math question
 

let say you have a 12v fan that does 100CFM at 50db.

so you drop the voltage to 6v(for simplicity)

does anyone know how to figure out the new numbers for CFM and db?

I know 50CFM and 25db is wishful thinking but what would the actual numbers be?

There are no equations.

Every fan is different, and they all behave differently out of spec. Some may not even turn.

Decibel is not a linear measurement of sound level.

it'll be 50cfm or there abouts +-20%, but the sound will be all differant, because dba is a logarithmic scale. 40db --> 50dba is twice as loud, not 25% louder

yeah for CFM you can USUALLY get a close estimate by setting up a proportion... X cfm/ Y volts = ? cfm / Z volts... However, that doesn't work when you lower the voltage too much. Even when you don't lower it too much, that equation isn't super accurate...
-Kev

i'll add that fans are not magic. there are actually equations (& many of them) to design & estimate CFM vs RPM for a given fan geometry. i won't put them together for you because i just got done investing years in college learning them & know it would take years to teach them to you. to oversimplify things a bit:

a fan's noise & CFM depend mainly on the aerodynamics of the rotor (yeah, you also have bearing vibration & structural resonance runing around too). regardless, most fans have a distinct "design point", where the rated RPM keeps the blades at the optimal angle of attack, so the energy transfer from the motor to the air is optimized for the required CFM (electrical power/kinetic energy is minimized). of course different fans have different design point RPMs, that's also why thier blade geometry is different, but not by happenstance.

it is mostly large scale turbulence created by flow separation at off design angles of attack that create noise. the below scan from my textbook sort of illustrates blade stall, which very much complicates any prediction of off-design noise & CFM levels:

http://www-personal.umich.edu/~kbern...DesgnBlade.gif

you can use your intuition to conceptualize how RPMs off the design operating point & back pressure (or foreward pressure) can lead to this separation on rotor blades. back or foreward pressure is another determining factor in noise & CFM calculations for both on & off design. to add these effects, we must consider many more variables that add up to the total Fanno Friction Factor (not worth doing for PC cooling systems, at least yet).

the bottom line, spinning rotors become essentially useless when the blades are in a stalled condition. if you know wings a little, you know you want to avoid stalling the wings of an aircraft. if forward moving wings (like spinning rotor blades) are beyond the stall angle of attack, the "lifting circulation" surrounding the wing stops & lift drops drastically, sending the aircraft barreling into the ground if there is not enough pilot skill, speed, & altitude to recover (unless you're like Patty Wagstaff, cheers Patty;-).

BTW, that image of rotor stall & the velocity vectors might give you a clue as to how us aeros would go about our "magic":). i reality none of it is majic. if you are curious enough, i have a bunch of those equations posted on my website (some with & some without context). look at the bottom "A Taste of..." section; "Propulsion" & "Aerodynamic Equations" are the files that have it all:
nemaste's Aero Page

nemaste
 

And your point is ........
What?

Damaster, some of the fans manufacturers provide data info related to your questions, see for example

See if the fan or fans in which your interested have that info available. Not all manufacturers provide it. :shrug: :rolleyes:

Dix, there were a few points i highlighted that may have not been easily grasped. sorry for saying too much here, but i'll briefly review the main points:

1) calculating CFM vs RPM is difficult, but very possible.
2) the calculations require higher education, but fundamental aerodynamic ideas that make up the calculations can still be conceptualized by the uneducated (or so i believed).
3) flow geometry around blades (angle of attack) is most important to the calculations.
4) fan blade angle of attack are designed to peak in efficiency at their rated RPM
5) running fans off of this design RPM makes fan blades operate off of the design angle of attack
6) off design angle of attack creates noise & inefficiency
7) filters & other restrictions ad the complexity of friction to the calculations (without major bluff-body obstructions, Fanno friction is insignificant & blade drag dominates)

In short, if you really want to calculate something, you can calculate cfm as a linear scale between 0 rpm (at whichever voltage your fan stops turning) and max rpm (full voltage).

As for noise, it'd be the same voltage range, but the scale would be exponential, not linear.

That should give you a "rough" idea of what to expect.

If you've got the tools you can do a simulation of the fan. But the fan's performance is dependent on the back pressure. That means you have to simulate the heat sink's resistance to the flow so you have to include it in your calculation.

Performance data as provided by the fan manufacturer is measured to allow one to select a fan based the resistance it may see.

No, in fact the fan can spin with no air flow. However a linear relationship between pressure and flow isn't typically too bad.

Right, but for practical purposes, I say a linear (rough, but should be close enough, without getting into some heavy math, or testing) at zero load.

How the fan behaves with a load is a whole different ballgame. Usually, fans are rated (specs on website) with a max pressure, measured in mm of mercury. That can be converted to psi (but what's the point?). Typically, I believe they are rated for .25mm mercury of pressure (can't remember if I'm right here, someone correct me).

The problem of course is that no one can easily measure the flow resistance of a rad, as far as air is concerned, which makes this whole issue quite pointless, if you ask me:D

i have a 0-1psi differential pressure transducer somewhere in my garage. i could determine Dp/Dvdot (change in pressure drop as a fuction of CFM) numerically using that device on my BIX, but i don't have my amp circuit ready to test yet.:shrug: i'll post results here whenever i get around to collecting accurate data.

Nemaste, your link is dead...

i dont know about converting rpms to db or anything but, with sound, decibles are logarithmic. this means that 60db is more than twice as loud as 30 db. also, if you have two 30db fans the total sound is something like 33db. dont know if this is relevant or not...

every 3 dB sound doubles.