you're welcome man! And taylorfade, no problem! Basically, the larger the area of height and width, the higher the efficiency of lower frequencies due to coupling. Think of coupling as a "close enough to 1:1 as possible" in this case, even though coupling is more complex. And think of the first 1 as being the area of the port and the second 1 in the ratio as the area of your vehicle. Length not included yet. So, the closer to the area of the vehicle that the port opening is, the better it will sound as long as all other factors are correct.
The average vehicle is 3ft tall and 5ft wide.
Since it is unlikely you will make the port a 3ft x 5ft opening, the 1/8 of this can be utilized for "close enough" efficiency for the frequency range that the example 3x5ft area will allow.
So, take for instance you want the efficiency to be highest around 30-50Hz. Then a cross sectional of this should be for a full wavelength cutoff of the average of 30-50hz(which is 40Hz) is going to be 1130/40...giving us a wavelength of 28.25ft. This equates to a cross sectional area greater than 3x5ft because the cutoff of 3x5ft is around 140hz. The actual cross section needed for a 40hz efficiency is around 9 ft squared but 1/8 cross section needed for a 40hz efficiency is about 3.25ft squared. So, this is more usable in a vehicle....though still very large. You can get about 30% the efficiency higher than the average 5% by using a 1/16th cross sectional area of the cutoff wavelength.
i know the math here was not simplified but the rest is said about as simple as I can.
Do you see how making the port opening smaller equates to less efficiency, without me getting into the specifics too much?
The more the opening allows for lower frequencies to travel, the higher the efficiency.
Interestingly enough.....in conventional design, we all know that a smaller port opening, if all else is constant, will result in lower tuning, right? Well, this is where people do get confused. The reason is, I said all else is constant. So making the opening larger will lead to higher tuning and overall higher efficiency at higher frequencies....the exact opposite of what we want. But when other variables such as length and compression volume and others are changed, the efficiency changes for where in the frequency spectrum it is by pure physics. So, yes, you do have to accommodate port length in this concept, but by using 1/8th scales, we can get pretty compact. And 1/8 scales are for great efficiency, around the 30-40% range as a smaller design with smaller ports will be around a 2-10% range, less than 1/10th the efficiency....which is not as great.
I really hope that was understandable.
The cutoff basically allows higher efficiency much like the tuning of a ported box. But, unlike tuning, it is for a broader range of frequencies.