I got sprung from the hospital about 90 minutes ago.
I think I have to give you a little tutorial.
I have to assume you know at least something about simple harmonic waves, resonance, and the propagation of sound. I also have to assume you have some acquaintance with the physics of the Helmholtz resonators and pipes.
Now a ported speaker enclosure is a Helmholtz resonator. The box has a resonance Fb. This is determined by the box volume and the dimensions of the port.
The speaker driver has two resonances. There is the mechanical resonance Qms determined by the weight of the cone and and the stiffness off the suspension. Now the Q of a resonances specifies the width of the resonance. The shape of the knee if you like.
Since the driver is an inductor it has electrical resonance specified by Qes.
Now there is a total resonance defined by Qts. These parameters determine the resonance of a speaker driver in free air, Fs. This pretty much sets the lowest frequency possible from the driver. For most deigns especially sealed it will be significantly above this frequency.
Now there is another very important driver parameter, that is the equivalent volume Vas. This is the volume of enclosed air at standard atmospheric pressure that has the same compliance, springyness, as the driver. It IS NOT the optimal box volume.
Now the driver parameters, and box parameters define the total performance of the system, which is defined by the point at which the system response is down 3 db form the average system level. This is the F3. The total system resonance is defined by the total Q which is Qtc, and in a ported system losses around the box resonant frequency Fb, this is QL.
I now want you to understand the specifications of the ported mid Qt insert ported sub.
Please download and print the pdf. in this post.
http://forums.audioholics.com/forums/showpost.php?p=408584&postcount=59
The Fs of the driver is 22.9 Hz. The Fb is 18.6 Hz. The tuning volume for for this is Vb is 4.63 cu. ft, and making allowance for driver volume and port we get a total enclosure volume of 5.79 cu.ft.
This enclosure is tuned by a slot vent, or port, that is 2.5 in X 12.5 inches and is 50.19 inches long. This long vent has to be built into the cabinet. The weight of air and its resistance is the weight on the spring so to speak determining Fb.
Now there is one aspect of this port you need to understand. The slot vent is in effect an open pipe. The resonant frequency of this pipe is the speed of sound divided by twice the vent length. This is 132 Hz. This fundamental, and the even harmonics of this fundamental have to be suppressed. You can do this by making sure that the crossover has fourth order, 24 db/octave slope, beginning at no higher than 66 Hz. Or else you need to make sure your plate amp has parametric Eq so you can notch out 132 Hz. The latter is best.
Now lets go through the graphs.
The first graph shows the relationship between frequency and output in db. Note that there is a 3db peak at 35 Hz. This is very good. The 3db point, F3, is about 18 Hz. Now I have played with QL to optimize the flatness of the response and at the same time extend it to 20 Hz.
The second graph shows the absolute db level possible with frequency for this system. It is 109 to 112 db through the sub operating range. This is plenty.
The third graph plots the absolute spl. versus frequency.
The fourth graph the absolute power input versus frequency.
The fourth graph is very important to understand and plots cone displacement against frequency. Note the maximum linear cone travel of this driver, xmax, is 16.75 mm. Please note that there is a cone displacement of 14.5 mm at around 30 Hz, at full power. Now note as the box tuning kicks in, the pressure really builds as port output becomes dominant and cone displacement drops to 6 mm at 20 Hz. Below system resonance the driver decouples and cone displacement goes off the clock and exceeds xmax by 15 Hz. This is wasted effort, and although the cone movement looks dramatic it is low spl highly distorted output. This graph shows why with a ported enclosure, a subsonic filter is a good idea.
The fifth graph is also important. It shows the air vent speed with frequency.
The maximum air velocity is 12 m/sec at 15 Hz. This is excellent and will not cause chuffing or port compression.
The sixth graph shows the impedance plotted against frequency and shows the classic double humped curve of a well aligned ported enclosure.
The final and seventh graph shows the phase response. You can see as the port takes over there is a maximum phase shift of around 130 degrees.
There is not enough space allowed on this site to post the group delay plot. They only allow 100 KB. It would be nice if those of us who get requests for this kind of help could have a slightly more generous allowance. There is significant group delay as the output transfers to the port, but nothing that is not typical for a ported sub.
So we are trusting the manufacturer, Infinity, to post reliable Thiel/Small parameters, and to build consistent drivers.
You are trusting a retired physician, who some members vow should be in the nursing home, to have optimally modeled this design. So you will have to take this design with some degree of trust. That will be your call.
