def lower_frequency(center, order=1):
    """
    Lower frequency of frequency band given a center frequency and order.
    
    :param center: Center frequencies.
    :param order: Fraction of octave.
    
    .. math:: f_l = f_c \cdot 2^{\\frac{-1}{2N}}
    
    """
    return center * 2.0**(-1.0/(2.0*order))