Noise Cancellation Experts? Have PC Muffler Idea-need help!

[cuban8]

Ok, I have been racking my brains for the last week on how to make a very silent computer and come up with the following wild idea.

I am pretty good making wood cabinets and thought of making a PC case - actually more like a stand about the size of a small filing cabinet maybe 18 inches wide, 24 deep and 29 tall.

My idea is this: I could put the board and all the components in it and build a "muffler" inside the cabinet using a noise-cancelling echo chamber. I know it's crude but thought it may just work.

Here are the details,
When my CPU fan runs at 10,000 rpm, it has it's first sound frequency spike at about 585 Hz which would make it's L/4 wavelength about 5.88 inches. I know the frequency because I used the following freeware program http://www.visualizationsoftware.com/gr ... /gram7.zip and my computer's microphone to find them.

If my understanding of noise cancellation is correct, all I have to do is tune all my other fans to run so that each of their natural frequeny spikes occur at multiples to one another.

If I adjust my 120mm case fan to run at 1650 rpm, it has it's first sound frequency spike at about 195 Hz and L/4 wavelength of about 17.6 inches - 3x the CPU L/4).

With these fans tuned to one another at these speeds,
the L/4 of my Case fan is 17.6 inches which is exactly 3 times the L/4 of the CPU fan.

So.... if I make some kind of echo chamber that is 17.6 inches long, I'm guessing that it would cancel out the noise from Both fans simultaniously.

Here is a rude & crude drawing of my concept.

Is there a sound-wave expert out there that would know if this concept would work?

My specific questions are:
1. Are there any guidlines to the dimensions/shape of the echo chamber?
2. What about the opening between the chamber and the main exhaust chamber?
3. If the echo chamber is vertical and waterproof on the bottom, could I use water to adjust the length in case I'm a little off?

By the way, if someone could design and manufacture a stand-alone "Active Noise Cancellation Box" with a little speaker (Like Bose does with headphones) that would fit inside a computer case, They'd take over the Silent Computer Market in a flash! That way, we could put a Tornado or Volcano 9 in our case at Max RPM and it would be silent!

Thanks for any advice.

Mike

[jafb2000]

Since you are designing a different enclosure,
the solution is largely done by two simple things:

o Use wood - so you have mass to absorb noise
---- 3/8" plywood is cheap and very effective
---- go to a marine supplier and get real laminate if you want :-)
o Use labyrinth - so noise has no straight path out
---- each 180-degree turn in airflow reduces noise by 6dB(A)

Don't go overboard, PC noise emitters don't have much power.

However you need to consider the Make or Buy:
o Why quieten a 10,000rpm CPU-cooler fan
---- if it really turns that fast, change to a larger fan
---- a 92mm fan will give 36cfm at 23dB(A)
---- 36cfm will cool around 175W of CPU remember.
o Are quieter fans or working to improve airflow cheaper?
---- some simple ducting or quiet fans may be cheaper

That said, the PC case is not designed for low noise.
o Any hole in the front of a case allows direct sound path out
---- absorption can only do so much
o Airflow should be ducted along the bottom of the case
---- then to drives in the front, no noise-path thro the front

Target noise caustion, not noise absorption first.
The former is easier & cheaper than the latter.

You could even site the intake & exhaust fans partly inside.
This does require the intake/exhaust ducts to be acoustically lined,
but in doing so you achieve the real soundproofing so often missed:
o You have the fan noise bouncing along a tube cavity
---- this will focus the noise
o Lining that cavity forces noise absorption over the LENGTH of absorber
---- so the time & distance interface of noise with absorber is "huge"

At least, huge relative to the 6mm between noise & outside in a PC case.
Interesting project, and done with care, will last multiple PCs.

Might be worth doing a nail-together prototype because you are very
likely to change the design vs building it perfect then scrapping it.
--
Dorothy Bradbury
http://homepage.ntlworld.com/dorothy.br ... anaflo.htm

[Rusty075]

I'd consider myself about halfway between an Expert and a Novice, but I don't think it's going to work.

Niose cancellation is a very tricky thing. If the waves don't meet perfectly you end up with a noise twice as loud, instead of quieter. It isn't like horseshoes, close doesn't count.

Think about it this way: For the 120mm fan the L/4 distance is 17.6", just like you figured. But the fan is 4.75" across. So if you put the hub 17.6" from the CPU fan, some of the noise is going to be produced 15.3" from the CPU, and some of it 20" from the CPU. Only the waves created exactly 17.6" away will be cancelled, the rest will be multiplied.

Now having said that let me say that I don't know for sure that it won't work, I just don't think it will. Maybe someone here who knows more will correct me.

The idea of noise cancellation has been discussed here before:
http://forums.silentpcreview.com/viewtopic.php?t=2503


I think it would be both easier and more effective to reduce the noise sources first. (What do you need a 10,000 rpm fan for?? I'm cooling an XP2100+ OC'd to 2.1Ghz with a fan that's running at 1200rpm's) Jafb2000 is on the right track. Use the mass to absorb the sound, rather than try to cancel it.

[Ralf Hutter]

I'm certainly no expert but my suggestion would be similar to Rusty075's and jafb2000's: Get rid of the 10,000rpm CPU fan, even if you have to spend some money to get a new HSF entirely. That would certainly be cheaper than building an entire box. Have you priced lumber lately?

[Alistair]

By the way, if someone could design and manufacture a stand-alone "Active Noise Cancellation Box" with a little speaker (Like Bose does with headphones) that would fit inside a computer case, They'd take over the Silent Computer Market in a flash! That way, we could put a Tornado or Volcano 9 in our case at Max RPM and it would be silent!

Sorry but, only in dreams...
http://forums.silentpcreview.com/viewtopic.php?t=2503

NielBlanchard puts it about right:

And as you move around, your ears change the distance from those noise sources -- and from the cancelation device, so again, there is no practical way that a cancelation device can work EXCEPT if it is attached to your head!

But regardless, you get full points for eagerness and creativity.

a.

[Phill77]

Thinking of a doing a similar thing myself. I'm going to use a small wooden chest 700x350x350mm in size, will put a shelf in it so the psu and hard discs are at the bottom, and the motherboard above.
I'll have loads of room for ducting, with fans sitting part way along from each end. I'm going to try and get all the openings in the case in the bottom to face my carpeted floor. The insides will be lined with foam or underlay or something.
I won't bother with a floppy and I'll make a seperate case to sit on top of the main box to house my cd drive, lights and switches.

Its going to be quite big, but that doesn't matter as it will double as a table.

Getting started this afternoon with a bit of luck.

[jafb2000]

Noise cancellation needs to be active to work, and as cited really next
to a persons head. It was tried on the Lotus Esprit, due to the noise
source & head being largely static - and predictable 4-cyl engine noise.

Using the labyrinth & baffle is by far the best solution:
o 180-degree turn knocks 6dB(A) off noise
---- remember 10dB(A) off noise is a halving of noise level
---- remember also that dB(A) gets quite poor as a metric <20dB(A)
o Baffle sized to reflect sound back on itself internally
---- open an earlier G series mac
---- they had a baffle ingress into the case near the fan
---- wavelength set to stop a direct noise path to the persons ear

A wooden enclosure, or wooden PC, is not particularly difficult.
o Remember flight cases are wood, but laminated
o Flight-case angle for edges, plastic ball corners & castors, handles
o Hybrid joint extrusion, selaed, luggable (needs case-in-case foam separator)

Very easy to work with, PC for life, tailored to be truly silent.
Can accommodate as many motherboards/PCs as you like & cheap.
With cases costing 150-200ukp/$US for one, it is ~25% less than one
and after that the saving grows very rapidly indeed - and quietly too.

Example:
o Housing 64-motherboards in rack cabinet/racks or server cases
---- total metwork 15,000ukp for racks & cases
o For density it used 1U which needed 640 40mm fans
---- a 1/2 life of <1yr at 3000ukp for a complete set re maintenance
o For PSU it forced 460W 1U PSU - 64x 140ukp discounted = 9,000ukp
---- plus lower 1/2 life of PSU re cooling
o Total was around 22,000ukp with every discount wrung out

Which is rather a lot to have achieved some lights on & fans spinning!

o Nylon/perf-alloy blow-thro case frames & flight case shock/soundproof
---- total metalwork now just 1,700ukp
o Use standard 92mm & 120mm fans - plus some 254mm
---- a 1/2 life of over 8yrs, power mgt controlled so even longer, 250ukp
o Standard boring 460W ATX PSU, 64x 19ukp discounted = 1250ukp
---- longer 1/2 life & quality Panaflo fans fitted for maintenance free
o Total was around 3500ukp, saving 18,500ukp
o 18,500ukp left 289ukp for each PC, P4 m/b & P4 & RAM for free
---- only dog was sorting out the boot-over-LAN re Intel PXE

Airflow path is straight thro as essentially no case fronts/rears. Just a
windtunnel which reduces the static resistance so fan noise & fans.

The same benefits can be applied to a smaller scale - particularly if you
wanted to put 3-4 servers in one box and make it silent & long lasting.

Within 5yrs we might well end up with heatpipe from CPU to alloy-case,
somewhat like the old Mainframe module design re thermal output/density.
Dies shrink, voltages drop, but wattage is climbing irrespectively and the
future CPUs are likely to be soon 250-500W each within that 5yr window.

[Yomat]

Second 'making a box' note in short time. Looks like its vacation times.

As for cancellation: http://www.bang-olufsen.com/sw2199.asp
If that puppy can deliver what they say. Maybe it could be configured to correct noise sources too. Or .. at least one should be able to do something with the same technology. But the speaker is in the +$10000 range. Go figure.

As for 500W CPUs. Should be the death of custom built PCs. At least mobo-cpu-HSF combos. The sheer size, weight and noise level of the heatsinks would be crazy. Man.. before they do anything else them processor designers have to figure out better cooling solutions. The ATX standard has to be scrapped too, AGP too and the sockets of course. When, when, when.

This technology is starting to get dirty. Environmentally, ergonomically. Even gaming consoles make noise these days, damnit.
Is it really necessary with increased power consumption? Is it not possible to stabilize the power and improve processors by other means such as transistor spacing. Stop the race of hens, I say. Go VIA!

[cuban8]

Well, I did a rather crude noise cancellation experiment with some PVC pipes and it didn't work that well... so I'm making a wood case.

It's about 24" deep, 20" wide & 32" tall with 4 casters (it's going to be heavy). I have the room so I'm not worried about making it small or usefull for anything other than the PC.

I spent about $40 for a 4x8 sheet of 3/4 Birch plywood and $5 for the wheels. I also have a friend that works at an auto supplier that sells acoustical absorbers so he's helping me get some of that material.

So far so good.
I have it designed in actually 4 layers in the case.
Bottom - battery backup.
Level 2 - Intake "Muffler Chamber" with Power Supply & it's own dampened exhaust port. I will also have my hard drives on this level.
Level 3 - Motherboard
Level 4 Back - "Muffler" chamber (all the turns & foam damping etc...)
Level 4 Front - CD Drives etc.

My biggest challenge has been to make it so I can actually work on my computer (assemble & disassemble). As you can imagine, it would be pretty difficult to work on a motherboard with it buried 15-29 inches deep in a small box with the only major opening being the top.

To that end, I have designed it so that only the bottom, sides and back are held together with screws and the rest is all held together by "friction" and is very quick and easy to take apart when needed without any tools.

ie... to get into it I....
Remove the lid (which is held down by four small hooks.
The lid has several features which hold the sides together and the lower layers in place with a compressed fit.
Remove the CD tray which is wedged into place by the lid
Remove the Motherboard tray so I can work on it on a bench.
One downside is that I will have to have tight tolerances and foam between all the matting layers to insure that no noise can find a short cut.

Some other ideas I have are "Soft Mounting" all the drives without any screws so that I can put them whereever I want.. whenever I want. I plan to do this with a little velcro.

My final challenge is going to be mounting the motherboard. I have some angle aluminum for attaching the video & sound card but I'm concerned about mounting the MoBo to a plywood panel. A friend of mine built a solid cherry case and had some problems with expansion of the wood at different temperatures and humidities. ie, the panel could expand farther than the board causing stress.

For those reasons, I want to have it mounted on something that is solidly attached (with rubber dampers) yet can expand and contract without any problems.

ANY IDEAs on that one?

The last detail I still have to work out is routting all the wires from the MoBo level out of the case without creating a bunch of holes for noise to escape.

I'm travelling this week out of town so when I get back, I'll start taking some pictures of the progress. Maybe the other guys building the wood case can share some of their ideas. With that in mind, I'll probably start a new thread so keep your eyes open for it!

Thanks for the help!

[Rusty075]

Rusty's Quiet In-Desk PC