import math


def tone_to_freq(tone):
  """
    returns the frequency of a tone. 

    formulas from
      * https://en.wikipedia.org/wiki/MIDI_Tuning_Standard
      * https://en.wikipedia.org/wiki/Cent_(music)
  """
  return math.pow(2, ((float(tone)-69.0)/12.0)) * 440.0