His point was that larger speakers can't play higher freqs and that is what I disagree with. That doesn't mean that an 18" midrange is practical in a car, and if you think I implied that then you are an idiot.
For the last time, the upper rolloff frequency for a driver is entirely dependant on the Le of the voicecoil. Nothing else comes into play. PERIOD. That doesn't mean that you can use the higher frequencies, but he usability is limited by cone breakup which will create ragged response peaks that are actually above the reference level, far from a rolloff. All the XXX series drivers (the pre '06 ones anyway) shared the same motor assembly and guess what, they had the exact same high end rolloff. Why? Because they all had the same Le coil. Did they have the same usable frequency range? Not necessarily, but only because of cone breakup creating out of band noise and distortion. Last I checked, SUBwoofers were meant to play SUB-bass and not midbass so the rolloff of an 18 is not really an issue. At the same time, an 8 with the same motor is going to have the same rolloff which would still make it unsuitable for midbass applications. You are not going to find a high excursion sub that will do well as a true midbass. You will find midwoofers that will provide good midbass and good midrange extension. These come in all sizes. The Speaker Works Grand National had 12" midranges in it IIRC.
For the final time, driver diameter does not have an effect on the upper frequency range of the driver in question. That is entirely a function of the voicecoil.
F=MA the basic equation of physics. D= 1/2*A*t^2 tells us how far an object will move in a given time with a constant acceleration. I'll don't feel like doing calc right now so the generalized form will work. Given two subs with the same motor, one being an 8 and the other a 12, the differences in the drivers can be reduced to a difference in mass for the moving assembly. The force will be the same for the same power input because the motors are the same. Using the first eq, we see that the lower mass will have the higher acceleration. Now you can plug the acceleration for each driver into the second eq and you will see that for a constant time (.01 sec for a half cycle of a 50hz sine wave) the lighter driver will move farther during the time period. But guess what the time in question is dictated by the frequency being fed to the driver. The smaller driver moves farther and faster regardless of the freq reaching a higher excursion with the same amount of power, but it still plays the same freq.
Now factor in Le. The effect that Le has is to reduce the force as the frequency gets higher. The voicecoil acts as a 1st order lowpass filter. Once the cutoff freq for the filter is reached, the coil begins to act as a variable resistor with the resistance proportional to the frequency. Since current and resistance are inversely proportional current will decrease with frequency increasing. Since current is proportional to force, force will decrease with an increase in frequency. Force is proportional to cone displacement so cone displacement decreases with an increase in frequency. The smaller driver will always move farther than the larger, but since the larger driver doesn't need to move as far to displace the same amount of air as the smaller driver because of a larger cone. If you normalize the response plots of the two drivers, the end result will be they roll off on the top end exactly the same.
Your generalization is worthless. For every 8 you show me that has better high freq response than a particular larger driver, I can show you a large woofer that has better high freq response than some 8s or even some 6s. Driver diameter dictates how much air a speaker moves for a given displacement. Larger drivers make better low frequency drivers because you need to move a lot of air to get good output at low freqencies and it is easy to get a low Fs with a larger driver as a matter of mass. Most dedicated high freq drivers have a small cone because it doesn't take a large coil to move the lighter cone and the smaller coil will have a lower Le and provides a higher rolloff freq on the top. The smaller driver can also be made realtively stiffer and thus less likely to demonstrate cone breakup at higher freqs. Tweeters typically use a dome shaped driver that is edge driven because a dome is the most inherently stiff shape for a given weight and the extremely light weight allows for a tiny coil with an extremely low Le.
