Port width help

[Inetba]

Trying to design a box, but first, the important bits:

Subwoofer: SP4 18. DVC 1 ohm.

Amplifier: m1d, soon to be two m1d strapped (3,400 watts)

Alternator: hairpin high output 320 amps.

Battery: XS power AGM.

What do I listen to?: Mostly music you would hear on the radio but I also looooooove decaf. Lowest song I listen to daily on the drive to work plays 18hz. Also, I love really peaky boxes.

Vehicle: Medium sized station wagon. Everything behind C pillar below window.

Maximum size of enclosure: 33" wide, 33" deep, 18" tall. This gives 4" of space between the hatch and the box. Sub facing up, port facing hatch.

I have access to round ports (absolutely tons of 6" PVC tubes laying around) and I'm not adverse to buying sonotube if that makes the design easier.

Now onto my actual question. How do you determine how wide to make your slotted port? Or even round ports?

And any design help is welcome. Thank you.

[Joe X]

Before laying out you have a target port area that you determined it will work for the application (say 50 square inches) then height x width needs to be that area so: width = port area / height.

[Inetba]

I can only seem to find formulas about total port area (including depth) and not just the perimeter of the port so I'm about to put in two 6" aero ports that are 2 feet long and call it a day.

[Triticum Agricolam]

So what exactly are you wanting to know? What's your target net volume and tuning for this box?

[Inetba]

I'm going to have to do multiple short messages because this web page keeps refreshing on me at random erasing everything I wrote so please bear with me.

[Inetba]

I would like to tune the sub in between the Fs of 25hz and not much higher than 30 hz.

I would like to have the sub in a box that's in the recommended specs. I know they say space makes bass so I would prefer to have it closer to the 8 cubic feet.​

[Inetba]

If you make your port longer, port area per foot goes up and tuning goes down. If you make your port wider port area per foot goes up but so does the tuning.

If you make an aero port longer, port area per foot goes up and tuning goes down. If you add more aero ports, port area per foot goes up but so does the tuning.

So how do you guys choos​​e how many aero ports or how wide to make your slot ports?

[Joe X]

Just forget "port area per cube" altogether, port area is chosen based on how fast air will move through the port, if it gets too fast your port will choke and you will lose output, choke it some more and you will have port noise, more port area will result in lower airspeed, less area in higher airspeed.

Get some modeling software like WinISD and check your peak airspeed to be below (ideally) 20 m/s for slot ports and maybe 30 m/s for flared round ports.

If you are doing a slot port just don't make it too thin, a 5:1 width to height ratio or less is better.

[Inetba]

I'm just gonna have to deal with some port noise below 30hz because I'm not making the port 15 x 15 x 60.

Either that or I'm doi​ng something WAY wrong.

[zxsonnyxz]

I think you're confusing "port area per foot" with volume rather than just the actual area of the port or something? Either way, you're sort of half way right, tuning changes the way you say but not for the reason mentioned.

Length really has nothing to do with port area and port area per foot. When you measure the width and height of your port you get the port area, right? And port area per foot is that area in relation to box volume, used as a general guidance of how big of a port is usually needed in certain size boxes etc. That in it self says nothing about tuning. The other part is right though, longer port = lower tuning, wider port (yes that's more area) = higher tuning. That's because tuning is a relation between port area, port lenght and box volume to put it simple. Play with some design software (WInISD) and you can easily change each variable to see what happends.

The easiest way I can explain is to take a say 2cuft box and an external 6" aeroport. The area of a 6" circle is then your port area and remains constand, and the box volume remains a constant 2cuft. But you can change just the port length alone: from say 5" that will give aroung 60hz tuning to 17" that will give 40hz tuning. Port area, port area per foot, and volume is still the same regardless. You could also change just volume or just the size of the port.

Is any of that understandable?

There's much more to it though so I'll advice you to read a about it on the forums, look through lots of box threads and build logs.

Now I'm tired of typing