Any wave can be decomposed into two counter-travelling component waves.

Evidence

This equation states that any wave, here y(t, x), can be expressed as a sum of two waveforms, g1 travelling in the negative x-axis direction and g2 in the positive x-axis direction. This travelling-wave solution was published by d’Alembert in 1747. — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 180

Categorization

• XREF: The traveling-wave superposition principle resonates with Fourier decomposition used throughout signal processing — a foundational result reappearing across the book.