Cone area X cone excursion = displacement potential. Does either one of those factors dominate the other in that equation? No, not when looking at the situation so vaguely. If you halve cone excursion, you can double cone area and still arrive at the exact same air displacement potential. But, there are other factors that affect perceived loudness, either to the human ear or to a mic.
Other factors would include the enclosure, how its alignment affects freq response of the speaker, and in the case of a ported enclosure, how its efficiency affects total air displaced. Another factor is speaker/enclosure efficiency versus power available.
There is one simple reason cone area tends to be viewed as more important than excursion, its potential is always maximized. With cone excursion, your sub might have a 30mm xmax potential (or xmech if we are talking raw burps here), but you may not have a system capable of achieving 30mm of excursion. This could be because you dont have the wattage required to overcome the compliance of the suspension and enclosure. It could be because the sub does not have the motor force (BL) to overcome the damping factor the enclosure places on the cone motion at a given frequency. Or it could be because your sub does not have the cooling capability to dissipate the heat required to be generated in order to get that full 30mm in the enclosure you've placed it in. But, cone area is a constant, its potential is always at its maximum. You do not have to consider your amp size or box alignment or tuning frequency in order to get the full 18" of cone diameter out of your 18" sub. So without necessarily even realizing why many times, people tend to perceive more cone area as an 'easier' way to achieve higher air displacement.
BL. AKA, motor force. This plays the most significant role when using a ported enclosure, to burp at or very near tuning (for maximum efficiency potential). As the signal moves closer and closer to the enclosure's tuning frequency, enclosure efficiency rises, but excursion damping also increases. This is why the seemingly odd situation of the speakers moving less, but get louder, occurs. Obviously an SPL guy wants to burp very near tuning, again because over all system efficiency is at its peak. But it also means cone damping is at its peak. So more motor force is require to overcome this maximized damping. Ideally the SPL competitor will strike the perfect balance between power available, cooling capability to dissipate the heat from that power, while reaching something just slightly less than xmech of the speaker. Again since that balance occurs near tuning, a lot of motor force is required.
When we move away from discussing SPL competitions, and simply discuss what is 'louder', as in what our ears think is louder, the frequency of the sound also becomes important because our ears do not have the same sensitivity to sound at all frequencies (unlike a mic). Some guy with a single 10 tuned high might have a system that seems louder than another guy with an 18 tuned lower, because our ears are more sensitive to higher freqs than lower ones (in terms of subbass, not in general).
The resonating frequency of the vehicle itself will also affect perceived loudness intensity, to the ear and to the mic.
Multiple motors versus single motors being used. No, there is no inherent addition of perceived loudness merely because there are 'more' subs. You are not 'doubling the signal' when you have two subs. The main advantage having more motors than less is, you have more potential for cooling. More motors means better cooling means less output lost to power compression.
That's all the points I can recall discussed in this thread I wanted to clear up. But the situation is even more complex than this post implies, as I barely even touched on power compression, how it affects output, and how speaker efficiency, frequency played, and excursion affect it. I also didnt mention the space the speaker resides in also affects output. Air temp and air pressure also affect it. Speaker FS versus the desired freq to be played affect it. The location and positioning of the speaker system within the vehicle will affect it Even the levels of background noise affect the perceived output potential of a speaker system. There are not so many factors that predictions are impossible, as someone suggested earlier in this thread. But there are far more than simply comparing excursion versus cone area.
Hope this reply helps.
