def impedance_delany_and_bazley(frequency, flow_resistivity):
    """
    Normalised specific acoustic impedance according to the empirical one-parameter model by Delany and Bazley.

    :param frequency: Frequency :math:`f`.
    :param flow_resistivity: Flow resistivity :math:`\\sigma`.

    The impedance :math:`Z` is given by

    .. math:: Z = 1 + 9.08 \\left( \\frac{1000f}{\\sigma}\\right)^{-0.75} - 11.9 j \\left( \\frac{1000f}{\\sigma}\\right)^{-0.73}

    """
    return 1.0 + 9.08 * (1000.0 * frequency / flow_resistivity
        ) ** -0.75 - 1.0j * 11.9 * (1000.0 * frequency / flow_resistivity
        ) ** -0.73
