You can get down to 25 dB by running two at half speed. Fan noise is proportional to RPM^5, so 50% speed will mean −15dB noise. The fans just need enough static pressure to maintain close to 50% airflow at 50% speed.
Do you have tested numbers? Going from high to low on 4 filter CR boxes shows a reduction in airflow in the range of 30%-40% and db reduction around ~13%.
So basically you subtract 13 dB when halving the CADR. I now realize that if you have two boxes, the sound energy will double (+3dB) and so you’ll actually only get −10 dB from running two at half speed. So a more accurate statement for the Airfanta would be that for −15dB noise at the same CADR, you need something like 2.8 purifiers running at 36% speed. It’s still definitely possible to markedly lower noise by adding more filter area.
Your box fan CR box data tell a similar story. If logarithmic scaling is accurate, the sound reduction for halving CADR would be ln(1/​2)/​ln(165/​239)*(8 dB) = 15 dB, or 12 dB for maintaining CADR with double the units. It just doesn’t have a speed low enough to get these low noise levels (and due to the box fan’s low static pressure you might need to add more filters per fan at low speeds).
Airfanta’s absolute noise levels are high for a CR box type design but this is a device that retails for 298 CNY = $41 USD in China, runs at high speed, and uses near-HEPA (95%) rather than MERV filters so is to be expected.
Bummer. Operating at 25 db would have been really impressive for a room filter but it looks like it’s not even close. Extrapolating from David Elfstrom’s airflow estimates it looks like ~1/​2 cfm would put it >40 db, which isn’t great. So if we want to create a satisfactory occupant experience we probably shouldn’t put down the tin snips just yet.
You can get down to 25 dB by running two at half speed. Fan noise is proportional to RPM^5, so 50% speed will mean −15dB noise. The fans just need enough static pressure to maintain close to 50% airflow at 50% speed.
Do you have tested numbers? Going from high to low on 4 filter CR boxes shows a reduction in airflow in the range of 30%-40% and db reduction around ~13%.
(edited to fix numbers, I forgot 2 boxes means +3dB)
dB is logarithmic so a proportional reduction in sound energy will mean subtracting an absolute number of dB, not a percentage reduction in dB.
HouseFresh tested the AirFanta 3Pro https://​​housefresh.com/​​airfanta-3pro-review/​​ at different voltage levels and found:
12.6 V: 56.3 dBA, 14 minutes
6.54 V: 43.3 dBA, 28 minutes
So basically you subtract 13 dB when halving the CADR. I now realize that if you have two boxes, the sound energy will double (+3dB) and so you’ll actually only get −10 dB from running two at half speed. So a more accurate statement for the Airfanta would be that for −15dB noise at the same CADR, you need something like 2.8 purifiers running at 36% speed. It’s still definitely possible to markedly lower noise by adding more filter area.
Your box fan CR box data tell a similar story. If logarithmic scaling is accurate, the sound reduction for halving CADR would be ln(1/​2)/​ln(165/​239)*(8 dB) = 15 dB, or 12 dB for maintaining CADR with double the units. It just doesn’t have a speed low enough to get these low noise levels (and due to the box fan’s low static pressure you might need to add more filters per fan at low speeds).
Airfanta’s absolute noise levels are high for a CR box type design but this is a device that retails for 298 CNY = $41 USD in China, runs at high speed, and uses near-HEPA (95%) rather than MERV filters so is to be expected.
Bummer. Operating at 25 db would have been really impressive for a room filter but it looks like it’s not even close. Extrapolating from David Elfstrom’s airflow estimates it looks like ~1/​2 cfm would put it >40 db, which isn’t great. So if we want to create a satisfactory occupant experience we probably shouldn’t put down the tin snips just yet.