PannerNode.rolloffFactor

var audioCtx = new AudioContext();
var panner = audioCtx.createPanner();
panner.rolloffFactor = 1;

const context = new AudioContext();
// all our test tones will last this many seconds
const NOTE_LENGTH = 4;
// this is how far we'll move the sound
const Z_DISTANCE = 20;

// this function creates a graph for the test tone with a given rolloffFactor
// and schedules it to move away from the listener along the Z (depth-wise) axis
// at the given start time, resulting in a decrease in volume (decay)
const scheduleTestTone = (rolloffFactor, startTime) => {
  const osc = new OscillatorNode(context);

  const panner = new PannerNode(context);
  panner.rolloffFactor = rolloffFactor;

  // set the initial Z position, then schedule the ramp
  panner.positionZ.setValueAtTime(0, startTime);
  panner.positionZ.linearRampToValueAtTime(Z_DISTANCE, startTime + NOTE_LENGTH);

  osc.connect(panner)
     .connect(context.destination);

  osc.start(startTime);
  osc.stop(startTime + NOTE_LENGTH);
};

// this tone should decay fairly quickly
scheduleTestTone(1, context.currentTime);
// this tone should decay slower than the previous one
scheduleTestTone(0.5, context.currentTime + NOTE_LENGTH);
// this tone should decay only slightly
scheduleTestTone(0.1, context.currentTime + NOTE_LENGTH * 2);