You really need an acoustic expert to decipher the noise levels.
There are several ways to indicate the noise level. Some manufacturers 'play' on this and give the most flattering figures.
In essence, the most reliable method of fan noise comparison is the SWL Sound Power Level (Sound Watt Level)
If you use this criteria, then all manufacturers are on a level playing field.
As soon as you introduce 'typical noise levels', or Sound Pressure Levels (SPL) then you can disguise or mould the noise levels into whatever you like.
You can for instance say that a SPL sound pressure level for a unit is only 40dBA but this might assume a room size of 'X' and a duct connection with a bend and a particular grille size....all of which are variable.
The room might also have carpets, curtains, furniture etc.
How do you do a comparison of one manufacturer with another when they might assume a diffetnt duct connection, grille size, or size of room etc.
An acoustics engineer will do the calculation to convert the AC unit SWL INTO SPL by doind a few simple calcs. (Its pretty easy when you know how)
Remember, a fan will have a constant sound power level , no matter where it is installed so comparisons between manufacturers are easy. The sound pressure level though will depend on individual circumstances as already mentioned.
Friar Tuck of Sherwood