Understanding Ports in Vented Cabinets

[Loren42]

Here is a question for those with experience with vented cabinets.

Ports can be circular or they can be rectangular. The tend to function the same, as far as I know, when the port protrudes into the cabinet.

However, if you have a rectangular port in the front baffle, what happens when you move the position of the port from some position that lies away from the cabinet wall to a position where one side of the long side of the port's rectangle becomes part of the cabinet wall?

Here is an example, imagine a cabinet with an internal width of 20" and 30" high.

Example 1 has a rectangular port that is 2" by 20" and stands vertically with none of the ports side making contact with the cabinet walls.

Example 2 has the same size port, but it is oriented horizontally and is positioned at the extreme bottom of the cabinet so that the port's bottom is coincident with the bottom panel of the cabinet.

Will both ports function the same and have the same tuning if the port lengths are the same?

 

[GlocksRock]

I believe the tuning will remain the same as long as the port length remains constant, not sure if it will function the same, but most likely it will.

 

[Paul_Apollonio]

Here is a question for those with experience with vented cabinets.

Ports can be circular or they can be rectangular. The tend to function the same, as far as I know, when the port protrudes into the cabinet.

However, if you have a rectangular port in the front baffle, what happens when you move the position of the port from some position that lies away from the cabinet wall to a position where one side of the long side of the port's rectangle becomes part of the cabinet wall?

Here is an example, imagine a cabinet with an internal width of 20" and 30" high.

Example 1 has a rectangular port that is 2" by 20" and stands vertically with none of the ports side making contact with the cabinet walls.

Example 2 has the same size port, but it is oriented horizontally and is positioned at the extreme bottom of the cabinet so that the port's bottom is coincident with the bottom panel of the cabinet.

Will both ports function the same and have the same tuning if the port lengths are the same?

Ports will act differently depending on where they are located in the box. While computer models give us good predictions of the response of a given speaker in a given cabinet tuned to a certain frequency, they are poor predictors of the actual frequency of tuning the port will create by and large. If the port is in the bottom of the box, with the wall of the enclosure along the edge of the port, the physical port can be smaller than predicted as the wall of the cabinet tends to extend the effective port lenghth. After 25 years, I still yank what I want out of my backside, and then test the woofer response and port response and impedance to verify the tuning frequency I get is what I wanted, almost without regard to the actual prediction of length required by computer.