AnalyserNode.minDecibels - Web APIs 编辑

The minDecibels property of the AnalyserNode interface is a double value representing the minimum power value in the scaling range for the FFT analysis data, for conversion to unsigned byte/float values — basically, this specifies the minimum value for the range of results when using getFloatFrequencyData() or getByteFrequencyData().

Syntax

var curValue = analyserNode.minDecibels;
analyserNode.minDecibels = newValue;

Value

A double, representing the minimum decibel value for scaling the FFT analysis data, where 0 dB is the loudest possible sound, -10 dB is a 10th of that, etc. The default value is -100 dB.

When getting data from getFloatFrequencyData() or getByteFrequencyData(), any frequencies with an amplitude of minDecibels or lower will be returned as 0.0 or 0, respectively.

Note: If a value greater than AnalyserNode.maxDecibels is set, an INDEX_SIZE_ERR exception is thrown.

Example

The following example shows basic usage of an AudioContext to create an AnalyserNode, then requestAnimationFrame and <canvas> to collect frequency data repeatedly and draw a "winamp bargraph style" output of the current audio input. For more complete applied examples/information, check out our Voice-change-O-matic demo (see app.js lines 128–205 for relevant code).

var audioCtx = new (window.AudioContext || window.webkitAudioContext)();
var analyser = audioCtx.createAnalyser();
analyser.minDecibels = -90;
analyser.maxDecibels = -10;

  ...

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();

Specifications

Browser compatibility

BCD tables only load in the browser

See also

• Using the Web Audio API