But that isn't the only reason to use a sealed enclosure. Air trapped within the enclosure acts as a restoring force to the speaker commonly referred to as an "acoustic spring." This acoustic spring is formed as the speaker cone is set in motion, helping it return to its original rest position when the air inside is compressed or rarefied. In other words, it sucks the cone back as it moves forward and presses back as the cone moves inward. This is greatly affected by the distance that the cone moves - the longer the stroke, the greater the force exerted by the acoustic spring in the opposite direction. The restoring nature of a sealed enclosure improves transient response over other box designs and can be used to better the overall damping of the speaker. This is one of the reasons many industry aficionados often refer consumers to sealed enclosures for "sound quality."
Power handling of a sealed system can be directly linked to the acoustic spring. As power is increased, so is the load on the cone of the woofer, helping control movement as its excursion increases. However, it's important that the enclosure is of proper size or tuning in order to keep the speaker operating within its mechanical limits. For example, say you have two enclosures using the same woofer make and model, but one enclosure measures 1ft3 while the other 2ft3. Now apply the same amount of power to each woofer. If you could measure the distance the cone is moving, you'll notice the cone excursion of the woofer in the 2ft3 enclosure is further. This is because there is less restoring force to resist the cone's movement. With less restoring force, the woofer is likely to reach its mechanical limits much quicker than it would in the smaller enclosure, resulting in lower power handling. On the other hand, if the 1ft3 enclosure is too small for the driver, it won't perform to its maximum potential. There has to be a happy medium so that the speaker can maintain good power handling and offer a good sound performance.
This where the Q factor comes in. The main purpose of fitting a woofer in a particular enclosure volume is to shape its response characteristics. General alignments describing the shape are referenced by the Q factor or Qtc, which represents the degree of which all the circuits of the woofer/box combination relate to control resonance in the system. This is also an invaluable tool in determining the damping (the means of oppressing resonance) in a speaker system. An optimally damped speaker system yields a Qtc of 0.707, otherwise known as a Butterworth response. In this alignment, the bass system would have a desirable 90-degree phase angle, no peak in the response and gradual roll-off of -10.5dB/octave below its f3 point. Bass systems with this alignment tend to exhibit good transients (Read: Sound very detailed) while sustaining the woofers power ratings.