Just found this good explanation from Steve/BlueWizard.
BUNGS -
When you place a speaker close to one or more boundary walls, you get a boost in the bass. But the bass is usually accompanied by timing errors in the midrange. In short, the excess bass reflecting off the wall muddies the midrange. You can bung/plug the port to soften the bass, and this will help clean up the midrange. Since the position boosts the bass, and plugging the port reduces the bass, it balances out.
I had the exact problem with some bass heavy JBL Stadium speakers. They did not like being 1 foot from the wall behind. Plugging the port did improve them, but these speakers really needed to be 18" minimum from the wall, and 24" would have been even better.
My new speakers, also rear ported, are quite happy to be 12" from the wall.
The port diameter and length are relative to the volume of the cabinet, much like the pipe on a pipe organ. In general, they are tuned to resonate that the resonance frequency of the bass driver. The resonance frequency of the bass driver is the point where all the resistive, capacitive, and inductive aspects of the speaker are at their peak. The effective impedance of the drive can be 10 time or more than the rated impedance of the speaker at the resonant point. This resonance typically occurs around 30hz.
A tuned ported cabinet is actually an anti-resonant chamber. Functionally, the resonance of the port/cabinet counters the resonance peak in the bass driver. Also, having a hole in the cabinet, reduces the spring tension of the air in the cabinet making it easier for the speaker to move and raising the efficiency.
The advantage is that the bass response is flatter to a lower frequency, but when it drops off, it drops off quickly. A seal cabinet, or acoustic suspension cabinet, will start dropping sooner, but will fade much slower.
In many cases, especially in bookshelf speakers, the port is in the rear simply because there isn't room on the front. In floorstanding, it is a question of appearance. Some people don't mind seeing the ports, some designers prefer to hid them.
Technically, it doesn't matter where the port is, it can be top, bottom, front, back, or side. A top port might seem a good idea, but if you throw a party and your guest set their drink on the speaker and spill them, it is going to run straight down in to the port. That and people will tend to use them for ashtrays.
Several speakers use bottom ports, few to none use side ports, none use top port, for most it is front or rear.
I personally feel most people completely underestimate how far forward they can have their speakers. In my case, I simply move my speakers forward until the front of the speaker is even with the front of the equipment cabinet, and that gives me 12" behind the speaker. Which for the floorstanding speakers I have now, works fine.
Steve/bluewizard
Simple question really. Why, if it then means that they are picky with positioning, are so many more expensive speakers rear ported?
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