Why a steady hiss can hide a quieter sound
White noise is made from rapidly changing sound energy spread across a broad range of frequencies. Because it contains many pitches at once, the ear hears it as a continuous hiss rather than as a single note. The familiar comparison is white light, which combines many visible wavelengths; white noise does something similar with audible frequencies.
The ear listens in overlapping bands
The inner ear sorts incoming sound into frequency bands, much like a set of overlapping filters. A sound is easiest to notice when it stands out from the sounds in the same band. If white noise is already filling that band, a quieter tone or rustle has less contrast and is more likely to be missed. This is called auditory masking.
Masking is strongest when the competing noise is near the quieter sound’s frequency, but a broad hiss can cover many bands at once. The result is not that the other sound has been erased: its vibrations still reach the ear. Rather, the brain has a harder time separating its signal from the surrounding sound.
That distinction explains why turning on a fan or a noise machine can make a room seem quieter without making it physically silent. A distant conversation or intermittent traffic may become less noticeable because its changes no longer stand out against the steady background. The noise works by changing what the ear can pick out, not by removing the sounds themselves.
