Bass is a wonderful thing, especially when you can feel it in your car. I'm not talking about bass that shakes the pictures off the wall, but solid bass that accentuates the music your listening to....Who am I kidding?! Most people want bass thats going to turn heads and crack the pavement right? I once built for a customer a ground pounding setup that consisted of three JL Audio 12W6 subwoofers powered by a 1200Wrms PPI amplifier and a horn loaded compression driver front stage powered by a 600wrms PPI amplifier. This system was tested at 146dB. Later on the owner wanted the car LOUDER and I ended up installing a single 18" Cerwin Vega Stroker that produced 156dB, which is louder than a fighter jet during take off. Just to give you an idea of how loud this car was, it physically hurt to be in this car at even half volume. Whilst it was loud and clear it wasn't enjoyable to listen to due to the physical discomfort...it literally blurred your vision. Imagine the hearing loss potential this thing had! A system that is built properly can be a delight to listen to during a drive home from work or school or a blast to have it cranked up at the beach or down the main drag. This is no easy feat to acheive, as the acoustics characterstics of the interior are a real bitch to work with. This is especially true with sedans, coupes and wagons, not so much with hatchbacks...go figure huh? The job of a subwoofer is to move air. Thats it. Nothing else. Just move air. The larger the surface area of the cone the more it air it will move and therefore will produce higher sound pressure levels (SPL) There are five different sized woofers that are commonly used to produce sub bass...8" 10" 12" 15" and the bad boy.....18". The size that suits you is determined by the size of your boot and the music you listen to. I don't recall the last time I used 8"s for bass...Only where space is a premium. eg. Porsche, MX5s and what not. More often then not it is either 10s or 12s being used. Both will play down to 20Hz (the threshold of human hearing on the lower end of the scale) but the 12" will move more air so generally is louder. It will really depend on what your musical preference is and how much space you can dedicate to the woofers enclosure. Once you have selected the woofer you have to decide what type of enclosure you want. A sealed enclosure is generally small in size and is the most acoustic in terms of sound quality. Since the air inside the enclosure acts as a cushion against bottoming out, a sealed enclosure has better power handling capability if you plan to use a high powered system. Ported enclosures are larger and more complex in design due to the port design involved. Port sizes and lengths have to be calculated in order to tune the enclosure. A properly ported enclosure will play lower then a sealed enclosure at the expense of the box being physically bigger. An attribute of ported enclosures is that they are fairly efficent and require less power than sealed enclosures but are great if you want loud, slamming sub bass. Another enclosure that isn't as popular as the sealed or ported enclosures it the band pass. A band pass has the actual subwoofer mounted INSIDE the box and the output of the woofer is directed out of a ported portion of an enclosure. This enclosure is designed to play LOUDLY within a specific frequency band that is generally narrower than most. e.g from 35Hz to 60Hz, but the frequencies outside of those areas decrease in amplitude very rapidly, and as a result, you can end up having a very booming box that isn't musical at all. Due to the narrow frequency response and the difficulty of designing and constructing a band pass properly, it's not a very desirable enclosure, but it's definetly the coolest looking one. Most bandpass enclosure have plexiglass fronts that allows the woofer to be seen or highlighted with creative lighting effects. All enclosures have a 3dB down point - the point where where a frequency is 3dB quiter than frequencies above this point. At this point, bass response begins to taper off considerably, and since every enclosure has this point be sure to bear in mind where you want that when beginning design of your enclosure. If you listen to rock the last thing you want is a bandpass box. If you are looking at building an all out SPL Monster and have the room to spare then go with multiples of 10s or 12s or a few 15s or 18s that will move massive amounts of air. Feel free to ask any questions.