Hi austinbirdman & chesseroo,
Some Axiom folks were at the NRC yesterday, I believe, and there may have been time to run some curves with grilles on and off (this relates to a thread on how loudspeaker frequency response is affected by leaving grilles in place). I did ask for that, and mentioned a couple of models, including the M22ti and M60ti but I'm not aware just yet which curves were run.
In any case, the answer to your question--the long version--will have to wait, because I'm just getting ready to leave for Las Vegas and the CES, as is Ian Colquhoun and several other Axiomites.
The short answer is this: it all relates to loudspeaker distortion as the woofers' voice coils are forced by loud signals to move farther out of the magnetic gap. It's a motor, remember, so when a woofer is producing substantial bass frequencies (without subwoofer assistance), its excursion--the amount of voice coil travel back and forth from its position of rest--becomes significant. In order to remain linear, you want the woofer's voice coil to remain in the portion of the speaker magnet's field where the field is linear, i.e. of equal strength. Now, as a loud, low-frequency signal forces the voice coil further "out of the gap," the cone's performance becomes non-linear, so it doesn't exactly reproduce the audio signal it is receiving. This is measured as Total Harmonic Distortion (THD), and if you scroll down and look at the NRC curves for THD&Noise at various sound levels, you'll note that with all speakers, the curve begins to climb as the frequencies get lower (and louder). On many smaller speakers, the NRC test signal is limited to 90 dB SPL, because if you try to measure the speaker at 95 dB SPL, the distortion will climb to very high levels--10% or greater. We hear this as a kind of fat, bloated bass sound (some enthusiasts even grow to like it!) and an increasing "edgy" quality as distortion climbs when SPL levels get really high.
Large floorstanding speakers will generally have much lower overall measured THD levels in the bass, typically 1% or less, than bookshelf systems because they usually have larger--and more--woofers, until you raise the SPL levels to 95 dB and much higher.
When you set your bass management to Small for the main speakers, and run a subwoofer, deep bass to the main speakers will be reduced, the woofer voice coil excursion becomes less, distortion falls, and they sound much cleaner. The sub, with a big driver and enclosure and its own amp, is better equipped to reproduce the deepest bass at lower THD levels. That way, you keep the main speakers' woofers more linear with much less THD.
Again, with smaller satellites, this is very beneficial. The sat's small woofers can operate within their ideal range and keep THD at audibly insignificant levels. It would be interesting to measure the differences and perhaps Axiom will have time to do so in the future. But loudspeaker distortion is well documented, and audible, especially with smaller bookshelf designs, and running a sub will keep the satellites THD at lower levels than would otherwise be the case.
Ian Colquhoun will have much more to say about this, as will Peter, one of our engineers. I've tried to simplify this but other factors come into play as well.
Regards,
