cliff design cd120c

I used the specs Qes, Qms, Fs, Vas and Xmax and came up with enough information to work up an enclosure.  But my OS (Microsoft, of course) crashed when I had WinISD up and was taking a picture of the response.  I'll see if I can bring it all up again tomorrow sometime.  But if I recall:

Use a box with net airspace of 1.5 cu ft...that's taking out the volume of the port and bracing, etc.  Build it vented with a PVC round 4" port that is 15.4" long.  You might have to elbow it inside the box, or you can use the equivalent 12.56 square inches of opening area and build it as a slotted port.  Tuning is 35 Hz.  This will give you about the "best of both" that you're looking for.

I suggest to stay away from the larger 3 cu ft box that has been suggested to you, and keep the tuning freq about where I suggested at 35 Hz.  There are help calculators directly to the left side of this page under "woofer calculators" to give you some more help.

You can build this box a variety of ways by changing size and tuning.  For maximum SPL (but no sound quality), you can build the box as described to you by the other.  The 3 cu ft with the 6" X 8" round port.  It will be loud, but you won't want to listen to it every day.  That's a comp box for SPL.  Tuning is at 45 Hz.

For SPL with the smaller box size, you'll give up some competition db's but can still be terribly loud.  Sound quality is diminished when you go for SPL...but you could build at 1.5 cu ft and port with a 6" round X 20" port. Tuning here is also at 45 Hz but the smaller box size decreases the db peak level from +6 to +3.

For everyday listening and still plenty loud, use what I described in the post above.

Just for clarity, because I told you two different port sizes of 4" and 6", decide whether you want a competition sub or whether you want a loud sub that you can enjoy listening to on a daily basis.  That will determine whether you go with a 6" (comp) port or a 4" (daily) port diameter.  There are two reasons for these different port diameters:  with the louder SPL you'll need the larger port opening to avoid too much port noise.  And, when you tune lower, the port length is longer.  A large 6" port would be too long to deal with, so a 4" size gets you by since you're not going for those extra db's anyway.

If you choose comp, decide how much space you can give up for the box.

If you choose daily, use the 4" with the ability to experiment a bit with the tuning.  Since it's a long enough port that you will have to put an elbow in it, you can make the first part of it about 8" long (tuning at 45 Hz), with the rest of it incl. the elbow the balance of the 15.4", or 7.4" (tuning at 35 Hz).  Listen to it with just the 8" straight port, then put on the elbow, secure with duct tape, and listen to it again.  You'll be experimenting with two entirely different sounding boxes but both in the same box.  If you decide you like the 35 Hz tuning best (longer port), glue the elbow in place and you can just enjoy your system from here on out.

Here's the deal, jeffliberty.  Rattling is caused by subpar car preparation, ie, not enough damping.  It's not the lowest tones that make these rattles but the ones closer to 50 Hz which corresponds with the vehicle frequencies.  That's why the higher tuning is used in SPL.

You don't want rattles.

Lower tuning equates with better music, at least in most cases.  This sub isn't going to go into home audio lows, but it will get low enough for the small environment of a car (where lows are better amplified).  So as I said above, ask yourself those questions.  Are you going to compete, actually in competitions?  And this is built for those?  No, I didn't think so.  You just want what everybody wants, a loud slamming sub system.  So do the 1.5 cu ft, 4" X 15.4" port, and experiment with the two tuning freqs like I suggested. 

The higher tuning freq at 45 Hz (8" port length) will cause more rattling than the lower 35 Hz (full 15.4"length) tuning.  But the lower tuning will make all your music sound better. 

ok when i build this box i would like the sub to face upwards instead of towards the back of the truck will that sound bad and another question should i have the port exiting towards the front of the truck or towards the back

the reason im asking is cause i have a 60/40 rear split seat so im not mounting the box in permanently all im gonna do is lift the seat up and put the box in and when i need more room ill just unplug the sub and fold the seat back down

The sag calculator confirms it is just fine for up or down firing.  I recommend that you place the port on the same baffle as the woofer, that is, on the face baffle.  Keep the other end of the port... the end on the inside of the box... free and clear by at least 4" of any obstructions.

(I edited out the last part.  I forgot you had a truck.)