def calc_impedance(density_soil, vs_soil, density_rock, vs_rock):
    """
    Calculates the impedance contrast between the soil and rock
    Ref: Eq 7.23 from Kramer (1996)

    :param density_soil: Soil mass density
    :param g_soil: Soil shear modulus
    :param density_rock: Rock mass density
    :param g_rock: Rock shear modulus
    :return:
    """
    impedance = density_soil * vs_soil / (density_rock * vs_rock)
    return impedance