Fans making th-th-th-th-th-th sound only at certain RPMs

Factual Questions
1

I have a 3D graphics card with two fans. The fans get louder and quieter as they spin up or down, naturally, but there’s also an odd side effect: At certain RPMs (I’ve noticed 1550 and 2350 so far, but not speeds in between them or above/below) the fans will make this th-th-th-th-th-th-th-th sound at about 6 times a second, almost like they’re brushing against a cable or something (but they’re not).

I’m already getting the card exchanged, but I’m curious: Physically, what would cause something like that? Is there some sort of interference pattern that would only happen at certain intervals? Anything like that on multi-rotor helicopters?

2

if there’s a slight difference in speed between the two, you could be hearing a “beat” noise from the slightly different pitch of each fan.

3

Hmm, Propeller synchronization - Wikipedia ?

Like this? I bet that’s exactly what it is! Now I gotta see if I can individually control the fans. Maybe even make some dubstep.

Thank you :slight_smile:

4

same phenomenon, yes. here’s an audio/visual demonstration of this:

5

[quote=“jz78817, post:4, topic:701540”]

same phenomenon, yes. here’s an audio/visual demonstration of this:

[/QUOTE]

Nailed it in one. Awesome, thank you! The Dope Delivers.

6

you callin’ me a dope?

7

Well, you do deliver.

8

you callin’ me FedEx?

9

Well, you’re not brown.