Even with jumps, the Fourier coefficients (\varepsilon_m) decay as (1/m) (for a step change). Meanwhile, the electric field or pressure wave is assumed to follow Bloch’s theorem:
[ \varepsilon(x) = \sum_{m=-\infty}^{\infty} \varepsilon_m , e^{i m K x}, \quad K = \frac{2\pi}{a} ] Even with jumps
The surprising answer is that when analyzing physical structures with abrupt changes—think square waves, step-index optical fibers, digital signals, or phononic crystals. e^{i m K x}