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Showing content from http://developer.mozilla.org/zh-CN/docs/Web/API/AnalyserNode/getByteFrequencyData below:

AnalyserNode.getByteFrequencyData() - Web API | MDN

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AnalyserNode接口的 getByteFrequencyData() 方法将当前频率数据复制到传入的 Uint8Array（无符号字节数组）中。

如果数组的长度小于 AnalyserNode.frequencyBinCount, 那么 Analyser 多出的元素会被删除。如果是大于，那么数组多余的元素会被忽略。

var audioCtx = new AudioContext();
var analyser = audioCtx.createAnalyser();
var dataArray = new Uint8Array(analyser.frequencyBinCount); // Uint8Array 的长度应该和 frequencyBinCount 相等
analyser.getByteFrequencyData(dataArray); // 调用 getByteFrequencyData 方法填充 Uint8Array

一个 Uint8Array（无符号字节数组）.

下面的例子展示了 AudioContext 创建一个 AnalyserNode, 然后用 requestAnimationFrame 和 <canvas> 去反复收集当前音频的频率数据，并绘制为一个柱状风格的输出 (频谱).

更多的例子/信息，查看 Voice-change-O-matic 演示 (相关代码在 app.js 的 128 行~205 行).

var audioCtx = new (window.AudioContext || window.webkitAudioContext)();
var analyser = audioCtx.createAnalyser();

  ...

analyser.fftSize = 256;
var bufferLength = analyser.frequencyBinCount;
console.log(bufferLength);
var dataArray = new Uint8Array(bufferLength);

canvasCtx.clearRect(0, 0, WIDTH, HEIGHT);

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

  analyser.getByteFrequencyData(dataArray);

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

  var barWidth = (WIDTH / bufferLength) * 2.5;
  var barHeight;
  var x = 0;

  for(var i = 0; i < bufferLength; i++) {
    barHeight = dataArray[i];

    canvasCtx.fillStyle = 'rgb(' + (barHeight+100) + ',50,50)';
    canvasCtx.fillRect(x,HEIGHT-barHeight/2,barWidth,barHeight/2);

    x += barWidth + 1;
  }
};

draw();

array (数组)

必须为Uint8Array, 频域数据将复制到该数组内。

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