What would you estimate?

krooton · Post by **krooton** » Fri Nov 19, 2004 2:28 am

Lets say you had a fan controller that did 12v-7v.

What effect would you say having a fan at 7v would be?

I doubt its as easy as saying half the cfm and dba, effectively doubling the quietness but halfing the cooling?

To elaborate, say I was giving fans percentage figures for cooling and quietness at 12v, how would I adjust these for being 7v?

cheers

Trip · Post by **Trip** » Fri Nov 19, 2004 3:03 am

Panaflo L1A:

VDC 12V 9V 7V 5V
RPM 1880 1520 1230 760
dBA@1m 21 17 12 10
CFM 24 19 14 9

From: http://www.silentpcreview.com/article132-page3.html

That article tells about cooling effect of L1A at the different speeds as well.

Also see: Choosing Fans for Quiet, High Airflow: A Scientific Approach

For noise remember that adding 10dB sounds twice as loud to the human ear. So 30dB sounds twice as loud as 20dB. Also, +3dB is actually twice as loud.

cansan · Post by **cansan** » Sat Nov 20, 2004 2:42 am

I think you have a slight mix-up with the dB levels:

+10 dB means twice the loudness as heard by the human ear, that is correct.

+3 dB is when you have two sound sources at the same (or almost same) location. It is not twice the loudness.

Trip · Post by **Trip** » Sat Nov 20, 2004 2:52 am

any idea of what twice the actual loudness is?

Becks · Post by **Becks** » Sat Nov 20, 2004 9:41 pm

As far as I know...

+3db is twice the energy and thus is twice as loud
+10db is about 10 times as loud...

here loud = "intensity"

how things sound to your ear however,
+3db is nothing significant
+10db is about twice as loud

ferdb · Post by **ferdb** » Sun Nov 21, 2004 4:53 am

A rough rule of thumb is that for free air (fan in an unrestricted system) the CFM is proportional to the RPM. Halve the RPM and you halve the CFM. As the system becomes more restricted this changes and if you halve the RPM the CFM drops by more than half.

Also Fan RPM is usually fairly linearly related to Voltage, Halve the Voltage, Halve the RPM. This is pretty true up till the point the electronics stop working.
These are rules of thumb and not completely accurate but they give you something to work with.

When people talk about noisy fans there are several factors going on.

There are 3 noise sources in a fan. Bearing Noise, Motor Noise, and Wind or Blade noise. Fans that get black marks for noise have lots of Bearing and Motor noise and there is no way to get rid of it. These become the dominant noise sources at lower rpms. This kind of info is never on a spec sheet, you need to look around SPCR for info on what's a good fan and what's not.
Wind noise is dependent on RPM and to a small degree on blade design. Wind noise is related to how much CFM x backpressure you are trying to get out of a fan that size, which is directly related to the RPM. For the same CFM, bigger fans run slower and thus make less noise. A less restrictive system also allows you to run your fans slower and get the same CFM.
The other problem is nobody seems to take noise readings the same way, and some people just make up numbers for their fans that have nothing to do with reality. Trying to compare published noise specs is almost a totally useless exercise.