Originally Posted by 60ndown
'In other words, a sealed setup will push the sub to its excursion limits much easier than a vented system"
i believe you, but it makes no sense to me?....in a sealed box driver moves forward-creates 'vacume' behind itself...forward motion is harder.....so surely with no box ...or a vented box the driver is able to reach its 'x max' more easily?? please help a struggleing not so new but vey interested in how and why car audion enthusiast...........
First off, forward motion of the cone in a sealed system is no harder than the rearward motion, its exactly the same. When the cone moves out, it creates a vacuum as you said, thereby restricting cone motion. But as it moves in, it compresses the air molecules, giving the same cusion effect with equal force. The amount of force it takes the push a specific sub to its excursion limit will be determined exclusively by the enclosure size. The bigger the box, the more air to compress (or create vacuum), there by giving less resistance.
Vented boxes are an entirely different animal. Many (if not most) people think a vented box simply allows the cone to move freely as the air from the back of the cone has an exit. But this is not true, the 'exit' (port) is designed in such a way that the amount of air the subwoofer moves cannot fully move in and out of the box without restriction. Simply put, the sub is capable of pumping more air faster than a properly designed port can accomodate in through-put. Follow me? Now another thing to consider is in vented systems its all frequency dependant. The closer your signal frequency moves toward the enclosure's tuning frequency (lower and lower), the more the cone is trying to move, and thus the more air the speaker is trying to push out the port, creating more and more back-pressure. This creates a situation where even as output is going up (reaching closer and closer to the boxes tuning), actual cone excursion is going down and down. Many people do not understand this phenomenon because they do not consider that a vented enclosure's efficiency is based on frequency. Closer and closer you move towards tuning gives more and more output even as cone motion is decreasing, BUT enclosure efficiency is rising, which explains the rise in over all output.
This also explains why it is easier to push a sub to its excursion limits in a sealed system rather than a vented one. A vented box actually has more restriction on cone motion over a given frequency spectrum than does its sealed counterpart (assuming a properly designed box). And considering that the lower the frequency the note is, the more air that's needed to be displaced to reproduce that note at a given decibel level. In a sealed box, the lower you go, the more excursion rises. Vented systems do just the opposite. If you try to reach maximum excursion at low freqs, at or near tuning, you will end up pumping so much wattage thru the sub that it will fail thermally before it reaches mechanical limits (overheat). And if you try to do it far from tuning where cone restriction is the least, by the time you get far enough away from tuning in the vented box so as excursion restriction from the port is gone, you've already gone well above the frequency range the sub is meant to play (or that your crossover will allow).