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How can you reduce the minimum fan speed?

I couldn't find any command line utilities for doing it with a quick search. There must be some though?

Neither smcFanControl or Fan Control are able to set the fan speed below 2000 RPM on my (Mid 2009, Core 2 Duo) MacBook Pro:

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SMCFanControl's FAQ:

Why can't I set my minimum fan speed to 0 rpm?

smcFanControl only lets you set the fan speed in the range of Apple's min and max values. Setting it to zero is possible from the technical perspective but could probably damage your machine.

But would reducing the base speed to say 1000 RPM be harmful? And no, it isn't that loud even now, but the perceived level of noise seems to have a roughly exponential correlation with the RPM.

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up vote 6 down vote accepted

Does the minimum level have some more complex effect in calculating the fan speed?

I did some experiments with oscilloscope, fan and a power supply with variable voltage.

Fan usually reports its speed using pulse width modulation. In the fan assembly there is a device called Hall effect sensor. It works by detecting magnetic field change as the fan rotates and it creates a number of pulses for each revolution of the fan. When the fan is operating very slowly or when the input voltage is very low, it may provide signal which is so deformed that the fan control circuitry could have serious problems getting any useful information form it. It could be that there is a software setting which would prevent fan from working so slow as to provide uninterpretable speed output.

Another point is fan starting voltage. Fan needs a certain voltage to start rotating. That voltage is lower than minimum voltage needed for the fan to rotate. It could be that software is designed in such way that it will not set fan's voltage below starting voltage for safety reasons.

Just to note: This is general information for computer fans and fan system in your computer may work a bit differently.

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How low is very low? SpeedFan's FAQ also mentions the problem with reporting the speed correctly with low voltages. How can you find out the operating voltage, besides using an oscilloscope? – user495470 Mar 22 '11 at 14:34
@Lri Actually, I used just a multimeter for voltage. Low depends on fan's rated voltage. Popular voltages are 5 V and 12 V. From what I've seen, 12 V fans won't start at around 5 V to 6 V, but that depends on the fan itself. I didn't have any 5 V fan to test. They are more common in laptops, so it could happen that your computer has one. As for reporting voltage, you'd need to disassemble the fan and see the Hall effect sensor. If you can read its markings, a simple Internet search will probably turn up its datasheet which will contain operating voltages. – AndrejaKo Mar 22 '11 at 16:41
@Lri Since you have a laptop, the fan assembly is very likely going to be small and it could be very difficult to read the writing on the sensor, if there is any at all. – AndrejaKo Mar 22 '11 at 16:42
I'll accept this as an answer for the is it safe part. Still looking for ways to drop the RPM though. :) – user495470 Apr 3 '11 at 23:57

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