Given a sub with *more* than one port, how is it that plugging one of the ports will make the sub reach lower? this seems counter intuitive to me. Anyone?

Not a designer of the sub, but... Plugging a port will cause higher pressure inside of the cabinet which should cause greater driver excursion (at the expense of losing clarity of the higher frequencies).

Edited... I take that back. Plugging a port would cause the driver to work harder to "work". It's a good question, I'd like to hear the answer.

I think the SVS cylinders offers this, maybe there website explains the purpose of plugging 1 or more ports.

On Outlaw's website, they show the measurements with both ports open and one port plugged, with the plugged one going noticeably lower. I checked SVS site and Outlaw's site, they didn't really give any info on it other than the numbers.

Let's do some math!

I found the following port-tuning formula on this page.



where

Lv is the length of your port in inches,
R is the inside radius of your vent tube,
Fb is the desired tuning frequency of your enclosure in Hertz,
Vb is the internal volume of your enclosure in cubic inches.

Solving the formula for Fb gives us



If the original sub design has two ports, plugging one port will only change one variable, R, effectively halving it. In this formula, you have an R^2 in the numerator and an R in the denominator. Without actually running the numbers, this relationship between the Rs should be enough to show you that halving the value of R will reduce the value of Fb, the desired tuning frequency.

Adrian, a ported sub has a lower tuned frequency than a sealed sub(other factors being equal)but rolls off at a rate of 24dB/octave below the tuned frequency while the sealed sub rolls off at 12dB/octave. Plugging one of two ports has the effect of moving the tuning part of the way toward being a sealed sub. The frequency where the sub begins to roll off is then actually higher, but because the roll-off is slower, the usable low frequency limit is extended. So, more output higher up with both ports open, but more extension with one plugged.

And thats exactly what happens. Same driver, same tuning same port lenght. The pink trace is a 1" smaller port.

Thanks for the input, Peter, John and Fred. That's interesting how much you can change the dynamics of the sub(or any ported speaker) by adjusting the port. So it would appear then, to generalize with all other parameters remaining the same...an "aggressively ported" box will have more bass support but drop off more quickly(not as low) and a more conservatively ported box would be a compromise between the two. Would it be safe to deduce that a sealed sub would go lower but on an even more gentle roll-off?

Well, that depends on the driver. In general, a ported design will go lower than a sealed design with the same driver.

However, some drivers will do very very well in a sealed design and with a lot of linear excursion can handle some very agressive eq to boost the low end.

It also depends what you believe about the audibility of distortion at low frequencies. Sealed subs tend to show more distortion the lower they go. EQing would further push up distortion.

Here is the same driver with two sealed models added. The one with the highest roll off is sealed with no boost. The blue line is with 6db of boost added at 25Hz. You can see that the ported designs outperfom sealed all around. This driver does best in a ported alignment though.

To follow on, a ported design is more efficient because the driver is not: a) fighting a sealed air mass to achieve its excursion, b) more efficiently coupling with the air in your room. For a given level of power you will get more loudness and/or extension out of a ported design.

On the other hand, the sealed mass acts as a natural damper and you don't have to worry about a rumble filter to keep the driver from unloading at tuning.