certain frequencies

Room acoustics have a huge effect on what you actually hear. In-room response will differ significantly from the simulated anechoic response, especially below 100 Hz.

Reverberation and decay:

  • How long sound persists in the room
  • Affects perceived bass quantity and "tightness"
  • Sealed boxes are often preferred in reverberant spaces

Advanced Enclosure Types

The simulator supports sealed, vented, and passive radiator designs. It does not model:

  • Bandpass enclosures: Driver is inside a dual-chamber box (planned feature)
  • Transmission lines: Long tapered tubes behind the driver
  • Horns: Complex acoustic waveguides
  • Dipoles: Open-baffle designs with front-to-back cancellation
  • Compound systems: Isobaric loading, push-pull configurations

These designs require more sophisticated modeling techniques (FEA, BEM, or specialized tools like Hornresp).

Port Velocity Limitations

Port velocity is calculated assuming laminar flow and simple tube geometry. This gives a useful guideline (keep under 17 m/s) but has limitations:

  • Flared ports: Reduce effective velocity, not accurately modeled
  • Turbulence onset: Depends on port surface roughness and geometry
  • Noise threshold: Subjective, depends on listening distance and content
  • Slot ports: Behave differently than round ports at high velocity

Use port velocity as a guide, but the 17 m/s threshold is approximate. Some designs tolerate higher velocities, others produce audible noise below this.

Parameter Accuracy

Manufacturer Specifications

Simulation accuracy depends on T/S parameter accuracy. Issues include:

  • Measurement tolerance: Parameters vary between samples (typically ±10%)
  • Measurement conditions: Pre or post break-in, different temperatures
  • Derived parameters: Some manufacturers calculate rather than measure
  • Outdated specs: Driver revisions may not update published parameters

For critical applications, measure T/S parameters yourself or verify against multiple sources.

Custom Driver Entry

When entering custom drivers, ensure parameters are self-consistent:

  • Qts must equal (Qms × Qes) / (Qms + Qes)
  • Fs must equal 1 / (2π√(Mms × Cms))
  • Vas must be consistent with Cms and Sd

The simulator doesn't extensively validate parameter consistency. Garbage in, garbage out.

Using Simulation Results

For Initial Design

The simulator is excellent for:

  • Comparing different drivers in the same enclosure
  • Exploring box size and tuning trade-offs
  • Avoiding obviously bad designs
  • Understanding fundamental behavior

For Final Verification

Before building, consider:

  • Measurement: If possible, measure the driver's actual T/S parameters
  • Prototyping: Build a test enclosure and measure the response
  • Adjustability: Design in some tuning adjustment (port length, fill material)
  • Conservative margins: Leave headroom on excursion and power limits

When to Use Advanced Tools

Consider specialized tools for:

  • Hornresp: Horn and transmission line designs
  • VituixCAD: Full-range speakers with crossovers and diffraction
  • COMSOL/ANSYS: FEA for complex geometries and nonlinear analysis
  • Klippel: Comprehensive nonlinear parameter measurement

Summary

This simulator is a powerful tool for low-frequency enclosure design. It's accurate within its domain (simple enclosures, low frequencies, linear range) and excellent for comparison and exploration.

Understand its limitations. It doesn't replace measurement or account for all real-world factors. Use it to narrow down designs and understand trade-offs, then verify with measurements or careful listening tests.

For most DIY subwoofer and woofer projects, it provides more than enough accuracy to achieve good results.

Further Reading