I can make you say "why"

[JeremyC]

Thanks 71buick, I just learned something new. Thats the same set up I used in my S10. I knew it sounded great, and hit hard for the sub and amp combo, but I had never heard it explained like that.

[Fishy]

A slot port and a duct port are generally considered the same.

Yeah but a slot port or a duct port is not the same as regular old rectangular port. A sqaure port in the middle of an enclosure behaves pretty much the same as a round port with the tuning frequency being dependent on net enclosure volume, cross sectional area and length, but a "slot" port of the same cross sectional area that uses an enclosure wall would need a slightly shorter length to acheive the same tuning frequency.

-Fishy

[71buickfreak]

How so? The physics are the same, the math is the same, the lengths are the same. What you don't get with a port that utilizes the enclosure wall is the same amount of port noise, as you have less air flowing over the end of the port. I disagree with your statement that simply using an enclosure wall reduces the required length of the port. The physics are the same. If properly built, the length:frequency would be the same.

[Jonathan]

Actually, Fishy is right about that. A slot ports effective length will be longer than that of a typical round port because one wall of the port is also one wall of the enclosure, and this wall extends beyond the end of the port thus effectively adding length to the port to a degree. A good example of this is the JL Audio specific enclosures, if you put the enclosure information in a calculator, the results won't be the same as JLs specifications because JL has already compensated for end length, unlike many manufacturers. The air immediately outside either end of the port acts in sympathy with the air mass in the port, this actually applies with both types of ports, but it affects slot ports by a larger degree because slot ports are usually designed to have a larger port area than others, so there is more to be compensated for, also the wall of the enclosure adds more friction to the movement of the air mass. The end correction factor is accomplished by adding 1/2 of the port width, and it is necesary because the driver can't see the actual length of the port (as in the exact location where the slot terminates), it can only go by what it feels is going on, based upon friction of the air mass. With round ports it isn't as much of an issue since you're terminating the port away from enclosure walls that would increase friction to the movement of air.

[Fishy]

Yeah and a flanged(baffled) termination results in greater effective mass as well. The new alpha version of WinISD allows you to compute vent length for 0,1, or 2 flanged ends, but unfortunately doesn't address slot ports.

My old acoustics book has "end correction" formulas to figure out the effective length of flanged and unflanged round ports. Normally you'd multiply Sd by the length by air density to get the mass of the vibrating collumn of air. With a flanged end you just multiply .82 times the radius to get the added length and .62 times the radius for unflanged. I guess its a little different for other cross section shapes, but the differences must be negligible because in WinISD they depend only on the area.

-Fishy