Go Back   AnandTech Forums > Hardware and Technology > Highly Technical

Forums
· Hardware and Technology
· CPUs and Overclocking
· Motherboards
· Video Cards and Graphics
· Memory and Storage
· Power Supplies
· Cases & Cooling
· SFF, Notebooks, Pre-Built/Barebones PCs
· Networking
· Peripherals
· General Hardware
· Highly Technical
· Computer Help
· Home Theater PCs
· Consumer Electronics
· Digital and Video Cameras
· Mobile Devices & Gadgets
· Audio/Video & Home Theater
· Software
· Software for Windows
· All Things Apple
· *nix Software
· Operating Systems
· Programming
· PC Gaming
· Console Gaming
· Distributed Computing
· Security
· Social
· Off Topic
· Politics and News
· Discussion Club
· Love and Relationships
· The Garage
· Health and Fitness
· Merchandise and Shopping
· For Sale/Trade
· Hot Deals
· Free Stuff
· Contests and Sweepstakes
· Black Friday 2013
· Forum Issues
· Technical Forum Issues
· Personal Forum Issues
· Suggestion Box
· Moderator Resources
· Moderator Discussions
   

Reply
 
Thread Tools
Old 03-15-2003, 10:29 PM   #1
RyanM
Platinum Member
 
Join Date: Feb 2001
Location: Ferndale, MI
Posts: 2,387
Default Relationship between CFM and RPM

I was wondering - What's the relationship between CFM, RPM, and dB levels?

Let's take a hypothetical dual-bearing fan. It puts out 100 CFM, 45 dB, and runs at 3000 RPM.

Now, let's cut the RPM in half. Is the relationship linear, and the CFM and dB would be cut in half, resulting in 50 CFM and somewhere around mid-30's dB, or is it exponential, and it would move less than 50 CFM?

Any ideas?
RyanM is offline   Reply With Quote
Old 03-15-2003, 10:37 PM   #2
Eskimo
Member
 
Join Date: Jun 2000
Posts: 134
Default Relationship between CFM and RPM

As far as speed versus noise there has been some emperical studies which have shown a logrithmic relationship, which makes sense since dB is a logarithmic unit of measure.
See the results here.
Eskimo is offline   Reply With Quote
Old 03-27-2003, 09:00 AM   #3
Mday
Lifer
 
Join Date: Oct 1999
Location: NYC
Posts: 18,641
Default Relationship between CFM and RPM

cfm and rpm

there is a relationship. it's highly non linear, and takes some random variables into consideration. so it's more of a probability distribution than anything else.

and it's fan dependent with respect to the motor, and the impaler design (fin shape, number of fins, fin size, impaler depth)
__________________
asjf;aiweadflkjblksj;l bkja dfba;s
Mday is offline   Reply With Quote
Old 03-27-2003, 01:21 PM   #4
RyanM
Platinum Member
 
Join Date: Feb 2001
Location: Ferndale, MI
Posts: 2,387
Default Relationship between CFM and RPM

So essentially, there's no general model for figuring dB drop with RPM/CFM drop, given that all other factors remain unstated and equal?

I would've assumed that there was some sort of general rule, which becomes effected by those various design aspects, but still served as a fairly accurate gauge for any fan when precision estimates aren't required.
RyanM is offline   Reply With Quote
Old 03-28-2003, 10:01 AM   #5
DivideBYZero
Banned
 
Join Date: May 2001
Posts: 24,119
Default Relationship between CFM and RPM

I have always understood that blade pitch and casing design can have an effect on CFM and noise independant of the RPM.

My 2p.
DivideBYZero is offline   Reply With Quote
Old 03-28-2003, 10:19 AM   #6
Mday
Lifer
 
Join Date: Oct 1999
Location: NYC
Posts: 18,641
Default Relationship between CFM and RPM

Quote:
Originally posted by: DivideBYZero
I have always understood that blade pitch and casing design can have an effect on CFM and noise independant of the RPM.

My 2p.
very correct.
__________________
asjf;aiweadflkjblksj;l bkja dfba;s
Mday is offline   Reply With Quote
Old 04-08-2003, 04:54 PM   #7
dawks
Diamond Member
 
Join Date: Oct 1999
Posts: 5,019
Default Relationship between CFM and RPM

Yoshis super quiet PC

He used bigger fans (larger diameter) with lower RPMs to move the same amount of air, with less noise produced.. He also made sort of a muffler.. but thats another discussion
dawks is offline   Reply With Quote
Old 04-08-2003, 11:23 PM   #8
RyanM
Platinum Member
 
Join Date: Feb 2001
Location: Ferndale, MI
Posts: 2,387
Default Relationship between CFM and RPM

Ugh. That's disgusting. He spent all that time sound-proofing, when he probably could've had the exact same effect by removing 2 out of the 3 fans on the power supply. Power supplies do not need anything more than the fan located at the exhaust section - If there is a place to intake the air located elsewhere on the power supply, it'll suck air through there and shoot it out the back.

You don't need to blow air INTO the power supply just to blow it out. It's a small enclosure that's much better sealed than a PC case, so you can rely on a single fan to take care of exhaust duties.

Idiot.
RyanM is offline   Reply With Quote
Old 04-09-2003, 12:29 PM   #9
gururu
Platinum Member
 
Join Date: Jul 2002
Posts: 2,402
Default Relationship between CFM and RPM

for autos:
theoretical cfm (Cubic Feet per Minute of Air Capacity)= (rpm x displacement) / 3456

may apply although I'm not sure what the displacement in cubic inches of a computer/computer fan would be in the equation.

but you can see that the relationship at least in this case is linear
gururu is offline   Reply With Quote
Old 04-10-2003, 05:02 AM   #10
Basse
Senior Member
 
Join Date: Oct 1999
Posts: 525
Default Relationship between CFM and RPM

Maybe a bit OT but quite interesting...

There is a maximum rpm figure for the fan, when the tips of the fins spin faster that the speed of sound, ~340m/s, they loose the ability to move air. This has to be taken into consideration with the fan diameter/rpm design.

Reg
Tomas

Edit: Maybe some of you mathgurus can calculate what the maximum rpm for a 10cm diameter fan would be? /Edit
__________________
Comprehensive Affiliate Program Directory
Basse is offline   Reply With Quote
Old 04-12-2003, 09:06 PM   #11
DannyBoy
Diamond Member
 
DannyBoy's Avatar
 
Join Date: Nov 2002
Location: London, UK
Posts: 8,755
Default Relationship between CFM and RPM

Quote:
Originally posted by: MachFive
Ugh. That's disgusting. He spent all that time sound-proofing, when he probably could've had the exact same effect by removing 2 out of the 3 fans on the power supply. Power supplies do not need anything more than the fan located at the exhaust section - If there is a place to intake the air located elsewhere on the power supply, it'll suck air through there and shoot it out the back.

You don't need to blow air INTO the power supply just to blow it out. It's a small enclosure that's much better sealed than a PC case, so you can rely on a single fan to take care of exhaust duties.

Idiot.
Well either way I wouldnt mind considering a project like that.

I would much rather have a silent PC than a hoover going 24/7 in my room.

I managed to take care of my server noise problems in a very technical way.

I modded it into the bottom of my pine wardrobe - EDIT: [b] And i use the waste heat to vent one half of my wardrobe which acts as an airing cupboard

Dan
__________________
DannyBoy is offline   Reply With Quote
Old 04-12-2003, 10:45 PM   #12
RyanM
Platinum Member
 
Join Date: Feb 2001
Location: Ferndale, MI
Posts: 2,387
Default Relationship between CFM and RPM

Hahahahahahah. That's hysterical. 2 birds with one stone is always a good way to go.

Seriously though - It's really easy to make a silent PC without any of that fancy soundproofing. You take some 92 CFM 120mm fans from Enermax, mod your front intake to accept a 120mm fan, cut a blowhole in the top of your PC and make it use the other 120mm fan, then you buy an SLK-800 and mount a 40 CFM 92mm fan.

Then you undervolt them until the whine of your hard drive is louder than the fans. Voila. Throw in an air filter for the front intake, and you're all set.
RyanM is offline   Reply With Quote
Old 04-12-2003, 10:54 PM   #13
DannyBoy
Diamond Member
 
DannyBoy's Avatar
 
Join Date: Nov 2002
Location: London, UK
Posts: 8,755
Default Relationship between CFM and RPM

If i lowered them to less noise than my hd i think id probably kill my processor and everything else in the comp :Q

Not a good thing to do when you have a barracuda *Thumb Down*

The barracuda dont make the slightest bit of :music:

I came up with the wardrobe idea by accident btw, perhaps i should market them

Want a self heated airing cupboard / wardrobe / pc all in one? Then try the new Compdrobe
__________________
DannyBoy is offline   Reply With Quote
Old 04-12-2003, 11:53 PM   #14
everman
Lifer
 
Join Date: Nov 2002
Posts: 11,290
Default Relationship between CFM and RPM

When you compare the relationship between rpm and db you need to take into account the type of hardware in the fan you're talking about. Some fans are just junk and will make noise no matter what. But if you were to make a fan that lets say used the same time of motor tech used in a seagate cuda V then you could get a rather high RPM with very low noise. (kinda dreaming about the perfect case fan there)

Personally I use a 120mm side fan in my case which is rated at 70cfm max. But I put it on a fan bus and is run silently unless I'm gaming w/ headphones. Greater blade surface area = greater cfm.
everman is offline   Reply With Quote
Old 04-13-2003, 01:20 AM   #15
RyanM
Platinum Member
 
Join Date: Feb 2001
Location: Ferndale, MI
Posts: 2,387
Default Relationship between CFM and RPM

Well, even more importantly:

Greater blade surface area = Greater CFM @ Lower RPMs w/ Even more lower noise.
Greater blade surface area = Even more greater CFM @ same RPMs w/ Lower noise.

;-)
RyanM is offline   Reply With Quote
Old 04-13-2003, 10:24 AM   #16
Howard
Lifer
 
Howard's Avatar
 
Join Date: Oct 1999
Posts: 46,833
Default Relationship between CFM and RPM

Quote:
Originally posted by: gururu
for autos:
theoretical cfm (Cubic Feet per Minute of Air Capacity)= (rpm x displacement) / 3456

may apply although I'm not sure what the displacement in cubic inches of a computer/computer fan would be in the equation.

but you can see that the relationship at least in this case is linear
Real CFM = (RPM x displacement (in^3)) / 3456 * volumetric efficiency

__________________
mugs, Crono for Elite

Kitchen stuff
Howard is offline   Reply With Quote
Reply

Thread Tools

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT -5. The time now is 04:50 AM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.