Amp question (Soundstream Class A 10.2)

Sealed box calculator seems odd for that driver :/

RE is probably suggesting the ported box for a few reasons. Probably the loudest results will be vented. Also vented boxes give the motor better cooling when driven hard for long periods (fewer warranty claims). That doesn't mean it won't work in a SQ box. It can be loosely calculated which subs are better suited to sealed or vented, based on Efficiency Bandwidth Product (EBP). A score below 50 favors a sealed enclosure, above 100 indicates vented is best, between 50 & 100 means the driver is flexible, ok to use in either type. The SX18 scores 65, based on the factory T/S specs, which may or may not actually be correct for your specific driver.

Here's some discussion on the topic: http://www.caraudioforum.com/showthread.php?t=195839

I dunno, I really don't like the low end on that driver, seems great for SPL, but how is it going to hit as low as 15 Hz with an FS of 32.2 Hz?

Maybe I'm just tired, but according to specs and what I've seen so far, that 18" driver won't go much lower than my two 12"s

Not sure how you came to decide on the SX18, but it is designed as an SPL driver. However lots of subs in this category can be employed for SQ duty - the answer to your question is cabin gain will reinforce frequencies, even below Fs. It may not be ruler flat, which is why someone in the link I posted suggested added some EQ boost around 20Hz. It would take some experimenting to do something like this. But keep in mind, we're talking about an 18" driver - it's not going to struggle to produce low notes in a car.

So right, maybe there are better drivers out there for SQ on paper. However in application I think you might be surprised at what this driver can do. The driver is but a small part of the final result. System tuning, installation, driver loading, enclosure design are all much bigger factors. There used to be a saying, "The best driver in the wrong enclosure won't sound as good as the worst driver in the right enclosure."

You do have really good point there, and for that price, it would be worth it even if it didn't push low notes as well as I wanted because I could use it for other great applications.

I got to hear a pretty interesting story a while back about how they hand tuned drivers in the factory from my friend's dad, and it actually taught me a lot.

Oh, I randomly saw that driver, and started pondering on it, the price is too cheap to ignore with nearly 93db efficiency, which is very good for a non PA subwoofer driver

The automated box design software online is probably not the best bet I'm guessing, and I'd have to play with it a lot more, or the acoustics of the car will pretty much augment it?

Also, is it a bad idea to have a huge ported enclosure tuned closer to 20Hz for a more passive EQ, and SPL gain solution, which would also help ventilate the voice coil and increase durability?

One thing that always helped me realize the value of going sealed: T/S specs are usually quite a bit off, so even if you calculate carefully, you can still be wrong. I've built quite a few sealed and vented boxes. For sealed, they usually end up being pretty close. a little polyfill or a wood block inside to dial it in. Sometimes they are perfect. For vented, they are almost always off, because of the complexity of designing the vent in relationship to the volume. A sine wave sweep from 100Hz down will reveal the Fb of the system - you can see the cone almost stop at this point. Using stepped Hz tracks you can identify the exact frequency, then trim the length of the tube to get it right. This means to do it right, you build a mock-up first, then a final copy with better finish. Lots of work.

I once installed a JL Audio 10W1 in a large vented box tuned to 18Hz. The vent fired up through the rear deck, and it was LONG (like 24"!). I tuned that one in the car and tested response over and over until it was right. The result was very low and very loud. It sounded like a 12". But the transient response was missing. I built mostly sealed after that. Make up for SPL efficiency with more power and good motor linearity, imo. That's why currently use an 8" with a true 200W RMS. Doesn't sound like much, but lets me have a low Qtc box in a very small space, and gets decently loud. Only place it lacks is below 25Hz.

I wouldn't worry about motor ventilation unless you are competing in SPL. One of the good parts about the SX18 is its toughness. It's built to take abuse. You can play music loudly all day and it shouldn't get too hot. Especially with larger sealed boxes, you'll never reach the thermal power rating of the driver before the mechanical Xsus. That means you'll hear the coil strike the top plate before it ever fries the coil.