Noise cancelling a fan
29 comments
·September 15, 2025jerf
Unfortunately, this is not possible with such a simple approach. In 2 and higher dimensions, the problem is that any attempt to create a cancelling wave from a position other than the source of the wave will not cancel the wave. Instead it will create a network of places where it cancels and places where it constructively interferes, depending on the wavelength and their relative positions, and there is no way to make the entire space be cancelling in such a short space. You can only get various arrangements of cancellation but also constructive feedback.
(Some other things happen as you get a large number of wavelengths away from the source, but given the wavelength of the audio in question, being in a room with it means you get that local behavior, not long-range behavior.)
Probably somewhere on the internet is a fantastic interactive diagram that would clearly demonstrate this for you, but I couldn't google one up. Links solicited. (I got a lot 1D stuff but this phenomenon doesn't show up in 1D. 2D is adequate, 3D just adds more nodes in more dimensions.)
The way noise cancelling headphones work is that they know where they are relative to your eardrum, and as such, they can arrange it so that for all incoming audible frequencies, your eardrum is in a cancellation location for that frequency, ignoring a lot of details. They'll still unavoidably create locations of constructive interference, you just won't have your sensors there.
In principle you may be able to do this with some very precise location of where your ears are, where your mics are, where your speakers are and the exact characteristics of all of these things, and some very clever coding; I've seen people kicking this idea around but I haven't yet heard of anyone pulling it off. I can say it's still yet harder than it sounds at first, because you have things like echos and all kinds of other fun effects to deal with. In theory it should be possible to echo cancel at a distance, but you'd be getting into super high end audio processing, not just a weekend project where you record a microphone or two and "just" invert it with a couple of speakers. You might need something as fancy as https://youtu.be/UPVcwDzhBZ8?t=463 just to get started, and an accurate room model, and all kinds of things, and you might still get something that only works as long as nothing in the room moves, including you or even parts of you. In practice, I'd guesstimate this at the level of difficulty of doing a PhD in audio processing at a minimum... but not necessarily impossible.
NobodyNada
> Probably somewhere on the internet is a fantastic interactive diagram that would clearly demonstrate this for you, but I couldn't google one up. Links solicited.
Here's one: https://apenwarr.ca/beamlab -- as well as the author's writeup: https://apenwarr.ca/log/20140801
The author is focused on beamforming WiFi signals, but the principle is exactly the same whether it's a radio wave or a sound wave.
jorvi
My thought immediately jumped to beamforming / phased speaker array.
What's more problematic is that its not the lower frequencies that are annoying (the 312Mhz drone), but the mid and high range. Think about it like this: fridge compressors suck to hear with their 2500Hz high-pitched electrical buzz, but once the compressor turns off, the gentle but deep slosh of the liquid being pumped around isn't annoying at all.
jerf
Exactly what I was looking for, thank you.
Interestingly, the wavelength of sound and the wavelength of wifi signals are in the same ballpark. 900MHz electromagnetic waves come out to ~30cm waves, which is about 1000Hz in sound-in-air.
posix86
It would be possible if you had a matrix of speakers covering all walls & ceiling. In that scenario you could control the entire sound landscape across the board, and cancel out or simulate arbitrary sound sources in the room.
armada651
> The way noise cancelling works is that a microphone picks up the sound-wave, and then another speaker plays a slightly delayed version of that wave, which cancels it out.
I always thought noise cancelling worked by playing an inverted version of the sound wave rather than just a delayed one.
In fact, wikipedia seems to back me up on this:
> A noise-cancellation speaker emits a sound wave with the same amplitude but with an inverted phase (also known as antiphase) relative to the original sound.
maeln
For a sound of a specific frequency, a delay of half the wavelength is equivalent to the invert of the wave
https://graphtoy.com/?f1(x,t)=sin(x)&v1=true&f2(x,t)=sin(x-%...
useless_foghorn
If you have a single amplitude wave then a delayed playback could be inversion of the wave. Not very sophisticated, but good enough for canceling a single frequency.
armada651
The original post stated unequivocally that noise cancellation works by playing a delayed version of the sound before deciding it was only going to cancel a single frequency.
So either the text has been re-ordered or OP is under the mistaken impression this would also work when dealing with a mix of frequencies.
rthz
Interesting problem, not sure it’s worth solving. A larger diameter and slower rotating fan would likely produce less noise for a comparable air flow.
LeifCarrotson
A 20" Lasko box fan is about as cheap as they come - and, consequently, noisy and inefficient. The thing retails for $20, the budget for quieting measures is $0. In particular, especially when placed at a window (where it's expected to generate a pressure differential) you get a counterflow effect in the corners, and the 5 blades create individual pressure pulses as they move near to the 4 flat sides of the enclosure and then out into open air.
A quality high-volume, low-speed industrial drum or axial HVAC wall fan costs a whole lot more than $20 but the quiet, low-frequency noise is so much less intrusive.
Another mechanism (if you don't want a 36", 1/2 HP galvanized industrial contraption on your desk) is to concentrate the airflow near the user. Less power, but more concentrated. I've got a big fan that helps in the morning and evening to exchange air through the entire house, but on my desk I've got the biggest PC case fan I could find (a 230mm monster) wired to a speed controller cable and then directly to a 12V wall wart. At ~300 RPM, you can almost keep up with the motion of a single blade with your eyes, at 500 or 800 RPM it's barely perceptible... but it's only about 8x8x1" and keeps the air moving over your skin!
anjel
Windows dimensions are a limiting factor to this approach
afry1
Those Lasko fans have pretty raw edges on the blades of the fan itself, which I think contributes a lot to the noise. If you take the cover off, sand down the nubs and bits of flaking plastic, and reassemble, I think that will take care of a lot of noise.
hopelite
My first approach would be to experiment with some of the developed low noise blade designs
Rooster61
Is the phenomenon here why when you lay a sound waveform over another which is exactly out of phase copy (try this in Audacity or similar), the final product is silence? Or is that another aspect of sound waves that is similar but not quite the same effect?
drjasonharrison
This is exactly the physics occuring. Amplitude of sound is pressure. When two signals are 180 degrees out of phase, one is "increases the pressure" while the other "decreases the pressure" of the air.
In an sound editor, the waveforms can be perfectly aligned.
In the physical world, the waveform created by the fan spreads out through space. Providing an opposite but equal sound waveform at your ears is very hard (impossible) with a single speaker but can be done with sound cancelling headphones.
moralestapia
>can be done with sound cancelling headphones
How/Why?
I guess its because all noise just enters through two holes? So you reduce the dimensions back to 1.
munificent
Yes.
If you have two speakers playing the same tone (with perhaps different phases), then the distance between you and each speakers will affect the phase at the point that they reach your ears. At different places in the room, the phases will either be out of alignment and cancel out, or in alignment and reinforce. Unless you can literally have both speakers at the exact same position, there's no way have their phase difference be the same across the whole room.
Imagine throwing two rocks into a pond and the way the ripples interact and overlap. It's the exact same phenomenon. If you could throw the two rocks at exactly the same place (and somehow throw a "negative" rock that causes ripples to go up instead of down), then the waves would all cancel out. But if they are in two different locations, you'll get a whole mess of different interactions at different places in the pond.
This isn't an issue with noise-cancelling headphones, because the distance between the incoming sound, the headphone speaker, and your ear is always fixed.
drooby
Note - fans should be at least two feet away from the window to leverage the Bernoulli effect
jraph
I'm interested about this for next summer, could you or someone else expand on this / give pointers?
pshc
apparently this is the source: https://youtu.be/1L2ef1CP-yw
The fan increases air speed at the centre of the rotor, creating a low pressure zone which then sucks in surrounding air. So it helps to place the fan away from the window (roughly far enough that the wind cone "fits" the opening).
jraph
I would have loved to see that video 2 months ago. Thanks for sharing.
I tried to put the same kind of desk fan at the window, one way and then the other, for a few hours, to see if it had any effect. It was a very hot day but colder outside than inside. The building's concrete was likely still radiating the heat from the day before and there was no wind.
I see now that my observation at the time was right: it did nothing to the temperature, and it might have worked better if I had put the fan 1-2 meters away from the window, directing it towards the window. Now, whether the effect would have been significant anyway… we'll have to wait for next summer to know, I guess. I'm not particularly looking forward to it, though.
0xdada
How would this work without headphones in a room? I'm guessing the noise would actually be amplified in some parts of the room.
jajuuka
Not very helpful, but an interesting idea nonetheless. Would be interested to read someone who had a working prototype of this.
fluder_tw
I doubt it can work. Noise cancelling in earphones works good only because it is very narrow ear channel and you are not dealing with any phase shifts due to changing position relative to noise source. Also there is no other things like reverberation and room acoustic which also contributes to phase of the signal that we actually recieve in our ear channel. In other words noise cancellation is possible only in one specific position of listener in the room.
privatelypublic
Yup. I'm still learning- but it seems like a Helmholtz resonator would be best in this case.
Since a room has reflections and diffraction of the sound waves, the 312hz they're trying to dampen w/ ANC will be at different phases at different parts of the room. It's also not a point source.
fluder_tw
I would propose funny solution to change the room where 312hz will fall into one of the standing wave frequency. Then if you combine properly where to place fan and where to place your sit, you can achieve effect of cancelling at this frequency/position (and amplifying in other positions)
This won't work. In addition to what others have said about different points in the room having different distances to the fan and speaker, there are other issues:
1. The fan's fundamental frequency isn't perfectly stable, so even if you are in a spot where there speaker's tone cancels with it, the fan will probably wander around that frequency enough that the cancellation won't work well.
2. The fan isn't just a fundamental tone + noise. There are also a whole series of harmonics above it. You'll need to cancel those out too. Even if you do cancel out the fundamental, you'll still "hear" it because of the missing fundamental effect [1] otherwise. Cancelling those overtones out gets harder and harder because the higher the frequency, the more precise you need to be with phase to get proper cancellation.
3. Obviously, none of this will help with the atonal noise components of the fan's sound, which are significant. Though arguably, if you get rid of the droning tonal part, the remaining whooshing noise might actually be a nice sound.
I believe the most effective fixes here are:
1. Get a better, quieter fan that produces less noise to begin with.
2. Move the fan farther away. You don't necessarily need to filter the air from the window closest to you. Put it in a farther window. Or go all the way and get a whole house fan that puts the fan in the attic.
[1]: https://en.wikipedia.org/wiki/Missing_fundamental