I have fans which on full are 23db and 25db.
Squirrel cage design so the can handle duct and filters no problem.
Its something ive been looking for years and after buying a few supposed silent fans that were 50db I gave up trusting what they claim until I found these.
Mine are from ebay.uk but I know they are just Chinese generic fans and can be found on Alibaba.
Literally my circulation fans are louder, with a speed controller I cant even hear them.
https://www.alibaba.com/product-detail/2017-hot-220V-Acoustic-extractor-fan_873479872.html?spm=a2700.7724838.2017115.23.4b0b600apleHn1&s=p
Posted the link to show you what they look like and the data sheet on db and airflow, there are a few resellers knocking them out on ali.
I can give you the ebay link if you like but like I say im in UK.