def spring_stiffness(rho, c, sn, vol):
    """This function computes the air spring stiffness
    according to Var der Aa (2010) eqs 2.5 in page 10.
    Parameters
    ----------
    rho     (float): Air density (kg/m3)
    c       (float): Speed of sound (m/s)
    sn      (float): Neck opening surface (m2)
    vol     (float): Total resonator volume (m3)
    Returns
    -------
    s       (float): Air spring stiffness (N/m)
    """
    s = (rho * (c ** 2) * (sn ** 2)) / vol
    return s