Was out today at the store I get all my woodworking stuff and while there I ran into a guy that made speaker cabinets for high end home audio customers.
We got to talking about different cabinet designs as to tl, horn, bandpass and what not.
Anyways, I had bought up the question about car audio sub enclosure, but more importantly I asked if he knew about ported enclosure's that use T type ports and the differences in how the port configuration effects the actual port tuning given which method as to the part of T that branches off to the sides.
He told me that the part that branches off can done done in either fashion:\
Maintain same port area and divide the left over centerline length and make those branches that length.
OR
Make the branches half the width of the port and what ever the left over centerline length is, both branches are that same length.
Then I asked the question of the port tuning based on how it what calculated ( program ).
He told me that for the type of port we're talking about the port tuning would not be what was calculated since this type of port doesn't create a resonance frequency in the same manner a " ported type cabinet " and the only true manner to determine what the " true " tuning frequency of the port is would be to build/test ( measure ) the acoustic response and change the port as such to find that tuning point.
I asked him then if he were " speculate " what the port tuning of such a port was and he said it would more than likely be lower than what was to be expected, but, the port tuning would not be what was calculated for the enclosure.
"He told me that for the type of port we're talking about the port tuning would not be what was calculated since this type of port doesn't create a resonance frequency in the same manner a " ported type cabinet " and the only true manner to determine what the " true " tuning frequency of the port is would be to build/test ( measure ) the acoustic response and change the port as such to find that tuning point. "
Hey, I didn't know either. Given what has been discussed on other forums about this type of port and having been used in sub boxes that the port was either done in one manner ( half the port area, branch lengths are what ever the left over length divided )which is not true given what he said.
But, since this T type of port given which method used to make/create the final branch lengths doesn't act in the same manner acoustically as to what we think is the port tuning frequency.
The only thing I didn't say in the post is what he also told me when i asked about how to model/calculate such a port as to it tuning.
He said a person with the skill and knowledge of fluid dynamics to model such an enclosure with such a port design to model it and measure it. There in no enclosure ( design program ) that can do this and he said forget trying to do the math thing. Too many variables to account for - ergo - a program that can model such and provide accurate data.
A slot port will never have an exact tuning. They have a range of tuning due to the way they like to load. It wont be always lower than what it is on paper, it could be higher also. But once the enclosure is in a room or a car, the tuning will be different.
Aero ports have an exact tuning though in a perfect world. But the same factor applies for it, in a room or car it will change.
With slot-vents, the wall of the box makes up part of the port. Tuning is usually lower than 'on paper'. Also if the port opening (internally) is close to a corner or back wall of the box, tuning will also be lower.
Loading effects of the cabin can GREATLY affect performance. I have seen boxes "tuned" to 40hz+ SLAM on the lows in some vehicles, but suck in others. Longer vehicles tend to like the lows
Some people say that aero-ports create more of a precise tuning. This could account for their ability to yield higher SPL in many situations.
Tuning will also change with temp and humidity. IIRC, higher humidity can create a lower tuning. Sometimes 1-2hz or even more.
