changing port diameter (external port)

[mat3833 10+ year member]

to make this question simple i am going to just give a basic scenario instead of my actual problem. i have a box that is currently tuned to ~ 42Hz. it is a prefab HT box using a small 6.5 inch sub. with the current settup it performs VERY well except when a building falls or whatnot. it just cant reproduce the lower frequencies that well with its current tuning. so i decided to do some tests using some cardboard to add an external port to the box to lower the tuning and the results are fantastic. adding ~ 30 inches of port(port diameter is only 3 inches) has lowered the tuning into the low 30s to high 20s( tested using a visual method of measuring cone excursion and a computer program playing a steady tone) the only problem is i dont have enough room for 30 inches of port where the sub normally sits. so here is the question:

can i make the external port LARGER than the internal port. if i take the 3 inch port and make it a 6 inch port i can effectivly half the required port and i can fit the sub where is currently is and still load the port properly in a corner load settup. BUT i dont know how the change i port diameter will afffect the airflow. i would be using a cone to adapt the larger pipe to the smaller one and using epoxy to secure the cone without leaving a "lip" on the insede of the pipe. then silicone will be applied to the interior of the joint to form a smooth transition, with the same thing being done to the cone-to-large pipe transition.

how will the transition from a 3 inch port to a 6 inch port affect the enclosure? i plan to do a test with more cardboard(actually its a fiberglass reinforced cardboard sheet, pretty darn stiff and only ~ 2 mm thick.) to see actual results, but i wanted to get your opinions before i wasted my time.

Matt

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[THATpurpleKUSH 5,000+ posts]

sounds like a waste of time if you ask me

 

[mat3833 10+ year member]

the actual measurements for the port diameter change are as follows:

current internal port Diameter - 2.5 inches

current Desired External port Diameter - 3.5 inches

"Adapter" section 2.5 inch--->3.5 inch cone with a 2 inch length

Matt

well to me it isnt a waste of time. with the ammount of space i have available this little 6.5 sub is plenty of punch. the only problem is the enclosure tuning. i could build a new enclosure, OR i can just use the materials i have and make a simple external port to change the tuning.

 

[jolly_26 10+ year member]

Best just to build a new enclosure I'd say