helper method to extract features for this chunk
function extractFeatures(chunk) {
    //make it a F32A for efficiency
    var frame = arrayToTyped(chunk);
    //run the extraction of selected features
    var fset = Meyda.extract(featuresToExtract, frame);
    for (let j = 0; j < featuresToExtract.length; j++) {
        var feature = fset[featuresToExtract[j]];
        features[featuresToExtract[j]].push(feature);
    }
  }