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Beatin'
10-27-2013, 01:34 PM
Let's say you need 15 square inches per cubic foot.

For a 4 cubic foot box, that would require 60square inches.

Let's say the port takes up 1 cubic foot, you have 3 left. Do you still need a full 60square inches of port area or can you drop it down to 45 square inches?

bbeljefe
10-27-2013, 01:55 PM
You can if you don't mind a noisy port.

For a 4 cubic foot box with a 1 cubic foot port, you need more than 5 cubes gross because you also need to add bracing and driver displacement. If you don't consider those things, your tuning will be off.

T3mpest
10-28-2013, 05:32 PM
Remember that's also just a general rule of thumb to get you in the ballpark. The excursion and BL (often overlooked, but honestly more important than xmax in this case) really determine how big of a port you will need. Most box programs can model you airspeed and give you a general idea where you need to be. Generally the 12-16 foot rule works well, sometimes you need to go higher or can even go lower. Your going to want to use your net airspace NOT including the port displacement. So if you have a 5 cubic foot box and the driver and bracing takes up .5, I'd do 4.5 cubes gives you a 55-72 range to play with, so somewhere in the 60's would be pretty normal.

No real downside to a slighty larger than needed port, but when you start getting to small issues compound fast. Frequency response changes as the port begins to compress and port noise adds distortion. I always do the largest port I can comfortably fit given the airspace I have and tuning that I want, or use passive radiators, I'm a fan of those in the right circumstances too.