Maybe there's a misunderstanding of terminology here: There are MANY different types of HVAC registers.
Here's a couple of simple "grill" type registers, such as the ones that Waka is talking about, that do not direct the air in any way: the air just passes through directly, and carries on straight out:
HVAC-hex-grill.jpg
HVAC-grill-Eggcrate-Grille-Standard.jpg
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HVAC-GRILLS-AND-REGISTERS.jpg
HVAC-decorative-grills.jpg
With the above types, the air just flows through without changing direction. So if you have your duct coming down through the ceiling, those just dump the air directly down on your head.
You can also get a type that changes the flow in to move in only one direction:
HVAC-1-way-register.jpg
Or two opposite directions:
HVAC-2-way-register.jpg
Or two perpendicular directions, such as you would use if your duct comes down in the corner of the room:
HVAC-2-way-corner-register.jpg
Or three mutually perpendicular directions, such as you would use if your duct comes down against the middle of a wall:
HVAC-3-way-register.jpg
Or the four-way "diffuser" type that Waka is talking about using in his case:
HVAC-4-way-register.jpg
In some cases the angle of the vanes and flow rate are fixed: you can't change them. But in other cases, there are controls on the register to allow you to adjust the rate, and also the direction of flow:
HVAC-adjustable-diffuser-s-l300.png
HVAC-adjustable-diffuser-2.jpg
HVAC-adjustable-diffuser-3.jpg
HVAC-adjustable-diffuser-5--three-way.jpg
So choose your register carefully, specifically for the job it has to do! The cheaper registers have a low "percentage open area", and are made with simple flat vanes that create a lot of turbulence in the air flow, thus increasing air noise and also static pressure. The more expensive type have smooth, curved aerodynamically shaped vanes and high "open area", so they are a lot less noisy, and do not increase your static pressure much.
Obviously, the more you change the angle of air flow, the more turbulence, noise, and static pressure increase you get.
So, long story short: choose your registers carefully! They have a surprisingly large effect on the performance of your HVAC system, as well as on noise, and comfort.
- Stuart -