Discussion in 'Amps and Cabs [BG]' started by daveihde, Apr 10, 2014.

1. ### daveihde

Jul 17, 2006
Am building a small vented 10/6 bass cab and have a design question. I apologize in advance if this has already been discussed elsewhere, but I couldn't find it. Eminence's suggested design called for a 3.5" port at a certain length. I already had some 3" PVC, so I did the math to calculate what the same area would be and applied that to two smaller, shorter ports, instead of one longer, bigger one. I have since been listening and not getting the response I expected, so wonder if I was mistaken in my approach. Using WINISD to model the box, whenever I change from 1 vent to 2, the length specified INCREASES instead of decreases, the opposite of what I would expect.

Being a horn player, I understand that a longer, bigger pipe equals lower frequencies. Am I getting this wrong, or is the physics of venting counter-intuitive; that longer = higher?

2. ### DukeLeJeunerational romantic mystic cynical idealistSupporting MemberCommercial User

Nov 24, 2008
Princeton, Texas
Owner & designer, AudioKinesis; Auth. mfg, Big E (Home Audio only)
Same diameter but longer vent = lower tuning frequency
Same length but smaller diameter = lower tuning frequency, and possible earlier onset of vent noise (chuffing)

So when you go from one 3.5" diameter port to two 3" diameter ports, the port length needs to be increased to maintain the same tuning frequency, because the two 3" diameter ports have more cross-sectional area (larger effective diameter, if you will) than the single 3.5" diameter port.

Aug 25, 2007