. The addition of an optimal amount of noise boosted a weak periodic signal, resulting in the greatest signal-to-noise ratio (top graph). In the power spectrum of the output (bottom graph), the weak signal, now de- tectable, appears as a narrow spike rising above a broad background of noise (Moss and Wiesenfeld 67).
Making the analogy to sound, which follows strict rules and laws, makes understanding literature easier for less creative types like me.