def note_to_frequency(note):
    """ Translates a MIDI sequential note value to its corresponding frequency
          in Hertz (Hz). Note values are of integer type in the range of 0 to
          127 (inclusive). Frequency values are floating point. If the input
          note value is less than 0 or greater than 127, the input is
          invalid and the value of None is returned. Ref: MIDI Tuning Standard
          formula.
    """
    if 0 <= note <= 127:
        return pow(2, (note - 69) / 12) * 440
    else: return None