Air Purifiers

We have been back in China now for a bit over 2 weeks and right away we were hit with some pretty heavy smog days. There were a few days when the PM2.5 particulate levels were consistently above 250 µg/m³ (this is ten times the WHO recommended safe levels) with peaks up to 500 µg/m³.

[This where Carole suggests warning you that this is a very technical entry.  You've been warned.]

For example, this shot looking down on our neighborhood from the hill behind us was taken on a day with approximately 250 µg/m³ particulate count. On days of 500 µg/m³ it is much soupier. On those days we can still see across the street but, if one we were to see a person about one block away, we probably couldn't tell if its a person or a bush.

This is just life here in China, something we all have to deal with. I've already mentioned here that we use our 3M face masks when we go outside in these conditions. How about when we are at home? Well the government recommends people stay indoors when levels are elevated. That helps, a little, but indoor particulate levels are still about 30% to 50% of outdoor levels, higher if you don't keep the doors and windows closed 24 hours/day. So we run air purifiers at home.

We discovered the best deal on air purifiers is from a Beijing social enterprise called Smart Air, http://smartairfilters.com/en and http://particlecounting.tumblr.com/. They make very simple but effective air purifiers by strapping a HEPA filter on to a fan. See the above links for details and all the research and tests they have performed. We are quite happy with our 2 Smart Air air purifiers (one "Original" and on "Cannon" and they do the job for our whole house up to around 250 µg/m³ outside levels. Above that they don't quite keep up. The only issue we have with them is that they are loud, especially the "Cannon" when run at maximum. 

Here is our Cannon in its out of the way location under a table in our entrance/kitchen area:

According to Smart Air, this fan on high speed puts out 56 dB of sound volume measured from 1.95 meters. It's loud (but apparently not louder than some of the commercial products costing 10 times as much). We are still waiting for Smart Air to release their quieter system that has been promised for a while now. I hear it is still under development and coming soon... In the meantime I decided to take matters into my own hands to see if we could make our Cannon quiet enough to not interfere with our sleep at night.

I started by measuring the sound levels of our Cannon:

I don't have a fancy decibel meter like Thomas at Smart Air used, but I have a cell phone which has a microphone and lots of processing power and of course one can get apps for everything these days. This is the app I found:

I don't expect that using this generic app on my cell phone will produce test results equal to an instrument designed for the job and calibrated against known sound levels, but I do expect it to give me reasonable data comparing one sound to another. You can see the tape measure in the above sound test photo, I made sure I was exactly one meter away from the fan for each test. This test showed that the ambient sound in the room was 30db and the Cannon on full speed fluctuated between 59 and 60 dB (so I'll call it 59.5 dB) 

We had some pieces of puzzle shaped soft plastic floor mat left over from another project so I quickly snapped a few of them together into a box to surround the Cannon:

This is not the thin stuff you see at every children's playthings store, what we have is a full 1 inch (2.5 cm) thick. Anyway, I tested the sound level of the canon on high inside this box to be 55.5 dB. wow a 4 dB drop, definitely worth cutting up the floor mats to make the box properly. I had to leave an opening front and back of course to let air through, so I made the top front stick out so I could hang a blanket over it without completely covering the hole:

It turned out that how I was measuring sound from the side of the Cannon, it made no difference to have the blanket on or off, but measured from the front, there was a 2 dB difference (55 with and 57 without the blanket).

There was a problem with my quick test - when I put my hand in front of the opening I could feel no airflow. I'm guessing a lot of the air is just circulating around inside the box. So I figured we needed a baffle inside the box to separate the front from the back. I made up a quick template out of cardboard before attempting to cut one out of a floor pad:

It fit fairly well and probably would have been good enough, but we had extra floor pads so I thought it would be a bit more stable for long term use to make it with the floor pad. This and the final cutting of the front and back sections meant getting out some fancy tools. fortunately we had the tools required in our kitchen drawer:

And here's the completed baffle:

View from the back with baffled installed:

View from the front:

Well, as long as we're going this far, why not through some padding into the box to further baffle the sound, so I added some old blankets and towels. Not sure how much of a difference they'll make but lets try it anyway:

As for the openings front and back, the Cannon has a diameter of 10 inches which makes an area of  79 in². I made opening of 4 inches front and back which, with the width of the box being 22 inches gives us an area of 88 in². This box should not be restricting the airflow in any significant way. I would have been confortable with an opening of 3.5 inches (77 in²) but then I could't reach my arm in to change the speed setting dial.

Finally, I used one of the cutoff's from the front and back opening to make and extension for the top so it can extend forward to let a blanket drape over it. 

And here is the finished box from the front. Obviously I'll have to pull the top off to change the filter but that only happens every 3 or 4 months. Oh, and yes, now with the divider in the middle, I can feel air blowing out the front.

and from the back:

And it is now much bigger but it still fits under our table:

Drum roll... the final sound test results: 53.5 dB for a full 6 dB drop compared to the naked Cannon. Wow, that is even better than I had expected based on the initial test run:

Now, with this finished product, testing the sound from the front with the blanket on and off shows no difference whatsoever on the meter. To the ear, there is a different quality of sound but the meter detects the same sound pressure.

We are thrilled with the results. The Cannon is now at a very comfortable sound level, easy to live with. We hardly even notice it set to medium all day long Additionally, under heavy pollution conditions, we find it acceptable to run full power. And on low power, it is so quiet as to be almost negligible, even for sleeping. We are definitely breathing cleaner air now at home because we are not turning off the fans all the time. Yes we are happy:

And now, if you have managed to stay with this report this far, then you might even be interested in all the sound data that I recorded. So here it is: