BiquadFilterNode.getFrequencyResponse()

The getFrequencyResponse() method of the BiquadFilterNode interface takes the current filtering algorithm's settings and calculates the frequency response for frequencies specified in a specified array of frequencies.

The two output arrays, magResponseOutput and phaseResponseOutput, must be created before calling this method; they must be the same size as the array of input frequency values (frequencyArray).

frequencyArray

A Float32Array containing an array of frequencies, specified in Hertz, which you want to filter.

magResponseOutput

A Float32Array to receive the computed magnitudes of the frequency response for each frequency value in the frequencyArray. For any frequency in frequencyArray whose value is outside the range 0.0 to sampleRate/2 (where sampleRate is the sample rate of the AudioContext), the corresponding value in this array is NaN. These are unitless values.

phaseResponseOutput

A Float32Array to receive the computed phase response values in radians for each frequency value in the input frequencyArray. For any frequency in frequencyArray whose value is outside the range 0.0 to sampleRate/2 (where sampleRate is the sample rate of the AudioContext), the corresponding value in this array is NaN.

InvalidAccessError

The three arrays provided are not all of the same length.

In the following example we are using a biquad filter on a media stream (for the full demo, see our stream-source-buffer demo live, or read the source.) As part of this demo, we get the frequency responses for this biquad filter, for five sample frequencies. We first create the Float32Arrays we need, one containing the input frequencies, and two to receive the output magnitude and phase values:

var myFrequencyArray = new Float32Array(5);
myFrequencyArray[0] = 1000;
myFrequencyArray[1] = 2000;
myFrequencyArray[2] = 3000;
myFrequencyArray[3] = 4000;
myFrequencyArray[4] = 5000;

var magResponseOutput = new Float32Array(5);
var phaseResponseOutput = new Float32Array(5);

Next we create a <ul> element in our HTML to contain our results, and grab a reference to it in our JavaScript:

<p>Biquad filter frequency response for: </p>
<ul class="freq-response-output">
</ul>

var freqResponseOutput = document.querySelector('.freq-response-output');

Finally, after creating our biquad filter, we use getFrequencyResponse() to generate the response data and put it in our arrays, then loop through each data set and output them in a human-readable list at the bottom of the page:

var biquadFilter = audioCtx.createBiquadFilter();
biquadFilter.type = "lowshelf";
biquadFilter.frequency.value = 1000;
biquadFilter.gain.value = range.value;

  ...

function calcFrequencyResponse() {
  biquadFilter.getFrequencyResponse(myFrequencyArray,magResponseOutput,phaseResponseOutput);

  for(i = 0; i <= myFrequencyArray.length-1;i++){
    var listItem = document.createElement('li');
    listItem.innerHTML = '<strong>' + myFrequencyArray[i] + 'Hz</strong>: Magnitude ' + magResponseOutput[i] + ', Phase ' + phaseResponseOutput[i] + ' radians.';
    freqResponseOutput.appendChild(listItem);
  }
}

calcFrequencyResponse();

• Using the Web Audio API