Case fans - "beat" frequency

[Gav]

Hi all,

Just put together my new system with 4 * 80mm case fans (its located
in a very warm room).

I am being irritated by what I have discovered is called the "beat"
frequency caused by the slight differences in fan speeds, resulting in
a "throbbing" sound...

The fans are marketed as quiet ones - its not the overall noise that
bothers me, its just the variation thats irritating.

Do I have any options other that ear plugs/loud music? Looking in the
BIOS I see that the fan speed vary in RPMs a little as I would expect
- what percentage variance is normal? I suspect the fans would all
have to run at "precisely" the same speed to avoid the noise in any
case....

Ultimately I am more concerned about having a cool system than a quiet
one so i can live with it but it seems to me that given the current
nature of PC cooling requirements many people must experience this
"beat" frequency annoyance.


thanks for any thoughts

 

[Apollo]

Hi all,

Just put together my new system with 4 * 80mm case fans (its located
in a very warm room).

I am being irritated by what I have discovered is called the "beat"
frequency caused by the slight differences in fan speeds, resulting in
a "throbbing" sound...

I had the same problem, but with 1 x 120mm, 2 x 80mm, 1 x 92mm + psu fans.
I don't think that the speeds have to be close even, the beats are iirc
certain harmonics resonating.

The fans are marketed as quiet ones - its not the overall noise that
bothers me, its just the variation thats irritating.

Do I have any options other that ear plugs/loud music? Looking in the
BIOS I see that the fan speed vary in RPMs a little as I would expect
- what percentage variance is normal? I suspect the fans would all
have to run at "precisely" the same speed to avoid the noise in any
case....

Without expensive control equipment / fans, you can't run them at exactly
the same speed.

Ultimately I am more concerned about having a cool system than a quiet
one so i can live with it but it seems to me that given the current
nature of PC cooling requirements many people must experience this
"beat" frequency annoyance.

I got two Zalman Fanmates and played with the speeds of the 120 and the 92,
I probably only droppped them by 100rpm and it cured the beats. You may
have to play around for a while over a few days to get it cured. Mine come
back slightly now and again, it's probably related to air temp, pressure and
all sorts of complicated variables.

thanks for any thoughts

HTH

 

[John McGaw]

Hi all,

Just put together my new system with 4 * 80mm case fans (its located
in a very warm room).

I am being irritated by what I have discovered is called the "beat"
frequency caused by the slight differences in fan speeds, resulting in
a "throbbing" sound...

The fans are marketed as quiet ones - its not the overall noise that
bothers me, its just the variation thats irritating.

Do I have any options other that ear plugs/loud music? Looking in the
BIOS I see that the fan speed vary in RPMs a little as I would expect
- what percentage variance is normal? I suspect the fans would all
have to run at "precisely" the same speed to avoid the noise in any
case....

Ultimately I am more concerned about having a cool system than a quiet
one so i can live with it but it seems to me that given the current
nature of PC cooling requirements many people must experience this
"beat" frequency annoyance.


thanks for any thoughts

A single quiet 120mm fan might provide you just as much cooling without all
the noise if your case will accept one in the correct top rear location.
More is not always better when it comes to fans.

 

[Biff]

To avoid the beat frequency problem you need to keep the fans from
running at speeds that are too close to one another. So, just use a
controller to stagger the speed of your fans. Fan controllers that
independently control the speed of up to 4 fans are fairly cheap. Some
good info on fan controllers can be found at:
http://www.silentpcreview.com/section13.html

 

[Matt]

Hi all,

Just put together my new system with 4 * 80mm case fans (its located
in a very warm room).

Does your case have unobstructed vents? Lots of cases have vents that
seem to be more decorative than functional. Compare photos of cases at
newegg.com. What case do you have?

 

[Patrick]

A single quiet 120mm fan might provide you just as much cooling without all
the noise if your case will accept one in the correct top rear location.
More is not always better when it comes to fans.

1. try putting a small half-drop of fingernail polish on the blade that
is uppermost when stopped, when you unplug the fans, one-by-one. That
might balance the blades better so as to stop any vibrations...

2. place a small piece of rubber or foam packing as a washer, on the
screws that hold down each fan, between the case and the fan body.

3. unplug the fans, one fan at a time, whilst running the other three,
and see if the beat frequency changes. The one with the greatest effect
is the one most out of balance.

4. Fans at the case front must suck in, whilst the fans to the case rear
must exhaust the case.

5. report back with your results. Others of us need to know!

 

[Dave]

Get a fan bus.

 

[Gav]

I had the same problem, but with 1 x 120mm, 2 x 80mm, 1 x 92mm + psu fans.
I don't think that the speeds have to be close even, the beats are iirc
certain harmonics resonating.

Without expensive control equipment / fans, you can't run them at exactly
the same speed.

I got two Zalman Fanmates and played with the speeds of the 120 and the 92,
I probably only droppped them by 100rpm and it cured the beats. You may
have to play around for a while over a few days to get it cured. Mine come
back slightly now and again, it's probably related to air temp, pressure and
all sorts of complicated variables.

hey thanks for your response.
Yeah I agree there has to be a whole bunch of variables involved - the
PC is located in a room that gets particularly hot, in fact getting a
new case with 4 fans has *drastically* reduced the system temp - the
increased noise is the the price I paid however. Still, its better
than an overheating system...

I try to adhere to the "if it aint broke dont fix it" philosophy so
rather than tinkering (and making things worse as seems to happen all
to often to me!) I might just play some music to cover the din!

A friend of mine has just opted for an expensive water cooling system
so if he has success with that I might look into it.

HTH

cheers

 

[Jon Danniken]

Get a fan bus.

Is anyone thinking about Neil Cassidy at this point?

Jon

 

[Spajky]

I am being irritated by what I have discovered is called the "beat"
frequency caused by the slight differences in fan speeds, resulting in
a "throbbing" sound...

The fans are marketed as quiet ones - its not the overall noise that
bothers me, its just the variation thats irritating.

on the "blowing out" fans serially connect a diode to reduce for
approx. 0,8V their voltage (10% less Rpm max) & silence them more like
I did 8see my site also under comp/mods section ...

 

[Gav]

on the "blowing out" fans serially connect a diode to reduce for
approx. 0,8V their voltage (10% less Rpm max) & silence them more like
I did 8see my site also under comp/mods section ...

hey guys,

Yeah my case fans came with little diode attachments to do just that
if they were too noisy. I am not keen to reduce the fans
effectiveness however, its been an ongoing battle to keep my PC cool -
its located in a room that is part of a "Mezzanine" layout
(split-level room) and all the hot air ends up there so its a really
hot environment. Currently the case temp is 34 degrees C and the CPU
52 - this is a VAST improvement over what it was before! Noise is
preferable to overheating in the end of the day.

reducing the fan speed will help the noise but at the moment the
system temp is only just within acceptible boundaries I think...