Using your program of choice, find a round port diameter that meets your desired frequency response, tuning frequency (Fb), length and port velocity. A slot port is always shorter than a round port so don’t worry too much about the round port length at this point. And I believe a slot port generally has less chuffing problems than a round port.
Convert your round port area to an equivalent slot port width and height. I assume 9 inches is your inside cabinet width and is fixed. So final height = (diameter squared) * pi / (4 * 9).
Go to this port calculator site: LINK. Enter your parameters, making sure the correct units are selected. Notice the default correction factor of 0.732 is for a round port. With that value in place the calculator’s port length should match the program you used in step 1.
Now change the correction factor to 2.227, the slot port value (see below). The length is now the slot port length. If you want, you can fine tune the value by subtracting the slot port volume from your box volume and re-calculating.
Lots of good info Ed. I’m not claiming that you are wrong but I’ve never read that a slot port is less likely to chuff than a round port with the same cross sectional area and length (same tuning). I’ve only built one project that I can remember with a slot port so I have next to no experience with this. To the contrary I remember reading somewhere that a typical round port has the best cross section area to boundry wall surface area ratio. Does that effect chuffing?
Probably depends on what you consider “equivalent” ports as far as which will be better for chuffing. Same cross sectional opening area? Same volume taken up inside the enclosure? Same port air velocity?
[edit] Not trying to gang up on ya. I can imagine scenarios where a slot could help with chuffing. Shorter length and odd shape might also help with port resonance?
Thanks guys.Chuffing sounds to me to be turbulence related. Flared ports help right? I plan to use a thumbnail roundover router bit at the port exit and a 1/2 inch roundover internally. Also I am changing it to 1.25x 7.5. by 7 long. I seem to remember reading somewhere that a ratio of 9 was the limit for slot ports.
Not accidental, I just decided to put more effort into the project, so as to minimize my potential regrets. I also intend to apply an 1/8 inch stucco coating over the MDF, because I have some, so its kinda free. Also as it is now the woofers are easily removeable and the back and bottom are easily removeable, so it is a good opportunity to experiment and document my findings. I have been measuring NF woofer, Terminus output and electrical impedance. The Tripath multiple taper can only be implemented in the rear portion of the cabinet, but it has It still has a large effect on the terminus output. Please stand by for squiggly lines.
