Sub Box Design - Graph of Curves

1.  Neither good nor bad.  In a real box-in-car installation, the actual curve will be increased in the positive region due to cabin gain.  The graph you are working with juat gives you a guideline so that you can compare different enclosure/port sizes.  You will want to try for a graph that shows ~30 Hz at or above -6 db if possible.

You should program a filter in on that project to cut off sublevel freqs at 20 Hz. to protect the sub from over-excursion.  Work with the project with that filter added.  Typical rumble filters are 24 db/octave, I think.  It would be best to actually search for the filter you will use and use its crossover order.  The amp may even have one.

2. The port will make the enclosure loudest at the tuned frequency.  You should try to get it as low as possible, and within one octave below Fs ( your case at or below 25 Hz (if that is the Fs) and above 13 Hz.)  Since freqs below 20 Hz are rumble and will be filtered out at a steep slope, you can tune anywhere around 25 Hz.

A good way to see how the WinISD program calculates the T/S parameters and decides on everything, is to just click on a few of the pre-programmed drivers that are listed, such as the Image Dynamics.  Click through all the dialogue boxes and look at parameters, volume, tuning freq, etc.  Then you'll see what the calculators are trying to achieve.  You'll also see that there are different allignments available for vented enclosures and have names like Quasi-Butterworth and Chebyshev.  Look at the parameters of the drivers the program chooses to put in the Chebyshev catagory, for instance. 

It's all related to Q.  I'm going to quote haemphyst here with what he wrote about Q a few months ago:

"Generally speaking, the lower the Q a system has, the thinner the bass will sound. A Q of .5 will sound very thin for most people, but it will offer the lowest overall cutoff frequency. It will have a higher F3, but due to the VERY shallow slope of the rolloff, the driver will go deeper in it's overall frequency response. It will have lower power handling, but you will have the absolute smoothest response with a minimum of resonant peaks, both output and impedance.

A Q of .707 is what most people call "max-flat". It will offer good power handling with reasonable rolloff and extension. It will do this also with a decent trade off in resonant peaks. It will usually sound pretty good.

A Q of 1.1 is a maximum power handling enclosure, and will sound like CRAP. Their response is VERY boomy, with many high level peaks in output AND impedance. They will not go very deep at all. It usually happens when an enclosure is too small for the driver."

haemphyst 11-22-04

The WinISD version that I'm familiar with is the newest alpha Pro version http://www.linearteam.dk/default.aspx?pageid=winisdpro and I am no WinISD pro myself.  But if you have an older version you may want to download this one.  They also have a message board for questions.

Many of the quality amps that are used for sub bass output have an active subsonic filter, usually variable from 20-50Hz.  If your amp has one set it to between 20-30Hz.  Subsonic filters can be built, too, but is fairly complicated as you would be building a passive type.  I'll list some links for you to study, but you should look for either an amp with subsonic filter or a separate filter if you intend to use a vented enclosure.  After the rumble is filtered out, you'll use the LPF on the amp to limit the high freq that the sub gets.  So, in effect, the sub is filtered at the low point  and crossed-over at the high point.  This high point crossover frequency is also the low point crossover freq for the mids and highs in the car.

1.  Use the info in your amp manual for what db/octave crossover slopes it uses.  Once you know what filters or crossover slopes you will be using, then you can input that into the program.  Find more from their help or help forum.

Here are some links for info in general:

http://www.users.bigpond.com/bcolliso/active-filter.htm

http://www.diysubwoofers.org/prt/ported4.htm

http://www.partsexpress.com/resources/crossover/xoverfaqs.html

http://www.users.bigpond.com/bcolliso/active-filter.htm

https://www.the12volt.com/caraudio/cross18db.asp

3.  How would the program know the details of your particular car or installation?  Remember, this program is also used for in-home woofer cabinets.

Other, please use the version I linked to.  As I said in the first post, choose one sub that is preloaded in the database.  I looked at the Image Dynamics IDQ12D4V.2 for reference as I wrote this.  Choose as a new project.  You get a dialogue box in the center of the screen.  Click through it from next to next....it chooses an allignment that is as smooth and close to .707 flat response as possible.  (Working with a vented project doesn't show allignment value, I think.  Working with a sealed box does...who knows?)

But, nevertheless, to compare one allignment to another....leave that graph on display and click "new project" again.  Choose the same sub again, and this time when the box displays "Super boom-box", change it to quasi-Butterworth instead.  Finish out the dialogue box and a new graph will appear with the first one.  You will see the difference in output:  one is a smoother rolloff, the other is a sharper bottom end rolloff but with extended low freq range. 

As I said, I am no guru with this program and you'll find other help through the 'winisd forum.  The links I gave you have a lot of info you can learn.  Remember that building a successful vented enclosure usually comes with a standard prerequisite of having first built successful sealed enclosures.  It's all about experience and knowledge, so learn as much as you can.  Here is one more article you will find useful:  http://www.ivhs.k12.il.us/~andrewt/veisd