As always, the more I learn about building speakers the more I realize I don't know. I am building an Adire alignment vented Tempest enclosure. I received the two 3" flared ports via UPS yesterday. Included in the box was the port instruction sheet that included the formula for calculating port length. As I read through it I discoverd that when using a flared port like the ones supplied by Adire or Madisound the finished port should have an overall length ONE INCH LONGER than calculated. I wonder if this would account for the tuning error in the 281 speakers tested by Joe DAppalito? In the Tempest application sheets, Adire says that when using their 3" flared ports the center tube should be 11" long. I am fairly sure that they are taking this extra inch into account. However, in the original 281 instruction, I am thinking they listed the port length without regard to which port is being used. I know that there are other builders who have made ports in the past. Ultimately, I have observed that calculated port lengths are "RARELY" exact. Depending on driver variation from spec they can be off quite a bit. Impedence sweeps or direct observation with a tone generator is a good sanity check if you have the equipment to do it. You may well be surprised at what you find.