How a steady hum hides changing sounds
A fan does not stop traffic noise from reaching your ears. Instead, its continuous sound makes it harder to distinguish the traffic from the surrounding soundscape. The ear and brain are especially good at noticing changes—a passing engine that grows louder, shifts in pitch, then fades. When a fan supplies a steady layer of sound across many pitches, some of those contrasts become less noticeable.
This is called sound masking: one sound makes another harder to hear, even though the quieter sound is still present. A fan is not necessarily true white noise, which has energy spread evenly across frequencies, but its broad, steady hum can have a similar masking effect.
Why the brain notices less
Hearing is not just a matter of detecting sound; the brain also sorts incoming sound into meaningful events. A sudden horn or a voice can stand out against silence because it differs sharply from the background. Add a constant hum, and that difference may shrink. The traffic is still there, but its changing details are less distinct, so it may attract less attention.
Masking works best when the background sound overlaps the frequencies of the sound you want to make less noticeable. It is not a soundproof barrier: louder noises, or sounds with a very different pitch, may still cut through.
What the fan can—and cannot—do
For someone trying to sleep or concentrate, a fan can make an irregular environment feel more even by reducing the contrast between quiet moments and passing noises. Its benefit comes from changing what is easiest to pick out, not from removing the source. Turn the fan off, and the traffic remains; turn it on, and the passing sounds may simply become less prominent.
