Stereo panning

<div class="wrap">
  <div class="meter l" id="ml"><i></i></div>
  <div class="field" id="field">
    <div class="mid"></div>
    <div class="dot" id="dot"></div>
    <button class="play" id="play" type="button" aria-label="play sound"><span class="tri"></span></button>
  </div>
  <div class="meter r" id="mr"><i></i></div>
</div>

.wrap{position:relative;width:94%;height:86%;display:flex;align-items:center;gap:4%}
.field{position:relative;flex:1;height:60%;border:1px solid var(--line);border-radius:12px;background:var(--surface);overflow:hidden}
.mid{position:absolute;left:50%;top:0;bottom:0;width:1px;background:var(--line)}
.dot{position:absolute;top:50%;width:14px;height:14px;border-radius:50%;background:var(--accent);transform:translate(-50%,-50%);left:50%}
.meter{width:10%;height:70%;border:1px solid var(--line);border-radius:6px;background:var(--surface);display:flex;align-items:flex-end;overflow:hidden}
.meter i{width:100%;height:10%;background:linear-gradient(0deg,var(--accent-3),var(--accent))}
.play{position:absolute;top:6px;right:6px;width:20px;height:20px;border-radius:50%;border:1px solid var(--line);background:var(--surface);display:grid;place-items:center;cursor:pointer;box-shadow:0 2px 6px rgba(0,0,0,.12);z-index:3}
.play .tri{width:0;height:0;border-style:solid;border-width:5px 0 5px 7px;border-color:transparent transparent transparent var(--fg);margin-left:1px}
.play:active{transform:scale(.92)}
@media (min-width:460px){
  .dot{width:26px;height:26px}
  .play{width:38px;height:38px;top:12px;right:12px}
  .play .tri{border-width:8px 0 8px 11px;margin-left:2px}
}

const dot = document.getElementById('dot');
const barL = document.querySelector('#ml i');
const barR = document.querySelector('#mr i');
let shotStart = null;
const IDLE_PERIOD = 3200;
function pos(t) {
  if (shotStart !== null) {
    const p = Math.min(1, (t - shotStart) / 1000);
    if (p >= 1) shotStart = null;
    return p * 2 - 1;
  }
  return Math.sin(((t % IDLE_PERIOD) / IDLE_PERIOD) * Math.PI * 2) * 0.85;
}
function frame(t) {
  const pan = pos(t);
  dot.style.left = (50 + pan * 42) + '%';
  const rNorm = (pan + 1) / 2;
  barL.style.height = (14 + (1 - rNorm) * 70) + '%';
  barR.style.height = (14 + rNorm * 70) + '%';
  requestAnimationFrame(frame);
}
requestAnimationFrame(frame);
document.getElementById('play').addEventListener('click', () => {
  shotStart = performance.now();
  const ctx = new (window.AudioContext || window.webkitAudioContext)();
  const now = ctx.currentTime;
  const osc = ctx.createOscillator();
  const gain = ctx.createGain();
  const panner = ctx.createStereoPanner();
  osc.type = 'sine';
  osc.frequency.value = 523.25;
  gain.gain.setValueAtTime(0.13, now);
  gain.gain.setValueAtTime(0.13, now + 0.9);
  gain.gain.exponentialRampToValueAtTime(0.0001, now + 1);
  panner.pan.setValueAtTime(-1, now);
  panner.pan.linearRampToValueAtTime(1, now + 1);
  osc.connect(gain).connect(panner).connect(ctx.destination);
  osc.start(now);
  osc.stop(now + 1);
  osc.onended = () => ctx.close();
});