Need sub box help

[monopoly9182]

this picture is a top view (port is on top of box)

Okay so About a week ago I decided I was going to build a sub box to fit next to the spare tire in my car. I planned it all out, got all the wood cut, built the box and put the sub in it to test it out. From inside the car the bass was decent but when it was turned up it sounded pretty bad. So I went to the trunk to listen to it and it sounded really bad no matter what the volume was. There's a lot of air noise and when it's turned up you can hear the sub making some type of thumping noise but not in a good way.
So I think I just did the port wrong. I did some math and found that the cubic ft was around 1.5, then I used a port calculator online and made a 3 inch circle port 7 inches deep. I just used pvc pipe for the port.
I'm not sure if I did any of this correctly this is my first time making a box and I don't have much experience with subwoofers anyhow.
So I came here to find out how I can fix it and make it sound better

The sub woofer is a Rockford fosgate p3d4-10 punch p3 1000 watt and I'm not sure what the amp is. I just got it from a friend he says its 1000 watt with 500rms.
I know this sub and amp work cause I've used them in another box before and it sounded fine

 

[BoredSysAdmin]

Hi monopoly9182 and welcome to AH. We are pretty much exclusively Home Audio/Video forums.
You would be probably better one seeking help for car audio design on a specialized auto forums

 

[lovinthehd]

FYI boxes are specific to drivers' T/S (Thiele/Small) parameters. So one box doesn't necessarily work well for two different drivers. Did you use any specific software to determine the match to your box/port to the driver? WinISD is popular and there are others....

 

[rojo]

@monopoly9182 I'm not used to modeling car subs and accounting for cabin gain, so pardon me if I mess this up. But I modeled your driver in a 1.5 cu. ft. box with a 3-inch dia, 7-inch length port. The vent air velocity is very high -- 32 meters / second at 100 watts. I've read as a rule of thumb, vent velocity needs to be less than 20m/s at 125 watts. You need a fatter, longer port to reduce the chuffing.

If you want to keep the same tuning, you need two 3" ports, each with 16 1/4" length. That would bring the velocity down to 16 meters / second at 100 watts, much more reasonable. If you'd rather do a single 4" vent, a length of 13.5" would give you the same tuning while bringing the velocity down to 18m/s @ 100w. Also consider flared ends to reduce the turbulence further.

You seem to have your box tuned to 37 Hz. That's very close to what Rockford Fosgate recommends -- except they suggest a 4" dia. vent rather than a 3. I'm sure they know what size enclosure and tuning makes their drivers sound best.