AnalyserNode: свойство fftSize

const audioCtx = new AudioContext();
const analyser = audioCtx.createAnalyser();

// …

analyser.fftSize = 2048;
const bufferLength = analyser.frequencyBinCount;
const dataArray = new Uint8Array(bufferLength);
analyser.getByteTimeDomainData(dataArray);

// draw an oscilloscope of the current audio source

function draw() {
  drawVisual = requestAnimationFrame(draw);

  analyser.getByteTimeDomainData(dataArray);

  canvasCtx.fillStyle = "rgb(200 200 200)";
  canvasCtx.fillRect(0, 0, WIDTH, HEIGHT);

  canvasCtx.lineWidth = 2;
  canvasCtx.strokeStyle = "rgb(0 0 0)";

  canvasCtx.beginPath();

  const sliceWidth = (WIDTH * 1.0) / bufferLength;
  let x = 0;

  for (let i = 0; i < bufferLength; i++) {
    const v = dataArray[i] / 128.0;
    const y = (v * HEIGHT) / 2;

    if (i === 0) {
      canvasCtx.moveTo(x, y);
    } else {
      canvasCtx.lineTo(x, y);
    }

    x += sliceWidth;
  }

  canvasCtx.lineTo(canvas.width, canvas.height / 2);
  canvasCtx.stroke();
}

draw();