Rear Center Console

In a 'loaded' system, the throat is physically as close as possible to the driver. That second 'ring' around the opening is for the flexible surround to operate within. The diameter of the opening is roughly the radius of the driver.
In the larger enclosures there is even something called a 'phase plug' which further restricts air movement and loads the driver even more. The depth of the acoustic passage is usally equal to the radius of the throat and flairs out in width first then in both directions. The design does not allow for reflected sound to get back to the driver. The ports cause the sound created by the rearward movement of the cone to have to travel twice the distance as the original sound and enter the horn behind the original sound further reinforcing it.
The reason you go to all the trouble of eliminating hard corners is to minimize the reflected sound within the 'air stream'
The other reason you create one large horn is to emphasize the low frequencies, the higher ones self cancel against the walls of the horn.
Does that help?
Andrew