function helpers_freqToMidi(f) {
  var mathlog2 = Math.log(f / 440) / Math.log(2);
  var m = Math.round(12 * mathlog2) + 69;
  return m;
}
Returns the closest MIDI note value for
 a given frequency.

 @method freqToMidi
 @param  {Number} frequency A freqeuncy, for example, the "A"
                            above Middle C is 440Hz
 @return {Number}   MIDI note value