Relative sound speed squared perturbation for two models with M = 1.3   M_⊙ and ages of 2.25 Gyr (smaller amplitude) and 4.0 Gyr (larger amplitude). The symbols show the perturbation obtained by subtracting the squares of the sound speeds of the original and smooth models and dividing by the the sound speed squared of the smooth model. The continuous lines show over-plots of the rhs of Eq. (1). Shown also are the quantities A_δ, Δ, and r_d, which appear in Eq. (1), for the model of 4.0 Gyr of age.