What is white noise?

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Why, when I go to an area that is blocked from radio signal, does my radio receive and/or output white noise or static instead of cutting out?

In: Technology

Radios have an “automatic gain control” (AGC). When the reception is weak, the circuit that deals with the radio frequencies turns up it’s amplification to try and get the best signal it can so it can then extract what actual audio it can. When the gain goes up to maximum, all it ends up amplifying is noise, radio interference from outside and random electrical noise generated inside the circuit components. There might be a weak signal hidden in the noise so you may hear a noisy radio signal as well, so it wouldn’t just cut out.

White noise is a random signal with equal intensity at every frequency…

It’s basically every sound at once.

So, for a human being who’s hearing range is 20Hz to 20kHz, when you hear white noise, you’re hearing all those frequencies at once at the same “volume”. Because of the way human hearing works, some frequencies are more intense than others at the same volume, 2kHz to 4kHz especially. So you hear a pretty annoying hiss sound that also seems to rumble.

A related type of noise is pink noise, which is similar to white noise but drops off at 3dB per octave, which more accurately models the “curve” of human hearing, and we actually find quite pleasant. Music is often mixed to a pink noise slope, and other things like rain also have a frequency response similar to pink noise.

When you tune a radio to a specific channel, it’s like telling it to only look for one “*color*” of radio wave. When music is coming through it’s like a radio station is flashing a light of that color in a way that will make the magnets in your speakers vibrate as if that song was being played right next to them.

When you’re close enough to the radio station that it’s light is brighter than any other of that color, other sources of that color get drown out. Like how you can’t see any stars when the sun is out.

When you’re far enough away that the light from one radio station is no brighter than another, your radio starts to get signals at random from any photon that happens to have the same “color” as the station you were tuned to. So, the magnet in your speakers starts to vibrate at random, and viola white noise.