Dave:

There are several programs here that predict response in both closed and ported boxes:

http://www.wssh.net/~wattsup/audio/
There is also WinISD which I don't use but many others do, and Unibox has drawn good reviews on this forum.

Here is a brief overview of the sealed box situation. Most unenclosed subwoofer speakers have have Qts of .4 or below. Let us use an example of a speaker with a Qts of .4 and an Fs of 23 Hz.

Every speaker has a Vas. That is the volume of the enclosure where the springiness of the air is equal to the springiness of the speaker's own suspension-when you press a speaker's cone down, it bounces back. Fs, Qts and Vas are your key numbers-sometimes Qts is broken up into it's key components, Qes and Qms, by some programs. But we can use Qts for our calculations.

As I am sure you know, SPL is the dB that a speaker plays at throughout most of it's range. A typical number would be "88 dB @1Meter/1Watt". Meaning that at a distance of 1 Meter, 1 Watt of power will drive the speaker to a volume of 88 dB throughout most of the speaker's frequency range. This number is called the "midpoint" of the speaker. We often consider the lowest frequency the speaker can adequately play-the low frequency "cutoff"-as the place where it is (-3 dB) from the "midpoint". In this case, that would be 85 dB.

Okay, so our key numbers-our "parameters"- for our imaginary 12 inch subwoofer are as follows:

Vas = 4 cu. ft

Qts = .4

Fs = 23 Hz

SPL = 88 dB @ 1Meter/1Watt

Suppose we want to put this speaker in a 3 cubic foot box. Then Vb = 3 cu. ft.

Let's see what happens.

When you put an unenclosed speaker in a sealed box, you raise both the Fs and the Qts. The new numbers, Fc and Qtc, (What Fs and Qts are converted to when placed in a closed box) are raised according to the following formula:

Fc = {square root of: [(Vas/Vb) + 1]} X Fs

Qtc = {square root of: [(Vas/Vb) + 1]} X Qts

In our 3 cubic foot box, this yields the following numbers:

Fc = {square root of: [(4/3) + 1]} X 23 means Fc equals 35 Hz

Qtc = {square root of: [(4/3) + 1]} X .4 means Qtc equals .61, or .6.

Plug these numbers into any speaker software, and you will see that at Fc, the speaker is about 4.4 dB down from the midpoint. That means that F3 will be somewhat above Fc, which is 35 Hz.

Your Qtc is going to determine how far below, or above, the "midpoint" your speaker is when it hits Fc.

Let's take this same speaker and put it into a 2 cubic foot box. That means our Vb equals 2 cu. ft.

Using the same formulas as above:

Fc = {square root of: [(Vas/Vb) + 1]} X Fs

Qtc = {square root of: [(Vas/Vb) + 1]} X Qts

We get:

Fc = {square root of: [(4/2) + 1]} X 23 means Fc = 39.8 Hz

Qtc = {square root of: [(4/2) + 1]} X .4 means Qtc = .69

Taking a look at our graph in any program, we find that at Fc, our speaker is down 3.2 dB from our midpoint. So F3 is right around Fc.-when our Qtc is around .69.

I am trying not to fill you up with too many formulas. I just want to give you some idea what to expect when you start plugging numbers into a speaker program, to save you a lot of confusion when you see the results.

To know just how far Fc will be below, (or above), the midpoint, we have the following formula:

20 Log Qtc.

Therefore:

If our Qtc is .5, we will be -6 dB from the midpoint at Fc.

If our Qtc is .7, we will be -3.1 dB from the midpoint at Fc.

If our Qtc is 1.0, we will be at the midpoint exactly.

If our Qtc is 1.25, we will be +1.93 dB from the midpoint.

To briefly summarize:

For any given speaker, the smaller the box, the higher the Fc and Qtc.

The higher the Qtc, the higher the Fc is compared to the midpoint.

One more thing. The lower the Qtc, the better the transient response-less "hangover". For many years, in a sealed box, builders used to go for a Qtc between .7 and 1.0. Above 1.0, and the bass becomes sloppy. Below .7, and the response droops. However, some builders have begun to build their subwoofers with a Qtc of .5. It has the droopiest response, but they make up for it by running a big amplifier. It does have the advantage of the best transient response.

Sorry for the length of the post. I just wanted to give you a general guideline.

Good luck!