output

$ node scope.js
6Hz: 4048
214Hz: 4048

scope.js

var ndarray = require('ndarray');
var fft = require('ndarray-fft');

var baseFrequencey = 220;
var floats = new Float64Array(8096);
for (var i = 0; i < floats.length; i++) {
    // floats[i] = (i/floats.length) * Math.sin(baseFrequencey * i / floats.length * 2 * Math.PI);
     floats[i] = Math.sin(baseFrequencey * i / floats.length * 2 * Math.PI);
}

var reals = ndarray(floats, [ floats.length, 1 ]);
var imns = ndarray.zeros([ floats.length, 1 ]);

fft(1, reals, imns);

var abs = function(re, im) {
  return Math.sqrt(Math.pow(re, 2) + Math.pow(im, 2));
};

for (var i = 0; i < reals.data.length; i++) {
  var amplitude = abs(reals.data[i], imns.data[i]),
      freq = baseFrequencey * i / floats.length;

  if(amplitude > 0.0001) {
    console.log('%dHz: %d', freq.toFixed(0), amplitude.toFixed(2));
  }
}