@classmethod
def release(cls, duration, start_level=1, curve_shape=None):
    """
        Construct an simple decaying envelope
        :param duration: total decay length
        :param start_level: level decayed from
        :param curve_shape: shape of the curve
        :return: an Envelope constructed accordingly
        """
    curve_shapes = (curve_shape,) if curve_shape is not None else None
    return cls.from_levels_and_durations((start_level, 0), (duration,),
        curve_shapes=curve_shapes)
