What determines loudness of a subwoofer??
 

Rookie questian as I may be blind haha but I'm still learning here and it's hard to find the answers, hoping you guys can shine some light on the situation.

Isn't sound kind of about surface area and excision of a speaker?
I don't understand how (in a sealed box) a sub that is smaller and has less excursion can be louder/more bass than a sub slightly bigger and that has more excursion?

I know it probably has a lot to do with wattage and motor structure etc... but in the end isn't it fundamentally about movement and area?

Cheers

There's a whole lot involved here.
Box is probably the biggest thing that affects loudness, as well as quality of the sound. The car also makes a huge difference. Tuning of the box, positioning of the box, type of box (ported, sealed, t-line, etc). Then you can also look at wattage, since doubling power (if I remember correctly, since I've been out of the scene for a long time now) increases something like 3db of sound, which is barely noticeable to the average human.
Excursion makes a big difference, but only if the box is designed properly for the sub. Surface area definitely helps to make it louder, but then the bigger the sub the harder it is to design a good sounding box that'll actually fit the car nicely.
I doubt I'm much help, but just my 2 cents.

outside of of car interactions, as mentioned, box type AND tuning makes a difference in output. again, as mentioned, surface area helps by letting a driver move more air. excursion helps when you have more power to push the woofer harder. one thing that WASN'T mentioned is driver efficiency which is VERY important if you're looking to get louder. if two speakers have the otherwise same exact frequency response and thiele small parameters, but one is rated 85dB and the other is rated 91dB, you'll need 4 times the power to make the low efficiency woofer sound as loud as the high efficiency one, BUT, extra high efficiency woofers also tend to roll off a lot faster below 100Hz and can actually move LESS air at 30hz than a lower efficiency woofer with less rolloff. magnet size makes a difference in both power handling and how much control an amp can have over a speaker to make it sound faster & tighter. finally, as well as getting 3dB every time you double your watts, you can also get 3 free dB out of an amp by doubling your drivers. so, back to those theoretical 85dB woofers, you'd need 4 of them to sound as loud as one 91dB sub with the same specs otherwise. when multiplying speakers though, you have to watch your impedance as when it's too low, it's easier to blow an amp, just as it's actually easier to blow a speaker with a lower power amp overdriven into clipping distortion which causes more blowouts and dead amps than overpowering speakers probably.