Just out of curiousity, what are you basing this on? experience? numbers? I'm finding it hard to believe that a simmilarly constructed 18" and 6.5" woofer will have Exactly the same responsiveness.
It all depends on the coil and the motor. Size has nothing to do with the speed with which the coil moves. The inductance of the coil has everything to do with how fast it responds. Inductance is the measure of a coil of wire's resistance to a change in current. The higher the number the slower the response. As I said it not just that simple but assuming that the motor is strong enough to move the cone and the cone is string enough to move the air without deforming then it is that simple.
Even if the cone weight isn't the problem, the bigger the cone, the more air it has to move, which means more resistance, which means less responsive.
Magnetism is funny in that as long as the power is sufficient to move what you want to move, it moves. Let's get into the physics of it all. F=ma. The force (F) we have to work with is the Bl of the motor and the current going through the turns of wire in the magnetic gap of the motor. Mass is basically a constant (enclosure loading plays a small part in a sealed enclosure and a larger part in a vented enclosure especially at high excusion levels but starting at rest it is effectively zero for both). For a given mass, Bl and current, we know what the acceleration is going to be. Take the derivative of the above with respect to time. Because we are working with AC, F is constantly changing with changes in current (Bl is constant and we will assume that the coil remains in the gap). Mass is not changing with time. Acceleration must change at the same rate as the current. At 60hz, the current fully cycles once every 1/60 of a second. It does this whether it is running through the coil of a 6.5 or an 18. What is the difference? If the voicecoil inductance of the two speakers is the same, there is no difference. Inductance is the resistance to changing current, remember.
Let's look at an example. Assume that two speakers have exactly the same motor and voicecoil. In fact the only difference between the two is the moving mass (think larger speaker, different enclosure, what-have-you). We feed them the same signal. Will you hear a difference? The rate of change of current is the same. The rate of change of acceleration is the same. The magnitude of the acceleration is less, but the rate of change is the same. The lighter speaker will exhibit greater excursion because of the greater acceleration, but the transient response will be identical because of the same rate of change of acceleration.
Note, there is a difference between saying "it is possible to make an 18 with the responsiveness of a 6.5" and "size has 0% to do with speaker response".
Size has zero to do with speaker response all other things being equal. This seems hard to swallow because you are thinking instantaneously. Transient response is a function of time. It is harder to move a larger cone, but that is not the measure of transient response. Transient response is an electrical function, not a mechanical one.
All other things are not usually equal, that's the rub and that makes it even harder to grasp. To get a coil with enough power to move a larger cone with authority and able to acheive high excursion levels, the coil usually ends up with a higher inductance compared to a smaller, lighter driver. The newer motor designs being used on modern subs overcome this hinderance by using a more powerful motor rather than a larger coil as used to be the case. This keeps the inductance low and allows for the transient response of a smaller driver using a conventional motor design.
Just to take a step back and look at everything logically, Tweeters are by far, the most responsive part of the stereo system, and they are by far the smallest. Is that just a coinsidence? By your logic, couldn't we make the tweeter 10" in diameter and have it be incredibly loud w/o loosing any speaker response?
They are the smallest, but that has nothing to do with moving mass. It has everything to due with piston breakup at high frequencies. As you start to vibrate a large diaphram at really high frequecies, the forces involved with starting and stopping the diaphram cause the diaphram to start to ditort and lose its shape. If you could make an infinitely stiff diaphram, then you could make a tweeter as large as you wanted. Of course the other problem you run into is the whole inductance thing again. Inductors are the main component in passive low pass filters. Whatever frequency the inductance of the voicecoil equates to, the driver will begin to roll off above that frequency, thus limiting its high frequency capability. Tweeters are small because the small size and mass 1) will not destroy itself by vibrating at high frequencies and 2) because the coil required to move them is sufficiently small that it has a low enough inductance to not limit its capabilities within the audible spectrum of sound.
