Advanced Engineering Tech - pic: purpose of this plenum on intake duct ?

http://www.freeimagehosting.net/t/8de5o.jpg (http://www.freeimagehosting.net/8de5o)

what is the purpose of this plenum or whatever it is called hanging off the bottom ? This is the duct that runs from the air filter box to the throttle body on an 8.1L in a 2006 Sierra. I mistakenly got a duct for the 6.0L gas engine which is slightly different shape but that also has a plenum thingy hanging off it.
I am going back to oem configuration on the truck. It came with a K&N cold air kit- cone filter replaced the air box and the K&N duct was just a straight pipe that went from air box & MAF sensor to the throttle body.
So my question is how come the OEM setups have this plenum thingy, what is it's purpose? And why does the K&N setup not have it?

here is a pic of the k&n setup
http://a.cdnbrm.com/images/products/schem/k%26n/large/k%26n_fipk_57-3030-1.jpg

wrong forum but its a silencer

what forum should it be posted in ?

how does it work? what kind of noise does it silence ?

It cuts down on intake air noise. For noise emission abatement. Basically so the truck isn't too loud inside.

Looks like basically the same thing as the 'donkey dong' silencer on the LT1 intake elbow.

It can be used to "silence" or to just cancel out frequencies that are annoying, or to give a cool power sound at WOT (wide open throttle) in the higher rpm ranges.

My gues is all of the above for the one pictured.

FWIW, the principle is the same one CORSA uses on their mufflers to tune the sound.

Jon

Corsa use 1/4 wave, that looks like a Helmholtz. Helmholtz have a much lower Q and wider bandwidth than 1/4 wave resonators.