Sub Box Port question

[mike49504]

Im building a box for a couple 12's and i need a 30" square opening 21 1/2" deep.

Does anyone know what need if i did pvc instead of slot ports.

If i use 3" PVC pipe how many would i need and how long do i need to make them?

Any help would greatly be appericated.

[jacampb2]

We need more information than that. We need net enclosure volume, and desired tuning frequency. Give me that and I can give you an idea of what you need for round ports.

If you don't know tuning frequency, and I'm guessing you are going by specs for a box that the manufacturer recommends, then just tell me the net volume of the recommended box and I'll back calculate it.

I just got off midnight shift, so it will be tonight sometime before I can help if no one else can do it before that.

Later,
Jason

[spooney]

I wouldn't use 3 inch pvc pipe for ports for a pair of 12's . I honestly wouldn't use pvc at all if I could avoid it. You will have to chamfer/ round over the ends of those ports pretty good to avoid port noise. I would look into those precision port kits they sell through parts express. Two four inch versions of those trimmed to the appropriate length should work well for most people. You can use less port area with round ports because they are more efficient with the air flow.

[Perry Babin]

With enough port area, the port velocity should be low enough that there would be no noise?

If you want to leave the port tuned to the same frequency, leave the port length the same and use round ports that have the same total area as the rectangular port.

Although I may be wrong, I don't think you can use less port area with round ports. The port is tuned by the mass of the air in the port and the volume of air in the enclosure. Why would the shape of the port make a difference?

[jacampb2]

Yeah, the port shape does not have a huge impact on anything, it is the cross sectional area of the vent that is important. As long as the port air velocity is low enough, there is not going to be audible port noise. Obviously you should take reasonable care to have a smooth uniform surface inside the vent, but you shouldn't need flares and chamfers unless your port velocity is approaching the audible range.

Port area can be reduced, but the port length will be shorter for the same tuning frequency. If port area is reduced, you need to take care that your port velocity does not get to high, but in my experience you can work with a lot less port area than most typical manufacturer recommendations and still achieve good results.

BTW, you cannot just double area and leave the length the same when you add a second driver. If your manufacturers specs say 15x15x21.5 for one driver, then you cannot just double the area to 30x30x21.5 when you add a second driver-- it doesn't work this way.

Later,
Jason