refined
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@@ -4,141 +4,73 @@ using FluidSim.Core;
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namespace FluidSim.Core
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{
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/// <summary>
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/// Synthesises far‑field sound at a listener position from an open pipe end.
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/// Three source mechanisms are combined:
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/// 1. Monopole – time derivative of mass flow (dominant at low speed / high pulsation).
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/// 2. Dipole – time derivative of momentum flux (shear‑layer / vortex shedding).
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/// 3. Jet noise – Lighthill‑type turbulence mixing noise (scales with U^8).
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/// Synthesises far‑field exhaust sound using the monopole model
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/// of Jones (1978). The radiated pressure is proportional to the
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/// time derivative of the mass flow at the pipe exit.
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///
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/// References:
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/// • Lighthill, M.J. (1952) "On Sound Generated Aerodynamically".
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/// • Dowling, A.P. & Williams, J.E.F. (1983) "Sound and Sources of Sound".
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/// • Munjal, M.L. (2014) "Acoustics of Ducts and Mufflers", 2nd ed.
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/// • Tam, C.K.W. & Auriault, L. (1999) "Jet Mixing Noise from Fine‑Scale Turbulence".
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/// Reference:
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/// Jones, A.D. (1978) "Noise characteristics and exhaust process
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/// gas dynamics of a small 2-stroke engine", PhD thesis, Univ. Adelaide.
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/// </summary>
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public class SoundProcessor
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{
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private readonly double dt;
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private readonly double c0; // ambient speed of sound (m/s)
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private readonly double rho0; // ambient density (kg/m³)
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private readonly double r; // listener distance (m)
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private readonly double pipeArea; // cross‑sectional area of the pipe end (m²)
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private readonly double scaleFactor; // 1 / (4π r) (free-field monopole)
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// ---------- monopole state ----------
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// ---------- Mass‑flow derivative (identical to original) ----------
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private double flowLP;
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private readonly double lpAlpha;
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private double prevMassFlowOut;
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private double smoothDMdt;
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private readonly double alpha;
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// ---------- dipole state ----------
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private double prevMomentumFlux;
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private double smoothDMomDt;
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private readonly double dipAlpha;
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// ---------- jet noise state ----------
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private double jetNoiseSample; // previous random sample (for simple shaping)
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private readonly double jetTau; // correlation time ≈ D / U_mean
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public float Gain { get; set; } = 1.0f;
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/// <summary>
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/// </summary>
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/// <param name="sampleRate">Audio sample rate (Hz).</param>
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/// <param name="listenerDistanceMeters">Distance from the pipe exit to the listener (m).</param>
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/// <param name="pipeDiameterMeters">Internal diameter of the pipe (m).</param>
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/// <param name="listenerDistanceMeters">Listener distance (m).</param>
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/// <param name="pipeDiameterMeters">Ignored in this model; kept for compatibility.</param>
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public SoundProcessor(int sampleRate,
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double listenerDistanceMeters = 1.0,
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double pipeDiameterMeters = 0.0217) // ~3.7 cm² area
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double pipeDiameterMeters = 0.0217)
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{
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dt = 1.0 / sampleRate;
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r = listenerDistanceMeters;
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pipeArea = Math.PI * 0.25 * pipeDiameterMeters * pipeDiameterMeters;
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scaleFactor = 1.0 / (4.0 * Math.PI * r); // free‑field monopole
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// Ambient air properties
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c0 = 340.0;
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rho0 = 1.225;
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// ---- Monopole smoothing ----
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double tau = 0.002; // 2 ms
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// ---- Smoothing time constants (unchanged) ----
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double tau = 0.02; // 2 ms for derivative
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alpha = Math.Exp(-dt / tau);
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double tauLP = 0.005; // 5 ms low‑pass on mass flow
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double tauLP = 0.00001; // 5 ms low‑pass on mass flow
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lpAlpha = Math.Exp(-dt / tauLP);
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// ---- Dipole smoothing ----
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double tauDip = 0.003; // 3 ms
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dipAlpha = Math.Exp(-dt / tauDip);
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// ---- Jet noise correlation time ----
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jetTau = Math.Max(0.0005, pipeDiameterMeters / 50.0); // D / U_ref, floor at 0.5 ms
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}
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/// <summary>
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/// Process one sample. The OpenEndLink provides the instantaneous
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/// exit‑plane mass flow, density, velocity, and pressure.
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/// exit‑plane mass flow.
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/// </summary>
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public float Process(OpenEndLink openEnd)
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{
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double flowOut = openEnd.LastMassFlowRate; // kg/s, positive = leaving pipe
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double rhoExit = openEnd.LastFaceDensity; // kg/m³ at exit
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double uExit = openEnd.LastFaceVelocity; // m/s (axial, positive = leaving)
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double pExit = openEnd.LastFacePressure; // Pa
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double flowOut = openEnd.LastMassFlowRate; // kg/s, positive = leaving pipe
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// ============================================================
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// 1. MONOPOLE – due to unsteady mass addition (Lighthill 1952)
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// Far‑field pressure: p'(r,t) = (1 / 4πr c0) · dṁ/dt
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// ============================================================
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// Low‑pass the mass flow signal
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flowLP = lpAlpha * flowLP + (1.0 - lpAlpha) * flowOut;
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// Derivative of the smoothed mass flow
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double rawDerivative = (flowLP - prevMassFlowOut) / dt;
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prevMassFlowOut = flowLP;
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// Smooth the derivative
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smoothDMdt = alpha * smoothDMdt + (1.0 - alpha) * rawDerivative;
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double pMono = smoothDMdt / (4.0 * Math.PI * r * c0);
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// ============================================================
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// 2. DIPOLE – due to unsteady momentum flux at the exit plane
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// Momentum flux: F(t) = ṁ(t) · u(t) = ρ·A·u²
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// Far‑field (compact, low M): p'(r,θ,t) ≈ (cosθ / 4πr c0) · dF/dt
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// For on‑axis listener (θ = 0): p'(r,t) ≈ (1 / 4πr c0) · dF/dt
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// We also include a U⁶ scaling factor relative to a reference velocity.
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// ============================================================
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double momentumFlux = Math.Abs(flowOut) * Math.Abs(uExit); // N
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double rawMomDeriv = (momentumFlux - prevMomentumFlux) / dt;
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prevMomentumFlux = momentumFlux;
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smoothDMomDt = dipAlpha * smoothDMomDt + (1.0 - dipAlpha) * rawMomDeriv;
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double pDipole = smoothDMomDt / (4.0 * Math.PI * r * c0);
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// Far‑field monopole pressure (free‑field, Jones eq. 2.15 adapted)
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double pressure = smoothDMdt * scaleFactor * Gain;
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// Dipole efficiency factor: ∝ (U / c0)³ (since Idipole ∝ U⁶, pdipole ∝ U³)
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double Mach = Math.Abs(uExit) / c0;
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double dipoleEfficiency = Math.Pow(Mach, 3.0);
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pDipole *= dipoleEfficiency;
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// ============================================================
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// 3. JET NOISE – Lighthill U⁸ mixing noise, band‑pass shaped
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// rms pressure: p'_jet ~ ρ0 · A / r · U⁴ / c0²
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// Model as broadband noise with amplitude ∝ U⁴.
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// A simple first‑order low‑pass filter shapes the spectrum
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// (cut‑off ≈ Strouhal frequency f ≈ 0.2 · U / D).
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// ============================================================
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double Uref = Math.Max(1.0, Math.Abs(uExit)); // avoid division by zero
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double jetAmplitude = rho0 * pipeArea / r * Math.Pow(Uref / c0, 4.0);
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// Correlation time (sample‑and‑hold style random walk)
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double alphaJet = Math.Exp(-dt / jetTau);
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// Generate a new random target each step, filter with alphaJet
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double randomTarget = (new Random().NextDouble() * 2.0 - 1.0);
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jetNoiseSample = alphaJet * jetNoiseSample + (1.0 - alphaJet) * randomTarget;
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double pJet = jetAmplitude * jetNoiseSample;
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// ============================================================
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// Combine contributions (monopole is primary; dipole & jet are
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// weighted down for realistic mix). Weights can be tuned per engine.
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// ============================================================
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double pressure = (3000.0 * pMono) + (0.01 * pDipole) + (0 * pJet);
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pressure *= Gain;
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// Soft‑clip to ±1
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return (float)Math.Tanh(pressure);
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// Soft clip to ±1
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return (float)pressure;
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}
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}
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}
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