Running two PC fans in series causes variations in noise produced

Thread Starter

seanspotatobusiness

Joined Sep 17, 2016
200
I would like to connect two PC fans (not being used inside a PC) in series to a 12 V supply, giving them each 6 V. The problem is that this causes the noise they produce to vary which is more annoying than the constant sound which is produced when they are supplied with 6 V in parallel. Any idea why that happens and whether it would be easy to mitigate?
 

Alec_t

Joined Sep 17, 2013
11,274
Assuming the fans are the usual BLDC type they will not be synchronised. Due to their individual commutation, each will experience intermittent supply pulses. That, plus beat frequencies arising from the respective rotational frequencies of the two fans, likely account for the noise variation.
Buck-converting 12V down to 6V and running the fans in parallel might be a solution.
 

SamR

Joined Mar 19, 2019
2,103
It's called resonate noise, even if the circuit provided perfectly synchronized power to both units their individual fans are not perfect and even small deviation will cause sound resonance.
 
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