Are you limiting your noise estimates to strictly airflow/static pressure? The research I’m aware of suggests Corsi boxes perform at >40db and up to 60db on high speed (~350 cfm). https://housefresh.com/corsi-rosenthal-box-review . During the pandemic this was widely discussed as a major impediment to their adoption.
Upsizing terminations substantially reduces room-facing air velocity. Assuming 400 cfm, the velocity at the termination I link will be roughly 175 fpm. This is extremely low, and I’d expect to achieve 20-25 db. Most residential return registers are undersized and thus outside ACCA’s duct design standard of 500 fpm. It’s also less than standards widely used for quiet spaces (200-300 fpm). You could continue pursuing improvements here via upsizing further if you’re so inclined, or using clever placement to put the filtration system away from the room activities or behind (offset) cabinetry or furnishings.
That’s a box fan CR box; the better design (and the one linked) uses PC fans which are better optimized for noise. I don’t have much first-hand experience with this, but physics suggests that noise from the fan will be proportional to power usage, which is pressure * airflow, if efficiency is constant, and this is roughly consistent with various tests I’ve found online.
Both further upsizing and better sound isolation would be great. What’s the best way to reduce duct noise in practice? Is an 8“ flexible duct quieter than a 6” rigid duct or will most of the noise improvement come from oversizing the termination, removing tight bends or installing some kind of silencer device? I might suggest this to a relative.
Ah, sorry, the CR reference threw me off. That link seems to suggest those are in the 40 db range.
Duct design uses equivalent length as a metric across duct types and fittings with 1 foot of straight metal pipe as a ‘1’. The problem with flex is that each foot has an equivalent length of 1.5, which adds a moderate amount to system length (although to keep this in perspective many near air handler transitions are >100′). For something like this I’d probably do a short run of flex near the motor (both sides) to dampen motor/vibration noise and transition to metal beyond. Avoid cornering with flex! In general, remote fans, eliminating line of sight on the motor and oversizing ducts/terminations will be shockingly quiet to most room occupants. If you want to get super clever, you can start doing offset openings in walls by cutting grilles high on one side and low on the other, but it can be difficult to pull sufficient air while keeping velocity low (400 cfm = 6 stud bays).
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.
Usage varies—the top five posts on /r/crboxes all use PC fans. Other guides do too, and CleanAirKits and Nukit both describe themselves as PC fan CR boxes.
Are you limiting your noise estimates to strictly airflow/static pressure? The research I’m aware of suggests Corsi boxes perform at >40db and up to 60db on high speed (~350 cfm). https://housefresh.com/corsi-rosenthal-box-review . During the pandemic this was widely discussed as a major impediment to their adoption.
Upsizing terminations substantially reduces room-facing air velocity. Assuming 400 cfm, the velocity at the termination I link will be roughly 175 fpm. This is extremely low, and I’d expect to achieve 20-25 db. Most residential return registers are undersized and thus outside ACCA’s duct design standard of 500 fpm. It’s also less than standards widely used for quiet spaces (200-300 fpm). You could continue pursuing improvements here via upsizing further if you’re so inclined, or using clever placement to put the filtration system away from the room activities or behind (offset) cabinetry or furnishings.
That’s a box fan CR box; the better design (and the one linked) uses PC fans which are better optimized for noise. I don’t have much first-hand experience with this, but physics suggests that noise from the fan will be proportional to power usage, which is pressure * airflow, if efficiency is constant, and this is roughly consistent with various tests I’ve found online.
Both further upsizing and better sound isolation would be great. What’s the best way to reduce duct noise in practice? Is an 8“ flexible duct quieter than a 6” rigid duct or will most of the noise improvement come from oversizing the termination, removing tight bends or installing some kind of silencer device? I might suggest this to a relative.
Ah, sorry, the CR reference threw me off. That link seems to suggest those are in the 40 db range.
Duct design uses equivalent length as a metric across duct types and fittings with 1 foot of straight metal pipe as a ‘1’. The problem with flex is that each foot has an equivalent length of 1.5, which adds a moderate amount to system length (although to keep this in perspective many near air handler transitions are >100′). For something like this I’d probably do a short run of flex near the motor (both sides) to dampen motor/vibration noise and transition to metal beyond. Avoid cornering with flex! In general, remote fans, eliminating line of sight on the motor and oversizing ducts/terminations will be shockingly quiet to most room occupants. If you want to get super clever, you can start doing offset openings in walls by cutting grilles high on one side and low on the other, but it can be difficult to pull sufficient air while keeping velocity low (400 cfm = 6 stud bays).
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.
A filter with PC fans isn’t a “CR box”—a Corsi-Rosenthal box is specifically a design based on box fans. For example, Wikipedia, US Davis, Clean Air Crew, and the Corsi-Rosenthal Foundation mention only box fans.
Usage varies—the top five posts on /r/crboxes all use PC fans. Other guides do too, and CleanAirKits and Nukit both describe themselves as PC fan CR boxes.
Interesting! I hadn’t realized people had started using the term this way!