As in Fig. 2, but comparing the sound speed profiles of solar models calculated with the 1-equation model and different values for the free parameters. One parameter is varied in each panel while keeping the other parameters at the default values. The parameter changed is specified in the legend. The quantity on the y-axis is defined as $\delta c^2/c^2=(c_{\text{helio}}^2-c_{\text{model}}^2)/c_{\text{helio}}^2$.