New Computer Case Design - Airflow Dynamics?

[cerestes]

Building a homemade computer case, designed mainly for silence.
General theory is computers generate heat, fighting heat (fans) make noise.

Heat moves, up, don't fight it. Case will be designed in a chimney fashion. Using many levels will force the sound to bounce back and forth (off of walls lined with sound deadening material), and hopefully not hinder air flow too much.

Question is, from an airflow perspective, should each tier have the same volume? What I'm worried about is if you go from a tier with a large volume to a smaller tier, does the air simply speed up?

I would expect if you halved the volume, their air speed would double. However, I'm concerned about creating a pressure buildup trying to force the air through the smaller tier which would decrease effeciency.

|^^^^^^^^^^^^^^^^|   <-- exhaust air filter, grill
|     DVDROM     |   <-- 5.25" bay
|  ____________  |
|   CPU  [ [ [   |   <-- upside-down motherboard
|   | |  [ [ [   |
|___| |__||______|   <-- two exhaust ducts
|                |
|_____________   |
|                |
|     HD         |
|_PSU____________|   <-- PSU and 120mm fan push air
|                |
|_____________   |
|                |
|                |
|^^^^^^^^^^^^^^^^|   <-- intake air filter
 V              V    <-- legs to raise above floor

Any input from someone real familar with how air flows?

 

[Cliff_J]

My fluid dynamics is too rusty, I'll let someone else tackle that. I'd say as long as you don't get too small with your air pathways, not much to worry about there.

The two main types of sound reduction are to absorb the sound energy (foam, fiberglass insulation, jute carpet padding, etc) and to minimize structure borne noise. The structure borne noise that is a result of the actual mechanical panels acting as 'speakers' creating the noise in the interior compartment can have the noise level reduced with viscoelastic asphalt based materials and decoupling materials. Works fine in cars and planes but that wouldn't help much in a computer case.

You need quiet fans (places like Grainger sometimes list fans with db rating) and then need to try to place absorbing material effectively to minimize direct sound (and reflect it back in) and this is a simple 90 degree or 180 bend with an absorber placed for the sound to 'run into' at the end of the turn. Here's a picture about 1/3 the way down this page that shows how to create a box to quiet down a HVAC register, and that's a fan blowing air in a manner similar to yours...
http://www.acoustics101.com/components_gateway.htm

Cliff

 

[russ_watters]

Its a little tough to tell from the pic, but are you showing everything horizontal? Motherboard, drives, etc? That'll completely destroy your airflow. The motherboard especially, must be vertical and there must be at least a small unobstructed vertical air path.

 

[Artman]

Neoprene washers on the screws between the fans and metal.

Yes, cutting the area in half doubles the velocity of the air and will increase the pressure some.

Some company (Dell I think?) just used a huge heatsink on their CPU and the PSU fan for their PIII cooling. Very quiet. Ran the heatsink to infront of the PSU fan intake. And I think Apple had a fairly recent model with no fan.

 

[Njorl]

Think about conservation of energy before you put sound absorption inside your case. What happens to that accoustic energy when it is absorbed? It is converted to heat. Also, probably more importantly, most sound absorbers act like thermal insulation. They restrict airflow, and prevent convection processes from moving heat to the case's metal exterior.

Njorl

 

[Gokul43201]

Some of the the Dell cases are quite silent. I think most of the sound comes from bad bearings in the fans. Papst (or Pabst, can't remember exactly) makes great fans. Perhaps a little pricey, but they're really nice ! Worth looking into, I think.