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.
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.
So it could be possible if the cancelling came from the source of the wave, such as making the spinning fan or its enclosure the speaker?
I guess the 'fan as the noise cancelling speaker' idea could be reworded as 'a fan with active stabilization that doesn't vibrate', making no mechanical noise.
A friend with a recording studio solved their problem by putting the fan at the end of a length of ducting with a couple of 90 degree bends, lined with foam.
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!
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?
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.
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.
> 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.
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.
Try using a microphone and closed loop generating sound and measuring the result in realtime. Randomly mutating and layering noise to find the best fit.
Even if this worked (it will not and cannot), it will only work in SOME places, whereas it will constructively ADD in others since the fan and the speaker are not co-located. To make it work in ALL places you'll need an infinite number of speakers with precise phase control for each. This is how beamforming works. Fun stuff. Mathematically quite a bit more complex than "put a BT speaker near a fan"
Why won't this work even if you could co-locate your speaker with the fan? Fan noise is neither stable in time nor as simple as a sine wave. This would need a complex active system to work.
A more interesting question is probably something like, what’s the quietest (practical) way to move air around the room? My money is on a high-end ceiling fan.
It is a solved problem for those who can install fans, filters, exhaust and intake valves far away from the room itself and just carry the stale and fresh air via ducts. Always fresh air without noise and mosquitoes.
What I don't get is why the lowest speed on an adjustable speed fan is always so high. It's like if your car could either be parked or go 70-120 MPH. Sometimes I just want a large light breeze, not the papers to fly off my desk.
This begs the question of what is the most quiet fan(s) on the market? If all of us sat down in a room together and had this mission, what would it look like? An electrostatic fan (using electrohydrodynamics) would be pretty silent, but provide very little airflow. It's a fun problem to solve and would love to see a company create better products than those overpriced Dyson fans (which are junk IMHO).
If you have a furnace or air handler, it should already be sized to change the air inside a building multiple times per hour which should keep the air nice and clean assuming you change the filters when needed. ‘Change the air’ in this case means the entire air volume of the building passes through the filter, not a total exchange of indoor and outdoor air.
If you want a quiet but expensive solution that doesn’t involve a properly sized air handler, read on.
The solution here is to put the fan outside the building mounted on ductwork that goes inside the building with a damper you can open and close. “Powered roof ventilator” is what you should google.
Then add in a makeup air fan that is interlocked with the exhaust fan (and the dampers on both the exhaust fan and makeup air unit) outside and mounted on ductwork so your house isn’t negatively pressurized which would have the opposite effect, dirt and dust would enter the house. “Makeup air fan” is what you should google.
If you’ve ever been in a commercial kitchen, the vent hood is connected to ductwork that goes outside the building where the exhaust fan in mounted, and there’s always a makeup air unit mounted outside sized roughly the same (cfm) as the exhaust fan to push air into the kitchen to maintain the air pressure and prevent negative pressurization. A (typical modern) large building has multiple intake and exhaust fans that work in concert with a building automation system to maintain air pressure. The idea is to maintain slight positive pressure with respect to the outside air pressure to prevent dust and dirt from being sucked into the building.
I actually use a LectroFan device _for_ noise cancelling. It's not a fan (originally it was) and I use the (Color)Noise sounds for the purpose but nonetheless. Also when traveling in Asia I always found the AC noise droning out the city noise to be a welcome side effect.
Since all comments seem to agree that noise cancelling wont work, 3 practical tips:
1. use the Bernoulli-Effect, aka get "up to 50+%"(depends on fan model) more airflow by placing the fan 0.2(0.5) - 1.5m away from the window.
2. Blow air out of the window, dont try to suck air in.
3. To avoid noise: Put the fan in another room and open the window in the room where you are.
As others mentioned this will only work in the right spot in the room.
This was an experiment in my high school physics, where our teacher played a constant tone from two sources and students were to walk around the class room searching for dead spots where the phases would cancel each other out.
It was quite surreal to take a step and suddenly the tones went quiet.
If it could work marginally, I suspect this would require at least a pair of giant subwoofers right next to the fan, but then I wouldn't expect it work very effectively.
Instead, it's probably a better idea to not create the noise in the first place.
PS: Typed while wearing a pair of fairly tattered Bose QC45 1st gen because I'm intent on repairing perpetually rather than $$$ replacement.
I noise canceled my fans, which were extremely loud, simply by doing less.
It had an on-off controller, and I replaced with a smooth pid controller. Now when it starts making noise it's making much less noise and much earlier. And mostly stays below 20%
29 comments
[ 2.6 ms ] story [ 51.8 ms ] thread(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.
I guess the 'fan as the noise cancelling speaker' idea could be reworded as 'a fan with active stabilization that doesn't vibrate', making no mechanical noise.
A friend with a recording studio solved their problem by putting the fan at the end of a length of ducting with a couple of 90 degree bends, lined with foam.
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!
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.
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.
https://en.wikipedia.org/wiki/Active_noise_control
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
Why won't this work even if you could co-locate your speaker with the fan? Fan noise is neither stable in time nor as simple as a sine wave. This would need a complex active system to work.
If you want a quiet but expensive solution that doesn’t involve a properly sized air handler, read on.
The solution here is to put the fan outside the building mounted on ductwork that goes inside the building with a damper you can open and close. “Powered roof ventilator” is what you should google.
Then add in a makeup air fan that is interlocked with the exhaust fan (and the dampers on both the exhaust fan and makeup air unit) outside and mounted on ductwork so your house isn’t negatively pressurized which would have the opposite effect, dirt and dust would enter the house. “Makeup air fan” is what you should google.
If you’ve ever been in a commercial kitchen, the vent hood is connected to ductwork that goes outside the building where the exhaust fan in mounted, and there’s always a makeup air unit mounted outside sized roughly the same (cfm) as the exhaust fan to push air into the kitchen to maintain the air pressure and prevent negative pressurization. A (typical modern) large building has multiple intake and exhaust fans that work in concert with a building automation system to maintain air pressure. The idea is to maintain slight positive pressure with respect to the outside air pressure to prevent dust and dirt from being sucked into the building.
This was an experiment in my high school physics, where our teacher played a constant tone from two sources and students were to walk around the class room searching for dead spots where the phases would cancel each other out.
It was quite surreal to take a step and suddenly the tones went quiet.
Instead, it's probably a better idea to not create the noise in the first place.
PS: Typed while wearing a pair of fairly tattered Bose QC45 1st gen because I'm intent on repairing perpetually rather than $$$ replacement.
It had an on-off controller, and I replaced with a smooth pid controller. Now when it starts making noise it's making much less noise and much earlier. And mostly stays below 20%