# Fans making th-th-th-th-th-th sound only at certain RPMs **URL:** \ **Category:** Factual Questions\ **Created:** [October 16, 2014, 11:17pm UTC](https://boards.straightdope.com/t/fans-making-th-th-th-th-th-th-sound-only-at-certain-rpms/701540 "2014-10-16T23:17:02Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![Reply](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/reply/32/15952_2.png) [@Reply](https://boards.straightdope.com/u/Reply)\ **Post date:** [October 16, 2014, 11:17pm UTC](https://boards.straightdope.com/t/fans-making-th-th-th-th-th-th-sound-only-at-certain-rpms/701540/1 "2014-10-16T23:17:02Z")
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? ---
**Author:** ![jz78817](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jz78817/32/1028_2.png) [@jz78817](https://boards.straightdope.com/u/jz78817)\ **Post date:** [October 16, 2014, 11:59pm UTC](https://boards.straightdope.com/t/fans-making-th-th-th-th-th-th-sound-only-at-certain-rpms/701540/2 "2014-10-16T23:59:12Z")
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. ---
**Author:** ![Reply](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/reply/32/15952_2.png) [@Reply](https://boards.straightdope.com/u/Reply)\ **Post date:** [October 17, 2014, 12:06am UTC](https://boards.straightdope.com/t/fans-making-th-th-th-th-th-th-sound-only-at-certain-rpms/701540/3 "2014-10-17T00:06:34Z")
Hmm, [Propeller synchronization - Wikipedia](https://en.wikipedia.org/wiki/Propeller_synchronization) ? Like [this](https://www.youtube.com/watch?v=Mv8WGfa1LqU)? 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 🙂 ---
**Author:** ![jz78817](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jz78817/32/1028_2.png) [@jz78817](https://boards.straightdope.com/u/jz78817)\ **Post date:** [October 17, 2014, 12:41am UTC](https://boards.straightdope.com/t/fans-making-th-th-th-th-th-th-sound-only-at-certain-rpms/701540/4 "2014-10-17T00:41:59Z")
> [@Reply](#): > > Hmm, [Propeller synchronization - Wikipedia](https://en.wikipedia.org/wiki/Propeller_synchronization) ? > > Like [this](https://www.youtube.com/watch?v=Mv8WGfa1LqU)? 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 🙂 same phenomenon, yes. here’s an audio/visual demonstration of this: [![](https://img.youtube.com/vi/5hxQDAmdNWE/hqdefault.jpg "Beat Frequency - amplitued modulation resulting from adding sinusoidal signals of similar frequency") ](https://www.youtube.com/watch?v=5hxQDAmdNWE) ---
**Author:** ![Reply](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/reply/32/15952_2.png) [@Reply](https://boards.straightdope.com/u/Reply)\ **Post date:** [October 17, 2014, 1:01am UTC](https://boards.straightdope.com/t/fans-making-th-th-th-th-th-th-sound-only-at-certain-rpms/701540/5 "2014-10-17T01:01:33Z")
[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. ---
**Author:** ![jz78817](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jz78817/32/1028_2.png) [@jz78817](https://boards.straightdope.com/u/jz78817)\ **Post date:** [October 17, 2014, 1:36am UTC](https://boards.straightdope.com/t/fans-making-th-th-th-th-th-th-sound-only-at-certain-rpms/701540/6 "2014-10-17T01:36:17Z")
you callin’ me a dope? ---
**Author:** ![Reply](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/reply/32/15952_2.png) [@Reply](https://boards.straightdope.com/u/Reply)\ **Post date:** [October 17, 2014, 1:36am UTC](https://boards.straightdope.com/t/fans-making-th-th-th-th-th-th-sound-only-at-certain-rpms/701540/7 "2014-10-17T01:36:46Z")
Well, you do deliver. ---
**Author:** ![jz78817](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jz78817/32/1028_2.png) [@jz78817](https://boards.straightdope.com/u/jz78817)\ **Post date:** [October 17, 2014, 1:46am UTC](https://boards.straightdope.com/t/fans-making-th-th-th-th-th-th-sound-only-at-certain-rpms/701540/8 "2014-10-17T01:46:59Z")
> [@Reply](#): > > Well, you do deliver. you callin’ me FedEx? ---
**Author:** ![Reply](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/reply/32/15952_2.png) [@Reply](https://boards.straightdope.com/u/Reply)\ **Post date:** [October 17, 2014, 1:52am UTC](https://boards.straightdope.com/t/fans-making-th-th-th-th-th-th-sound-only-at-certain-rpms/701540/9 "2014-10-17T01:52:40Z")
Well, you’re not brown.