Originally Posted by Mik James
It would be nice to know some examples of what effect diffferent sub frequencies at different sound levels generally have in a 150 sq' room.
Mik, it can't be said what kind of effects different freq's will have in a room. Each room is different. Things like furniture, drapes, carpeting, windows, material the floors and walls are made out of, the shape and volume of a room, etc all play a role in effecting sound waves and how they are absorbed or reflected. Also, every thing on this planet has what's called a resonant frequency. This is the natural frequency at which an object resonates, or vibrates, at. Look up the Tacoma Narrows Bridge for a good example. The bridge did this due to the wind traveling over, and under, it at just the right speed. In this case, think of blowing across the opening of a bottle. Except in our case, things will do this at very specific frequencies. I could make my cat vibrate apart if I played the right frequency loud enough. All of these variables effect the acoustics of the listening environment.
Back in the 90's when I worked for a local car audio chain, JL was just cutting their teeth in the industry. Nobody had really heard of them. I bought 4 12w1-8's and put them in the hatch of my Ford Escort in 1.25ft3 sealed enclosures and ran them bridged to 2 ohms with about 500w RMS. After those, only 1 sub I had ever came close to performing as well as the JL's did (Alpine ZR's). The W0 series is what replaced the original W1's. Back then, there were no W0's. Price was about $100 each and they handled 100 or 125w RMS each. If JL still makes subs like they used to back then, even the bottom of the line W0's won't let down. (As a side note, the W7's are what the W6's used to be, top of the line.)
What you get with the more expensive models, like the W7's (and most drivers really), is cast baskets (which won't flex under high power and stress) vs the W0's stamped baskets. Also higher Xmax, (how far a driver can travel in one direction before exceeding it's mechanical limits), larger voice coils to handle higher power, etc. In short, the ability to handle gobs more power. You don't want to run a sub rated for 500+w rms with only 100w rms. Bad things can happen and cause damage to the driver, amp or both. It's actually better to run a sub with more power than it's rated than less.
With a sealed enclosure, aka acoustic suspension, the enclosure is more forgiving when it comes to errors or compromises on internal volume. A ported enclosure, aka bass reflex, must be made to pretty close manufacturer recommendations. In sealed, the air in the box acts to damp and control the driver. Kind of like a spring on a cars suspension. It helps to keep the driver from exceeding xmax and tearing itself apart. In a ported box, you don't have that "air spring" because the box isn't sealed. This is where the term "unloading" that RLJ mentioned comes from. If the box size and port aren't carefully calculated and made, the air in the box no longer acts as a spring to help control the driver. At this point, Xmax becomes exceeded and the driver destroys itself. This is why the box and port size are critical.
On the subject of using a sonotube and the wall thickness. The thickness isn't as critical with a tube as it is with a regular box shaped enclosure. When we put a driver in a box type shape, there are 6 surfaces for the sound waves to reflect off of inside. Remember that bit above about resonant frequencies of an object? Well that comes into play here along with something called standing waves. The super thick (or thicker) 1 inch panels you see standard enclosures made from are that thick for a reason. To keep enclosure resonances down, which have a tendency to make things sound not so great. With a tube, we don't have all of those flat surfaces for the waves to reflect off of, thus not nearly as great a chance for the enclosure to resonate. The exceptions being the baffle where the driver is mounted, and the opposite end panel.
If I'm wrong in any of this in the slightest way, please, someone correct me. It's been a few years since I've built a sub and my knowledge may be a bit rusty.
Mik, what is the freq your mains play down to effectively without struggling? This point and lower is where I would focus. Me for example, my mains (Infinity IL40's) do a great job down to between 30-35Hz. I've been thinking about building a sub and focusing strictly on the 15/20-35Hz region. Some people are obsessed and insist on getting down to 10Hz or lower. This will cost some big $$$$ usually. If this becomes an obsession for you (as it does for most of us), it's inherent that you start reading and learning. Things only get more difficult, confusing, and expensive from here on out.