70% of your sound of your sub woofer is due to the design and build quality of your sub enclosure.  When picking a sub woofer box, deciding to either go sealed or ported will determine how your system will sound.

Sealed sub woofer boxes tend to be much smaller than ported enclosures, since they use the air inside the box as a spring for the enclosure.  If you don't have much space, it would be a good idea to go with a sealed box.  Not only is there a size difference, but there's also a difference in sound compared to a ported box.  The advantage of a sealed box is that the trapped air acts like a cushion, this air restricts movement of the woofer so that it is very unlikely the woofer will over exert itself in terms of cone travel.  That cushion also help with bass response time, i.e. listening to a rock song with a fast hard hitting bass drum will sound much more natural compared to a ported box.  Generally the larger the box gets the lower the tone of bass is achieved.  However if the box gets too large the "spring" effect that the air creates may be lost losing on loudness and crispness.  Sealed enclosures produce tight, accurate bass and have a flat frequency response curve.  They are the enclosure of choice when quality is the ultimate goal of the system set up.
                                                                                                      


Ported sub woofer boxes can get very large and have many varying factors that will determine their size and bass output.  For example the port has to be relative in size to the boxes volume depending on how much loudness vs quality is desired.  Ported enclosures create  louder bass than sealed enclosures, and allow you to tune the box to a specific frequency to determine how the bass will sound.  Higher tuning  on a ported box will get louder, but you will lose on sound quality.  Lower tuning will still get louder than a sealed box, and at the same time deliver good sound quality.  Generally this is a box for a set up where "boom" is the ultimate goal.  Port area plays a major role in these types of enclosures.  Too little a port area can result in port noise, which can severely detract from the sound quality.  Port design is also a huge factor.  Just like an intake or exhaust system, you want a smooth pathway for air to travel.  This allows air to travel more efficiently and drastically improves sound quality.  If you get a lot of turbulence when the air travels through the port all you'll get is a lot of distortion. So just drilling a hole with a hole saw in your sealed box is a very bad idea!  Make sure you do it right!  There is another disadvantage to running a ported box.  A ported box will suffer from a delay due to there being no air spring so music with very fast hitting bass will lose quality.  However, if your goal is to get loud bass and you don't care too much about the sound quality, a ported box, tuned high, with a large amount of port area is the way  to go.
                                                                     

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