biquad.js

// Second-order IIR filter with arbitrary coefficient configurations
class IIRFilter {
  constructor(a0, a1, a2, b1, b2) {
    this.recalcCoeffs(a0, a1, a2, b1, b2);
  }
  
  recalcCoeffs(a0 = 0, a1 = 0, a2 = 0, b1 = 0, b2 = 0) {
    this.a0 = a0;
    this.a1 = a1;
    this.a2 = a2;
    this.b1 = b1;
    this.b2 = b2;
    this.z1 = 0;
    this.z2 = 0;
  }
  
  process(x) {
    const output = x * this.a0 + this.z1;
    this.z1 = x * this.a1 + this.z2 - this.b1 * output;
    this.z2 = x * this.a2 - this.b2 * output;
    return output;
  }
}

k-weighting.js

// K-weighting filter, used for LUFS metering
class KWeighting {
  constructor(sampleRate) {
    this.highpass = new IIRFilter();
    this.highshelf = new IIRFilter();
    this.recalcCoeffs(sampleRate);
  }
  
  recalcCoeffs(sampleRate = 44100) {
    const hpFreq = 80,
          hpQ = Math.SQRT2/2,
          hsFreq = 1500,
          hsGain = 10 ** (4/20),
          k1 = Math.tan(Math.PI * hpFreq/sampleRate),
          k2 = Math.tan(Math.PI * hsFreq/sampleRate),
          norm1 = 1 / (1 + k1 / hpQ + k1 ** 2),
          norm2 = 1 / (1 + Math.SQRT2 * k2 + k2 ** 2);
    
    // highpass part
    const hpA0 = norm1,
          hpA1 = -hpA0 * 2,
          hpA2 = norm1,
          hpB1 = 2 * (k1 ** 2 - 1) * norm1,
          hpB2 = (1 - k1 / hpQ + k1 ** 2) * norm1,
    // highshelf part
          hsA0 = (hsGain + Math.sqrt(hsGain*2) * k2 + k2 ** 2) * norm2,
          hsA1 = 2 * (k2 ** 2 - hsGain) * norm2,
          hsA2 = (hsGain - Math.sqrt(hsGain*2) * k2 + k2 ** 2) * norm2,
          hsB1 = 2 * (k2 ** 2 - 1) * norm2,
          hsB2 = (1 - Math.SQRT2 * k2 + k2 ** 2) * norm2;
    
    this.highpass.recalcCoeffs(
      hpA0,
      hpA1,
      hpA2,
      hpB1,
      hpB2
    );
    this.highshelf.recalcCoeffs(
      hsA0,
      hsA1,
      hsA2,
      hsB1,
      hsB2
    );
  }
  
  process(x) {
    return this.highshelf.process(this.highpass.process(x));
  }
}