Indoor air quality is important. It increases comfort. Potentially, it might improve cognition as well as reducing infection risk in a space. And it would be cool to have Swiss quality air in my house!
One can increase air quality by improving ventilation. One way to do this is by turning on a fan near your window. The CDC recommends facing them towards the outside to exhaust air out the house. (I will be writing more about this later).
When it comes to fans, the stronger the better. But there is a trade-off. The annoying thing about strong fans is that they are loud.
I have this big 20 inch Lasko box fan and it's loud to the point of being annoying when I set it on max level, and so I don’t. But if it was silent I would!
Inspired by noise cancelling headphones, I was thinking of a DIY way to make my quieter and I did an experiment. 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.
The problem is that fans make sound from air turbulence (the whoosh sound), and this is hard to cancel since its scrambled and irregular.
However, when I listened to my fan more closely, I realised that there was a droning low tone that sounded like a constant frequency. I wondered if I could at least cancel that part out. Using my laptop microphone, I wrote a rough script to get the dominant frequency of the fan via a recording and then Fourier transforming it.
Doing that confirms that the fan does actually have a dominant tone that you can audibly hear (312Hz), shown by the highest peak below. But it’s not obviously sticking out as the main component of the sound. The whoosh from the turbulence still causes a broad spectrum of frequencies
But I tried to cancel it out anyway. I just tried to play a 312Hz tone through a bluetooth speaker near the fan to get antiphase cancellation of this dominant tone
It didn’t really work… Haha
The hard part is trying to sweep the delay so that the phases cancel out on time, and doesn’t become resonant instead. And I think this calibration is a little too complicated for one afternoon.
But thought the idea was cool, and would love to hear from you guys the first obvious steps to an improvement. My first thought is to just make a box around the fan with some corrugated cardboard to absorb the sound before it escapes.
Disclaimer: none of this is medical advice.
One of the other issues your experiencing is the fan is also resonating already with what ever object it’s on. In your case, it’s a window sill made of wood. So adding a speaker on the same location is just gonna resonate even more.
As mentioned by others, phase cancellations is all about timing the sound waveforms to be inverse. There are quite a lot of variables that have an effect on sound in a room environment, including other frequencies generated by the room and reverb it creates.
Have you considered having a second (identical) fan that you locate strategically to obtain some destructive interference over some region of the room? It’s probably very difficult to obtain a good result, and even then it would be over limited regions of the room, but perhaps it’s worth a try.