In this paper, we study theoretically the propagation of waves in locally periodic systems. To this
end, we employ a simple method based on the impedance function which is similar to the one used in
the transmission line theory. Results are presented for the particular cases of mass-loaded strings
arrangements. These systems are easily reproducible in laboratory of introductory and/or advanced
courses of physics, and contain all the main characteristics of locally periodic media. Transmission
coefficient and normal frequencies are calculated for both mono and diatomic chains, and compared
with previously computed and experimental data. Moreover, we show that the corresponding
eigenfunctions can be obtained easily from the knowledge of the impedance function.