A lot of people have slept with a fan on for twenty years and have never once asked why it works. It is worth knowing, because it tells you what to look for when you want the sound without the draught.
The short answer: a fan works because there is nothing in it. That sounds like a joke and it is the entire mechanism.
A fan is the same every second. No drops, no gusts, no melody, nothing that begins or ends. There is no event in it for your attention to catch on, nothing to follow and nothing to wait for, so within a minute or two most people stop hearing it at all.
Rain is not like that. Rain is broadband noise with texture: individual drops, density that changes, the odd gust across the window. Waves have a rhythm. Anything with a tempo has a next beat coming. For a lot of people that movement is the whole appeal and their attention slides straight off it. For others, attention keeps catching, and the sound they put on to settle becomes the thing they are tracking.
If you have ever wondered why you can sleep through a fan but not through rain, that is the difference, and it is not a fault on either side. It just means a steadier sound suits you.
This is the most useful thing on this page if white noise has ever felt harsh to you.
White noise carries equal energy at every frequency, including the very top of your hearing, which is why it reads as bright and hissy, like a radio between stations. A fan is band limited. The motor and the blades put out a lot of low and mid energy and very little of that top end, and a wall or a door takes off more again.
So the sound most people are picturing when they say "white noise" is usually closer to a fan than to actual white noise. If you have tried a white noise track and found it sharp, you very probably did not dislike noise. You disliked the top end. A fan, or pink or brown noise, is the same idea with that part taken off.
People talk about "a fan" as though it were one sound. The single biggest split is not the size of the fan, it is whether the fan pulses.
A ceiling fan is a large slow blade passing over your head. Each pass gives a small rise and fall in the sound, and any fan that is slightly out of balance adds a wobble, a tick or a faint scrape on top of it. That is a repeating event, which is precisely the thing a steady sound is supposed to not have.
Some people find that rhythm settling. Others find their attention locks onto it and starts counting, which is the worst possible outcome from a sound you put on to stop noticing things. If a ceiling fan has never worked for you but a box fan does, you are not imagining it, and the fix is a sound with no pulse in it rather than a quieter fan.
Broad, low, plenty of body, very little pulse. The default meaning of "fan sound" for most people and the steadiest of the real fans.
Airy and open, with the blade pass in it. The most divisive one, for the reason above.
A laptop or a desk fan. Higher pitched, narrower, one insistent note. Good for covering speech at a desk, a lot for a bedroom.
A wall and a doorway roll the top end off it and blur it. Softer and further away in a way that turning the volume down does not reproduce, because turning it down takes the low end off, not the high end.
Air conditioning, extraction, forced air heating. Steadier than any bladed fan because you are hearing moving air rather than a rotating object, though a wide duct adds a resonance of its own.
The largest version of the same thing. Enveloping, weighted toward the low end, and famous for putting people to sleep who insist they cannot sleep sitting up.
Almost never because of the sound. Because of the air.
A recording gives the sound and none of that, and it goes where the fan does not.
Fan noise is the easiest sound in the world to loop invisibly, because there is no memorable moment in it to hear come around a second time. Rain is full of recognisable gusts and runs of drops. A fan has none.
The catch is that a recording of a real fan is not only the fan. It also has the motor's rattle in it, a bearing tick, mains hum, the shape of the room it was recorded in, and whatever went past the window during the take. Those repeat. Once you have heard the same little tick twice, you are listening to the loop instead of the fan, and you cannot go back.
This is the argument for a fan sound that is built rather than captured: nothing gets in that was not put in.
That is where the join lives, and where an outro voice or a fade would be. It is the least tested part of any sound app and it tells you most of what you need to know.
A tick, a wobble, a scrape, a rise and fall. If you can count it, you will keep counting it at two in the morning.
Quiet fan noise loses its low end first, so a track that felt warm and full can turn thin and hissy at the level you would really sleep with.
The right one is the one you stop hearing, not the one that sounds richest in the first four seconds.
Some apps announce the next track, resume a queue, or throw an advertisement in when the thing you chose finishes. Better to find that out now than then.
Less than the internet implies. Fan sounds are not a treatment and nothing here is advice about a sleep problem. What holds up without stretching anything:
Luten is sound for the mind, and fan noise is where the difference between built and captured is easiest to hear, because there is nowhere for a flaw to hide.
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Luten is a wellness app, not a medical device. Nothing here is advice about treating a sleep or attention problem, and how long anyone listens is their own business.
Because there is nothing in it. A fan is the same every second: no drops, no gusts, no melody, nothing that starts or stops. There is no event for your attention to catch on and nothing to wait for, so after a minute or two most people stop hearing it entirely. Rain, waves and anything with a tempo all have events in them, and an ear that keeps catching on events is not doing anything wrong, it just wants a steadier sound.
No, and this is the most useful thing to know if white noise has ever felt harsh to you. White noise carries equal energy at every frequency, including the very top, which is why it reads as bright and hissy. A fan is band limited: the motor and the blades produce a lot of low and mid energy and very little of that top end, and the room takes off more again. So a fan is closer to what most people picture when they say white noise than white noise itself is. If white noise feels sharp, a fan is usually the thing you actually wanted.
Because a ceiling fan is not steady. A big slow blade passing overhead gives a regular rise and fall in the sound, and a slightly unbalanced one adds a wobble or a tick on top of that. That is a repeating event, which is exactly the thing a steady sound is supposed to not have. Some people find the rhythm soothing and some find their attention locks onto it and starts counting. A box fan, a vent or a fan heard from another room has far less of that pulse in it.
Usually because of the air rather than the sound. A real fan cools the room, dries your eyes and throat, and is unwelcome in winter or when someone else is in the bed. It also moves the curtains and the papers on the desk. People who have slept with a fan for years often want the sound with none of that, and a recording is also the only version available in a hotel room, on a plane or in an office.
Better than almost anything else, because there are no memorable moments in a fan to hear come around a second time. The catch is that a recording of a real fan is not only the fan. It also carries the motor's rattle, a bearing tick, mains hum, the room, and whatever passed the window while it was being recorded. Those repeat, and once you have heard one twice you are listening to the loop rather than the fan. The last ten seconds of a track is where the join lives, so it is the quickest place to check.
Fan Noise is the steady band limited hush of a fan running in another room. Laptop Fan is a fan running close by, one note with no change in it. Air Through A Vent is air moving steadily through a wide duct with the duct in it. Cabin Noise is the low enveloping hush of an aircraft cabin at altitude, and Wide Air is open air spread across the stereo field. All of them are looped so the join is not there to find, and none of them has a voice, music or an advertisement in it.
Also worth reading: rain sounds with no thunder, white noise, pink noise and brown noise, sleep sounds with no talking and sounds for waking up at 3am.