Vas question

[3-LockBox]

How is it that a woofer, say a 12" woofer, with an FS of 21hz, a Qts of .25, and a Vas of 10.21^3', can have an optimized sealed cab of only 1.65^3', but a ported cab (tuned to FS), has to be nearly twice as big. I would have thought it would be the other way around.

Also, if the Vas requirement of two speakers is additive, does that hold true for other parameters, such as the Q factors? (I'm referring to designs employing more than one woofer in a cabinet).

[madinoz]

My understanding is that the sealed box requires a smaller box so that the constraints of the enclosure act as a resistance (mechanical) against the woofer. This obviously increases as frequency decreases (lower freq= more movement=increased resistance). The internal air acts as a spring (a very tight one mind-you which is one the reasons for the perceived increase in bass clarity of sealed enclosures).
Ported boxes have a number of alignments which are commonly used but they are (generally) optimised to get maximal bass extension (though I know there are many theories about how to make ported boxed sound better and a lot of them revolve around tweaking the box / port relationship). Generally a ported box with a smaller enclosure will have a high peak in the bass response so this would mostly be undesirable.
There is no obligation to use the manufacturer's recommended enclosure sizes but they are a good choice for the novice.
That's my 2 bobs worth anyway!

[Svante]

Originally posted by 3-LockBox
How is it that a woofer, say a 12" woofer, with an FS of 21hz, a Qts of .25, and a Vas of 10.21^3', can have an optimized sealed cab of only 1.65^3', but a ported cab (tuned to FS), has to be nearly twice as big. I would have thought it would be the other way around.

Also, if the Vas requirement of two speakers is additive, does that hold true for other parameters, such as the Q factors? (I'm referring to designs employing more than one woofer in a cabinet).

Well, the question which box type that requires the smallest volume is not an easy one to answer. It depends on a number of things.

It might be that some software suggests a twice as big box for a ported box, but that is probably based on that the particular driver is the same. I find it more interesting to specify properties of the system that should be the same, such as low cut off frequency, distortion or maximum output level and allow different drivers.

Doing so, a vented box can typically be smaller.

[richie00boy]

Vas is the only common parameter that adds.

[sreten]

Originally posted by 3-LockBox

I would have thought it would be the other way around.

Hi,

Why ?

The smaller cabinet raises the Q to around 0.7, venting this box
will give you a peak. The sealed box Q for venting is 0.5 to 0.6
which means you need a bigger box, typically as you say double.

/sreten.