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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

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.

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.

71buickfreak wrote: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

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.

A slot port and a duct port are generally considered the same. Where you might be getting confused is Slot loading, which is the act of firing the woofer side of a sealed enclosure into a 3-sided slot to gain SPL. this is how most Q-Logic Q-Customs are loaded. they down fire into the floor and the 3 sides around the woofer force the air moved by the woofer out the slot, in most cases resulting in a 3-db gain vs. an otherwise unobstructed path.

I thought a "slot" port and a "rectangular" port were two different beasts with a slot port using an enclosure wall at one or more sides of its internal termination. Because more air is forced to vibrate along with the rest of the collumn of air the effective mass is increased resulting in a slightly lower tuning frequency as a regular rectangular jobby of equal length.

Here's a much better calculator that does both types:

http://www.carstereo.com/help/Articles.cfm?id=31

JL Audio refers to them as "slot ports" in some of their driver specific recommended enclosures sizes:

http://www.jlaudio.com/subwoofers/pdfs/W6v2_MAN.pdf

and as "ducts" in one of their tutorials:

http://www.jlaudio.com/tutorials/ports/index.html

The inconsistent nomenclature sure had me confused for a while and I'm still not sure I got it right. Maybe all ducted ports can be considered "slotted" but not all slot ports are ducts.



-Fishy

Round ports are nice when you are short on space, but yeah, most perfer slot ports. Really though, both have their advantages and disadvantages, both work well if done right.

The only round ports Id use are precision aero ports. Other than that slot ported is the way to go IMO.

Not really circle, but cylindrical ports. They are most common on prefab boxes.