Port Noise

[SkyAustin]

Ok, so I built a ported box for my Trans Am. The box sits in the T-Top area and extends to behind the back seat. I ended up with 3.67 cubic feet of air space. This box houses 2 p2d4-12's. I have a 7" long 4" diameter pvc port. The box sounds great and thumps until you get to lower frequencies around 35hz. This is where i start to get a lot of port noise. Box calculators say the box is tuned to around 31-33hz. This is where it gets weird. On Rockford's website they have a box adviser for my sub. It states for a single sub, I should use an 11.8" long 4" diameter pvc port and that would tune the box to 40hz. I input their specs into the calculator I used and it said the port should be 8.9" wtf. I have no idea how this ported stuff works since this is my first ported box. My question is how big does the port need to be to eliminate my port noise? I've experimented with different lengths of ports and its still there. Do I need to go bigger in the diameter department or is it something else? Here a link to Rockford site for my subs.http://www.rockfordfosgate.com/support/boxadvisor.aspx

[Triticum Agricolam]

So port tuning is based on the relationship between the port area (diameter of the port), port length, and box volume. If you make the port longer, decrease the port area, or increase the box volume it will tune the port lower. Unless your box is the same size as what Rockford recommends, you are going to need a different length port to tune the same.

So do you have a single 4" port for those two 12"s? Or do you have two 4" ports? Either way your port area is grossly undersized. Around the tuning frequency ALL the output comes out those ports, you want that air to flow as easily as possible. How much power are you running?

To fix your problem you are going to need more port area, either through larger ports or more of them. How much you will need to add depends on how much power you are running. The more port area you have the longer the ports need to be to keep tuning the same, so fitting longer ports may be difficult.

[Joe X]

A single 6" flared round port (like s PSP port) would work, you could make it partially external so that you lose less internal volume. Here's PSP calculator, it asks for net internal volume: http://www.psp-inc.com/tools2.html

[Skullz]

I think he made the mistake of doubling the box volume but forgetting to add a second port to the box, quite possible by adding a second port it should be close to what your after.

[SkyAustin]

Ok so what im getting from this is I need more port area either by adding more ports or making them longer.

Yes its a single 4" diameter 7" long port that is shared by both subs. Only running 880 watt amp to them at 1 ohm so about 440 split between the two.

[SkyAustin]

I think he made the mistake of doubling the box volume but forgetting to add a second port to the box, quite possible by adding a second port it should be close to what your after.

I calculated the port length on a different site using the 3.67ft^3 of the box. Is it a general rule of thumb to use as many ports as subs?

[SkyAustin]

So port tuning is based on the relationship between the port area (diameter of the port), port length, and box volume. If you make the port longer, decrease the port area, or increase the box volume it will tune the port lower. Unless your box is the same size as what Rockford recommends, you are going to need a different length port to tune the same.

So do you have a single 4" port for those two 12"s? Or do you have two 4" ports? Either way your port area is grossly undersized. Around the tuning frequency ALL the output comes out those ports, you want that air to flow as easily as possible. How much power are you running?

To fix your problem you are going to need more port area, either through larger ports or more of them. How much you will need to add depends on how much power you are running. The more port area you have the longer the ports need to be to keep tuning the same, so fitting longer ports may be difficult.

880 watt rms amp powering both subs. Single port shared by both subs. What size/length port do you recommend?

[Vice03]

Ok so what im getting from this is I need more port area either by adding more ports or making them longer.

Yes its a single 4" diameter 7" long port that is shared by both subs. Only running 880 watt amp to them at 1 ohm so about 440 split between the two.

More port area is needed to reduce port noise. Port length is to do with the tuning frequency of the port - if you add more port area, the port(s) will need to be longer to be the same frequency you have now. Just lengthening your current port will not reduce port noise - it would just lower the frequency.

[Jessica]

port area can be tough. You dont have much room in that car, 2 12's ported with enough port area will be a challenge iirc. Frame of reference, i am running a single 10 with a 4" aero port and i get some port noise at low frequencies. I would do like others have said, or maybe run 3 4" ports

[Triticum Agricolam]

880 watt rms amp powering both subs. Single port shared by both subs. What size/length port do you recommend?

Ideally you would want to have 35 or more sq in of port area. Three 4" ports would get you there, though they will need to be about 24" long to tune you to 33 Hz. I don't know if you will be able to fit that or not.

If you absolutely can't fit three 4" ports, two 4" ports 14" long will also tune you to about 33 Hz. You might still get some port noise down low and you will most likely still lose some output to port compression, but it will be a lot better than what you have going on right now.