Fan Engineers: What factors determine how quiet a given fan design is?

Those days are over with the new Dyson fan. Blows like a jet engine and is virtually silent. Sorry I can’t help you on the answer.

There are so many variables impacting the physics of your three fans.

If the blades are not balanced or fastened tightly, there will be wobble which begets noise.
Your first fan has the additional elements of the variable box shape and a filter (additional static pressure).

The CFM (cubic feet per minute) of each fan is inherently related to the size of the room in which it is placed. This is a linear measurement. Some folks have tried to compare CFM to decibels but as db is measured using a logarithmic scale, this solution doesn’t really work too well.

You haven’t indicated (except for the filter in the first fan) if any of the fans have any sort of insulation (which will also dampen sound).

@Cruiser Cool. Where can I get one of these wonder fan? I don’t have to visit a Dyson’s Sphere to find one do I?

@Kayak8 Thanks for pointing me toward some of the influencing factors. None of the fans appear to have any sort of sound insulation. The smaller two have what appear to be ABS plastic cases, while the large one has a sheet-metal outer housing with plastic front and rear grilles. The small one has a molded plastic one-piece blade assembly with 3 blades. I can’t see the blades on the rectangular box ones, on the window fan, the blades are a one-piece molded plastic assembly with 5 blades.

Factors that I know of that affect fan noise are:
– Balance of the blades ; unbalanced fans are noisier
– Blade pitch ; affects how much air is moved per fan revolution
– Fan speed ; Faster fans are usually louder but part of it depends on teh balance and harmonics of various things, so this is not always true.
– The housing ; Simple acoustic engineering, though a housing that rattles will be noisy anyways
– Fan size ; At a given CFM, the bigger fan will generally be quieter.

I have an 80mm case fan that can move 53 CFM, which is more than many larger fans can muster, but that requires spinning about twice as fast as a 120mm case fan would for the same CFM, and also moving that amount of air through a smaller hole. While it doesn’t make nearly as much noise as the air hoses I use at work (80 PSI through a ¼” hole), just the airflow alone is audible, though it doesn’t drown out the sound of a small motor spinning at 5400 RPM. Bolt that to a panel that acts as a sounding board (like the side panel of my PC case) attached to an echo chamber (the case itself; kind of like a speaker cabinet) and you can see why I don’t use that fan unless I really need something cooled off rather severely. (I think it’s rated at 42 decibels at full-speed; about as loud as a “stage whisper”, and not much quieter than a normal conversation.)

@jerv Thanks. I’m getting the picture of what makes my low performing fans such loudmouths. Kind of like some people we all know. ;-)

@ETpro that link is to Meijers I don’t see why you couldn’t order it there or amamzon

@Cruiser Thanks.