Port, Alpine SWR 1521D

Hey guys, I just finished a nice sealed enclosure for my Alpine SWR 1521D, I have it wired to 4 ohms on my Alpine MRV 1507 (bridged). The enclosure turned out to be around 2.8 cubes after all displacement. It sounds real nice, but I would love to see what it can do ported. What would your recommendations be for port size and length? I would like to do 2 ports. The sub is facing the rear, but the ports would have to face the passengers. Your input is welcome.

Sub specs are:
Fs:23hz
Vas:98L
Qms:9.84
Qes:.4
Qts:.38
X max:16.5mm
X peak: 33mm

I have pics of the whole build if your interested.  Trying to get an answer on the port question too!

That's some real nice work, but you are going about it bass-ackwords.  It's kind of difficult to transform a sealed enclosure into a vented one after the fit and finish has already be done.  But if you're willing to pull it all out and start over it can be done, as you have plenty of room (it appears) to work with.

Those are minimal specs and I didn't do any research to find more info for you.  Just using those as the bare minimum in WinISD shows a vented box with 2.3 cubes net and a slot port 2 X 7 (or equivalent of 14 sq inches opening) X 28 inches long.  Yes, it wants a long port because the parameters are just over the line from sealed specs.  But the vented box will definitely provide much more deep bass extension than that large sealed is giving you.  You will need to get very, very accurate with the internal measurements to do this right.

Stevdart, maybe I'm doing something wrong. I downloaded WinISD and put in the above information and it gave me a box of 2.854 and if I tune it to 35hz I can get 2 4" ports 16.1" long or 2 3" ports 8.5" long.

Qts:.38

QMS: 9.84

Vas:98

Qes:.4

Fs:23

SPL:98

Re:4ohm   its a dual 2 ohm, but wired to a single 4ohm load

Pe: 500wt rms  1000 max

Le:3.72

Xmax:16.5    or 33max

Sd:754.77

Maybe I'm doing something wrong.

John

I agree with Jeff on the above comment.

John64, be sure to read the WinISD guide stickied above once or twice so that you can become more familiar with using the program.  There are several things in there that don't need to be repeated here.

Look over the following two file pics.  The first shows frequency response for the 2.8 cu ft 35Hz Fb box you stated that WinISD gave you, which is the blue line.  The other, the yellow line, is a 2.3 cu ft box tuned to 23 Hz.  You'll also see port noise for the two alignments.  If you will follow up later and also look at cone excursion for the two, you will see that the "yellow" is safe, but the "blue" is unsafe without a highpass filter.

alpine15response.jpg

alpine15noise.jpg

As you are working on this project, bear in mind that whatever you add to the inside of the enclosure will count as displacement against the airspace.  If there truly is 2.8 cu ft net now, the port structure that you add will take away from that space.  Now, this box will need a somewhat sizeable port and you will have to be careful in how big you let it get.  In a series of trade-offs, you want the best natural reponse you can get with the least amount of port structure.  The port is very short in the "blue" box (at 7"), but the response is not acceptable, nor is the port noise, as you will begin to understand as you look at the graph I provided.  The "yellow" has a longish 24" long, 4" round port but the displacement is workable with your existing box and the port noise is acceptable.  You will also find that if you use two ports, each port will be twice the length of the single port...which will add too much to displacement.  I suggest one port about 4" round for this box given the driver receives up to 500 watts RMS.

As you will read in the guide, you can make changes to your graph parameters.  For port noise, you want it to read feet/second (in the "rear port air velocity" chart).  Make the top end of your graph 140 ft/sec and the bottom end 0.  Right-click on any graph to get the "options" dialogue box (shown in one of the pics I provided).  Click on the parameters you want to adust.  For port noise, you don't want it to go higher than 110 ft/sec for a car audio sub.  That's 1/10 Mach.  Ideal is half or less than that, but that is where the never-ending trade-offs come into play.  Increasing the port opening and, thus, the volume will lower the port noise but will result in more displacement of air taken away from your enclosure.

You have some things to work with and it seems you're on your way, so good luck.