synthesis at https://acarabott.github.io/audio-dsp-playground/

hmm.js

let time = 0;

function getSine(freq, time) {
  const period = 2;
  const phase = freq * (time % period);
  return phase - Math.floor(phase);
	//return Math.sin(2 * Math.PI * freq * time) > 0.5 ? 1 : 0.0;
}

function loop(numFrames, outL, outR, sampleRate) {
  const freq = 40;
  const amp = 0.1;

  for (let i = 0; i < numFrames; i++) {
    let sample = getSine(freq, time) * (amp / 2.0);
    
   	sample += getSine(freq * 1.002, time) * (amp / 2.0);
    sample += getSine(freq * 0.998, time) * (amp / 2.0);

    sample += getSine(freq * 1.004, time) * (amp / 2.0);
    sample += getSine(freq * 0.996, time) * (amp / 2.0);
    
    outL[i] = sample;
    outR[i] = sample;

    time += 1 / sampleRate;
  }
}