It's a case of inertia, plain and simple. A woofer cone with a lot of mass takes its sweet time getting started and stopping. And here's the thing. The power amplifier running the woofer has no idea what's going on; that the woofer isn't doing what it's told. And when woofers don't do what they're told that's called distortion. It's not a pretty thing.
In an amplifier circuit we have means to compare the output of the amplifier with the input to make sure one's doing what the other demands of it. It's called feedback. It's the same thing a comedian uses onstage. He rips a joke and if the laughter's loud, the feedback's good, no correction need be applied. But if silence greets humor then the output did not meet the expectations of the input. Time to change parameters. Or, on another level, when I used to bring home a poor report card I'd get my butt whipped with my father's belt. Both are examples of feedback.
The vast majority of subwoofers on the market today haven't any feedback. If their box size is too small for the woofer, resulting in reduced low frequency bass, designers simply EQ the amp to compensate. But still, the one doesn't know what the other is doing and, in nearly all cases, the output certainly does not match the input. This is clearly a case where it'd be great to know what's happening on the woofer so the amp can try and fix its motion to more closely match the input signal.
Such a feedback arrangement isn't trivial. In an amplifier it takes nothing more than a resistor to loop the amp's output back to its input for comparison and correction. But imagine the difficulty of measuring mechanical motion and comparing that motion to the electrical signals that drive it. That's a whole different ballgame.
Tomorrow let's start to look at servo systems; motional feedback measuring devices coupled with amplifier's that compare inputs to outputs.
We'll also look at why so few subs have this technology; which is a pity since it's the only true means of getting great results.