def critical_frequency(subsystem_plate, subsystem_cavity):
    """
    Critical frequency.
    
    .. math:: f_c = \\frac{ c_0^2 \\sqrt{3}  } {\\ pi c_L h}
    
    .. math:: f_c = \\frac{f c_0^2}{c_B^2}
    
    See Craik, table 3.3, page 51.
    """
    return subsystem_plate.frequency.center * (subsystem_cavity.
        soundspeed_group / subsystem_plate.component.subsystem_bend.
        soundspeed_phase) ** 2.0
