Thoughts on box design?

[Kukuraai]

Hey all, I drew up a pretty simple design for a vented enclosure for a 12" Polk subwoofer in my car, and before building it I wanted to run it through people with experience to see if anything immediately stands out as a problem. This is my first build so bear with me 

 

So the outside dimensions are 28" x 15" x 14", using .75" MDF. The net volume is 1.54 cu. ft., and the port is a 12.5" x 2" slot, tuned to 27.4Hz at 54.75" long (I'm probably going to extend the port length to get it to an even 27Hz, but that's gonna change the net volume slightly so I haven't bothered yet). The sub is a Polk Audio MM1240DVC, and the suggested ported box volume is 1.58 cu. ft.. I put in the values to Speaker Box for android and came up with a few graphs. 

 

Ignore the green line, the red is the ported box I designed and the blue is a 0.8 cu. ft. sealed enclosure, which is what I used to have the sub in. 

 

 

I'm not really familiar with phase response, does this all look good? My goal was to get a smoother (less punchy), deeper sound from my sub. The sealed enclosure I have sounds tight and clean, but I wasn't satisfied with how early it rolled off. I wanted to try and create a vented enclosure that was much deeper but still relatively responsive, and clean sound is just as important. 

 

Another thought was to keep the time delay as small / low (out of music frequencies) as possible. My personal goal was 20ms at 30Hz or less / lower, which is right where I ended up. But I've never had much experience actually listening to delay, will that even be noticeable? Only a few songs I have actually hit 30Hz, and at that it's pretty much just a sine wave in hip-hop so I wouldn't imagine so. And, any fast beat (rapid kick drum) I would think is like 40-60hz, which only has 11-4ms delay. But again, is that noticeable?

 

I guess the main thing I'm looking for is if there's anything with the design that'll give me poor sound quality

 

Also here's a rough sketch of the design itself, and a CAD half-drawing (also first time using Autodesk lmao).

On the final design I planned on putting angles in the corners of the vent to allow the air to travel more smoothly. 

 

http://imgur.com/yX1UZRy

 

[ckeeler11]

What model sub? How much power? what is excursion like?