I’m looking for some help with this low frequency noise

Enclosures and acoustic damping to help quiet them.

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HokieJoe
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Joined: Sat Apr 08, 2006 12:04 pm

I’m looking for some help with this low frequency noise

Post by HokieJoe » Sat Mar 01, 2008 2:02 pm

Hello all,

My aim for this project was to use a handmade walnut cabinet I had built years ago. I realize this isn’t exactly PC cooling oriented, but I wanted to leverage the collective knowledge from the cats here at silentpc. If this subject is too far off the beaten path for you all I understand. After putting my equipment in the case it didn’t take long to realize that I needed some ventilation through the cabinet. The equipment was getting quite hot even though spacing/natural airflow didn’t seem to be much of an issue. I sought out four 120mm fans and some universal power supplies, and then set about designing a way to mount them to the case.

I wanted to decouple them from the case, but still maintain a relatively airtight seal. My common sense approach was to build a rectangular frame and use s-hooks and o-rings to suspend the fans in the frame. Then the frame would be mounted to the back of the entertainment cabinet with some sort of gasket seal.

Here’s some pics of my progress:

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You can see in the pics above and below, I decided to use flexible PVC shower mat underliner. It's 40mil(?) thick Oatey brand found at Home Depot. I chose to use it over hardboard because I wanted something flexible. I glued hardboard to the back inside of the PVC mat so that I could mount 5/8" door gasket seal to make the fans airtight as possible in the fan box. I then used black RTV to seal the fans to the door gasket seals as there was some gap remaining there.

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I drilled some more 1" holes for better ventilation. (Notice the difference between the top and bottom set of holes)


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As you can see, I’ve tried to decouple the fans and the frame from the cabinet as much as possible. I have both fans hooked up to a Radio Shack universal power supply. I have the fans set on 4.5v so the fans aren’t spinning that fast. Heck, in free air they’re practically inaudible. Once mounted to the cabinet, I could hear a low frequency hum overlayed by a slight buzz from the cabinet. To combat the buzz I installed Dynamat Extreme under the shelves and on the back panel of the cabinet. That eliminated practically all of the buzz. I think the primary problem now is some type of airflow/chamber resonance and excessive noise from the fans.

Here's a view from the front side (please excuse the mess):

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I think one partial solution would be to buy fans with a better blade design like the Yate Loon perhaps. What I bought was off-the-shelf Thermaltake fans I found at Radio Shack. Anyway, all suggestions are welcome and sorry for the long post.

ntavlas
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Post by ntavlas » Sat Mar 01, 2008 6:22 pm

It is possible that the noise is due to very unbalanced rotors. Some yate loons should fix it, though this is not the only possible cause.

At this speed the fans produce little vibration so I don`t think suspension is necessary. I actually suspect that it`s the suspension that causes this hum.
You could mount the fans directly on the cabinet, maybe with thin pieces of rubber in between.

Finally you could try to have the fans sucking air through the cabinet, it might improve cooling a little.

HokieJoe
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Joined: Sat Apr 08, 2006 12:04 pm

Post by HokieJoe » Sat Mar 01, 2008 7:39 pm

Thanks for the reply ntavlas. Initially, I tested several mounting techniques. I used simple door gasket, velcro, and double-sided tape. All three caused a pronounced buzzing sound in the cabinet. I think part of the issue is the backing of the cabinet covers such a large area and it's only 1/8" thick. It's like a big acoustic guitar.

As for fan direction, the fans are pulling air through back of the cabinet.

Before I applied the Dynamat I used a Radio Shack noise meter to get an idea of the hum's resonant frequency. The range on the meter only goes down to 60Hz, and at that setting the "hum" pegged the meter at +6db (while sitting inside the cabinet). At this point, I'm just fishing for suggestions, but I definitely foresee a fan upgrade coming.

NeilBlanchard
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Post by NeilBlanchard » Sat Mar 01, 2008 7:57 pm

Greetings,

What about a slightly different approach: instead of having the fans pull air directly through the holes, what about having a plenum and pull the air out of that? Then, the air would flow out of all the holes. The low frequency noise may be the obstruction of the blades by the wood "webs" that are between the holes, being so close to the blades.

[Edit: the RS Sound Pressure meter is measuring ~66dBA; not 66 Hz. It doesn't measure frequency; only sound pressure.]

MikeC
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Post by MikeC » Sat Mar 01, 2008 8:11 pm

What you describe sounds more like cavity air resonance than any problem intrinsic to the fans. That's the AIR in the cabinet accentuating low frequency noise at & around its resonance points. There are usually at least a few, in multiples of the base frequency.

It could also be that the flexible PVC shower mat underliner is too dense/stiff and allows vibration to be transmitted into the cabinet, causing its panels to vibrate... plus cavity air resonance effects.

Personally I would have used a piece of open cell foam (like sponge). Cut holes in your frame about an inch or two bigger than the fans, then cut foam to fit into those holes, and fit the fans in the foam. Now each fan is sitting in a frame of soft foam. The foam is not airtight, but it doesn't have to be, and it actually helps absorb some noise. This technique works really well.

Here is a pic of this technique used in the SPCR PSU tester:

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Also, doesn't the gear you have sitting in the cabinet make noise? Like with fans and hard drives? Could they not also contribute to both of the above? You might have to turn all the gear off and listen with just the fans alone. If the gear contributes, try placing them on soft cushy stuff and see what happens.

Try changing the fan speed and see what that does to the noise. Run your hands all over the cabinet panels and try to determine where/when there is vibration that coincides with the noise.

It's a matter of analyze carefully first; the solution usually comes naturally when you figure out what's going on.

Neil's comments may also be worth exploring. You could replace the holey wood panels with something like black screen material?

HokieJoe
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Joined: Sat Apr 08, 2006 12:04 pm

Post by HokieJoe » Sat Mar 01, 2008 8:21 pm

NeilBlanchard wrote:Greetings,

What about a slightly different approach: instead of having the fans pull air directly through the holes, what about having a plenum and pull the air out of that? Then, the air would flow out of all the holes. The low frequency noise may be the obstruction of the blades by the wood "webs" that are between the holes, being so close to the blades.

[Edit: the RS Sound Pressure meter is measuring ~66dBA; not 66 Hz. It doesn't measure frequency; only sound pressure.]

Thanks for the correction on the RS sound pressure meter.

The wood "webs" are something I have thought of. I considered simply sawing them out completely and fixing a screen of sorts over the opening. I really didn't want to alter the cabinet unless I had to though. On the subject of a plenum, when I was coming up with ideas I considered gluing some of these:

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Roof Vent Stack Flashing

directly to the back of the cabinet and attaching the fan at the small end. I have sourced some of these locally. That idea is not out of the question because I can unmount the fan boxes and alter them as needed. I would probably have to run the fans at a higher voltage though to pull an equivalent amount of air- no?

HokieJoe
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Joined: Sat Apr 08, 2006 12:04 pm

Post by HokieJoe » Sat Mar 01, 2008 8:52 pm

MikeC wrote:What you describe sounds more like cavity air resonance than any problem intrinsic to the fans. That's the AIR in the cabinet accentuating low frequency noise at & around its resonance points. There are usually at least a few, in multiples of the base frequency.

Although I didn’t explain it as well as you, that is what I'm thinking.
MikeC wrote:It could also be that the flexible PVC shower mat underliner is too dense/stiff and allows vibration to be transmitted into the cabinet, causing its panels to vibrate... plus cavity air resonance effects.

Personally I would have used a piece of open cell foam (like sponge). Cut holes in your frame about an inch or two bigger than the fans, then cut foam to fit into those holes, and fit the fans in the foam. Now each fan is sitting in a frame of soft foam. The foam is not airtight, but it doesn't have to be, and it actually helps absorb some noise. This technique works really well.

Here is a pic of this technique used in the SPCR PSU tester:

Image
That's a good idea. I suppose I could fashion another frame using solid 3/4" piece of plywood as backing. I could cut square holes out big enough for the foam/fans. The question is, where can I source foam like that?

MikeC wrote:Also, doesn't the gear you have sitting in the cabinet make noise? Like with fans and hard drives? Could they not also contribute to both of the above? You might have to turn all the gear off and listen with just the fans alone. If the gear contributes, try placing them on soft cushy stuff and see what happens.

Try changing the fan speed and see what that does to the noise. Run your hands all over the cabinet panels and try to determine where/when there is vibration that coincides with the noise.

It's a matter of analyze carefully first; the solution usually comes naturally when you figure out what's going on.

Neil's comments may also be worth exploring. You could replace the holey wood panels with something like black screen material?



Indeed, the components definitely add more noise than the cabinet itself, but in combination, the result is more noise than I would like. I wanted to minimize the cabinet noise first and then attack the other sources as needed. The RS universal adapters I have offer: 3, 4.5, 6, 7.5, 9, and 12 volts. I’ve tried them all (12 volts sounds like a vacuum cleaner), and 4.5v seems to be the sweet spot for moving air at minimum noise. Surprisingly, the vibration in the cabinet’s panels are quite tame, but they are perceptible. That’s what makes me think it’s some sort of chamber effect like you mentioned; or some sort of turbulence effect from the fans’ close proximity to the holes like NeilBlanchard mentioned.

MikeC
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Post by MikeC » Sat Mar 01, 2008 9:37 pm

HokieJoe wrote: On the subject of a plenum, when I was coming up with ideas I considered gluing some of these:

Image
Roof Vent Stack Flashing

directly to the back of the cabinet and attaching the fan at the small end. I have sourced some of these locally. That idea is not out of the question because I can unmount the fan boxes and alter them as needed. I would probably have to run the fans at a higher voltage though to pull an equivalent amount of air- no?
Definitely DON'T do this. You'll add Helmholz resonance noise from the cylinder to the mix.

HokieJoe
Posts: 9
Joined: Sat Apr 08, 2006 12:04 pm

Post by HokieJoe » Sun Mar 02, 2008 10:53 pm

Okay guys, lots of things to consider at this point. The first thing I want to do is buy some of the Yate Loon's. Should my little experiment find no use for them, I figure it won't hurt to have few extra hanging around for future projects. My first alteration will be to make new fan boxes. I'll utilize the foam surround concept you illustrated above Mike. If that doesn't satisfy me I look at other options.

Here are some of the other options I've considered:

Fashioning a plywood box to surround the 360 and PS3. The ideal here is to build it big enough to shroud the noise from the 360/PS3 and the cabinet fans. It will be open in the back with strategic openings along the front and sides. I may try to fashion a plenum silencer for the fresh air inlets. Something that won't restrict airflow too much, while also muffling direct sound from all of my associated equipment.

It's possible (I think) that I'm picking up some flutter echo off the sides of the cabinet. I need to pick up some 1" rigid fiberglass and try lining the inside of each shelf (on the sides). This may help to some degree, but I doubt that it will the be all cure.

Saw out the web of holes in the back of the cabinet. Then, replace it with screen door material and frame it out. I can maintain my current mounting system this way, and remount the fans per the SPRC PSU tester.

My final solution will be to attach doors to cabinet shelves. Obviously, the issue here is getting fresh air to pull through the case. Hopefully I can fashion a plenum around the fresh air intakes similar to above.

Anyway, I've lots more to read here before deciding on a final solution. One step at a time...Hey, process of elimination- right? :D Thanks for the suggestions thus far. Everyone has given me some good ideas to work with.

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