FluidSim/Core/SoundProcessor.cs

using System;

namespace FluidSim.Core
{
    public class SoundProcessor
    {
        // Monopole state
        private double lastMassFlow = 0.0;
        private double dt;

        // Resonant band‑pass filter (second‑order)
        private double b0, b1, b2, a1, a2;
        private double x1 = 0, x2 = 0, y1 = 0, y2 = 0;
        private double pipeLength;

        // Reverb (outdoor)
        private float[] delayLine;
        private int writeIndex;
        private float feedback = 0.50f;
        private float lowpassCoeff = 0.70f;
        private float lastFeedbackSample = 0f;

        // Turbulence (pink noise scaled by U³)
        private PinkNoiseGenerator pinkNoise;
        private float turbulenceGain = 0.05f;
        private double pipeArea;
        private double ambientPressure = 101325.0;

        // Gains
        private float masterGain = 0.0005f;
        private float pressureGain = 0.12f;

        public SoundProcessor(int sampleRate, double pipeLengthMeters, double pipeDiameterMeters = 0.04, float reverbTimeMs = 200.0f)
        {
            dt = 1.0 / sampleRate;
            pipeLength = pipeLengthMeters;
            pipeArea = Math.PI * Math.Pow(pipeDiameterMeters / 2.0, 2.0);

            // Design resonant filter at pipe fundamental frequency
            double c = 340.0;
            double f0 = c / (4.0 * pipeLength);
            double Q = 15.0;
            double omega = 2.0 * Math.PI * f0;
            double alpha = Math.Sin(omega * dt) / (2.0 * Q);
            double norm = 1.0 / (1.0 + alpha);
            b0 = alpha * norm;
            b1 = 0.0;
            b2 = -alpha * norm;
            a1 = -2.0 * Math.Cos(omega * dt) * norm;
            a2 = (1.0 - alpha) * norm;

            // Reverb delay line
            int delaySamples = (int)(sampleRate * reverbTimeMs / 1000.0);
            delayLine = new float[delaySamples];
            writeIndex = 0;

            pinkNoise = new PinkNoiseGenerator();
        }

        public float MasterGain
        {
            get => masterGain;
            set => masterGain = value;
        }

        public float PressureGain
        {
            get => pressureGain;
            set => pressureGain = value;
        }

        public float TurbulenceGain
        {
            get => turbulenceGain;
            set => turbulenceGain = value;
        }

        public void SetAmbientPressure(double p) => ambientPressure = p;
        public void SetPipeDiameter(double diameterMeters) => pipeArea = Math.PI * Math.Pow(diameterMeters / 2.0, 2.0);

        public float Process(float massFlow, float pipeEndPressure)
        {
            // 1. Monopole source: d(mdot)/dt
            double derivative = (massFlow - lastMassFlow) / dt;
            lastMassFlow = massFlow;
            float monopole = (float)(derivative * masterGain);

            // 2. Pressure difference (low‑frequency component)
            float pressureDiff = (float)((pipeEndPressure - ambientPressure) / ambientPressure) * pressureGain;
            float mixed = monopole + pressureDiff;
            // DO NOT clamp here – let the filter and final clamp handle dynamics

            // 3. Resonant band‑pass filter
            double y = b0 * mixed + b1 * x1 + b2 * x2 - a1 * y1 - a2 * y2;
            x2 = x1; x1 = mixed;
            y2 = y1; y1 = y;
            float resonant = (float)Math.Clamp(y, -1f, 1f);

            // 4. Turbulence noise: amplitude ∝ U³ (empirical for low speeds)
            double velocity = massFlow / (pipeArea * 1.225);
            double Uref = 100.0;
            double turbulenceAmp = Math.Pow(Math.Abs(velocity) / Uref, 3.0);
            float pink = pinkNoise.Next() * turbulenceGain * (float)turbulenceAmp;

            resonant += pink;
            resonant = Math.Clamp(resonant, -1f, 1f);

            // 5. Outdoor reverb
            float delayed = delayLine[writeIndex];
            float filteredDelay = delayed * lowpassCoeff + lastFeedbackSample * (1f - lowpassCoeff);
            lastFeedbackSample = filteredDelay;
            float wet = delayed + filteredDelay * feedback;
            delayLine[writeIndex] = resonant + filteredDelay * feedback;
            writeIndex = (writeIndex + 1) % delayLine.Length;

            // 6. Dry/wet mix
            float output = resonant * 0.7f + wet * 0.3f;
            output = Math.Clamp(output, -1f, 1f);
            return output;
        }
    }

    internal class PinkNoiseGenerator
    {
        private readonly Random random = new Random();
        private readonly float[] whiteNoise = new float[7];
        private int currentIndex = 0;

        public PinkNoiseGenerator()
        {
            for (int i = 0; i < 7; i++)
                whiteNoise[i] = (float)(random.NextDouble() * 2.0 - 1.0);
        }

        public float Next()
        {
            whiteNoise[0] = (float)(random.NextDouble() * 2.0 - 1.0);
            currentIndex = (currentIndex + 1) & 0x7F;
            int updateMask = 0;
            int temp = currentIndex;
            for (int i = 0; i < 7; i++)
            {
                if ((temp & 1) == 0)
                    updateMask |= (1 << i);
                temp >>= 1;
            }
            for (int i = 1; i < 7; i++)
                if ((updateMask & (1 << i)) != 0)
                    whiteNoise[i] = (float)(random.NextDouble() * 2.0 - 1.0);
            float sum = 0f;
            for (int i = 0; i < 7; i++) sum += whiteNoise[i];
            return sum / 3.5f;
        }
    }
}