point of low-frequency modes is deeper than the upper turning point of high-frequency modes. At a fixed frequency lower-$l$ modes penetrate more deeply into the solar interior than higher-$l$ modes. Therefore, higher-frequency modes, and to a lesser extent higher-$l$ modes, are more sensitive to surface perturbations.

@1990Natur.345..779L discuss the origin of the perturbation. If the perturbations were to extend over a significant fraction of the solar interior, asymptotic theory implies that the fractional mode frequency shift would depend mainly on $\nu_{n,l}/l$, which is not what we observe. This implies that the relevant structural changes occur mainly in a thin layer. @1988ESASP.286..321T found that the effect of perturbing a thin layer in the propagating regions of modes, such as the layer where the second-ionization of helium occurs, is an oscillatory frequency dependence in $\delta\nu_{n,l}$. Such a frequency dependence is also not observed, implying that the dominant frequency dependence is not the direct result of, for example, changes in the magnetic field at the base of the convection zone. If the perturbation was confined to the centre of the Sun the size of the frequency shift would increase with decreasing $l$ as low-$l$ modes penetrate deeper into the solar interior than high-$l$ 