AnalyserNode.getFloatFrequencyData() 返回负值

Question

I'm trying to get the volume of a microphone input using the Web Audio API using AnalyserNode.getFloatFrequencyData() .

The spec states that "each item in the array represents the decibel value for a specific frequency" but it returns only negative values although they do look like they are reacting to the level of sound - a whistle will return a value of around -23 and silence around -80 (the values in the dataArray are also all negative, so I don't think it's to do with how I've added them together). The same code gives the values I'd expect (positive) with AnalyserNode.getByeFrequencyData() but the decibel values returned have been normalised between 0-255 so are more difficult to add together to determine the overall volume.

Why am I not getting the values I expect? And/or is this perhaps not a good way of getting the volume of the microphone input in the first place?

function getVolume(analyser) {
  analyser.fftSize = 32; 
  let bufferLength = analyser.frequencyBinCount;
  let dataArray = new Float32Array(bufferLength);
  analyser.getFloatFrequencyData(dataArray);
  let totalAntilogAmplitude = 0;
  for (let i = 0; i < bufferLength; i++) {
    let thisAmp = dataArray[i]; // amplitude of current bin
      let thisAmpAntilog = Math.pow(10, (thisAmp / 10)) // antilog amplitude for adding
      totalAntilogAmplitude = totalAntilogAmplitude + thisAmpAntilog; 
  }
  let amplitude = 10 * Math.log10(totalAntilogAmplitude);
  return amplitude;
}

Answer 1

Your code looks correct. But without an example, it's hard to tell if it's producing the values you expect. Also, since you're just computing (basically), the sum of all the values of the transform coefficients, you've just done aa more expensive version of summing the squares of the time domain signal.

Another alternative would square the signal, filter it a bit to smooth out variations, and get the output value at various times. Something like the following, where s is the node that has the signal you're interested in.

let g = new GainNode(context, {gain: 0});
s.connect(g);
s.connect(g.gain);
// Output of g is now the square of s

let f = new BiquadFilterNode(context, {frequency: 10});
// May want to adjust the frequency some to other value for your needs.
// I arbitrarily chose 10 Hz.
g.connect(f).connect(analyser)

// Now get the time-domain value from the analyser and just take the first 
// value from the signal.  This is the energy of the signal and represents 
// the volume.