Quote Originally Posted by Pogre View Post
Servo subs are cool, but a modern approach with computer modeling, newer drivers and advanced design the tech has narrowed the gap very significantly in the last 15 years and has pretty much caught up.
I have been following the technical reviews of subwoofers reviewed on Audioholics and Data-bass, since they have been doing such technical measurements, rather than previously where there was simply impressions of how it sounds after listening to several different well known audio tracks and such. I agree that the measurements that are taken in such reviews shows only small differences at best between a servo sub and a sub that using DSP.

I do think that there are things that measurements can't capture that make a servo sub superior still though. The best analogy is with ribbon tweeters. I own Sierra 2s for critical listening in one room and CMT 340s for causal listening in the other. There is something about the ribbon tweeter where I can listen at -20 to -15 dB for several hours without fatigue. On a dome tweeter like that of the CMT 340 fatigue sets in for me much earlier at lower volume levels. Looking at the CSD measurements there is not a huge difference between the two tweeters though to explain this. I'm not at all talking bad about the tweeter in the CMT 340. It is far superior to other tweeters where fatigue sets in much earlier still, for me, and at even lower volume levels. From a theoretical standpoint we know that the ribbon tweeter is going to start and stop much more quickly than just about any other kind of tweeter, which partially explains what I am experiencing. But, the measurements are quite similar outside of the a few small resonant frequencies that don't die out quite like the ribbon tweeter. Maybe it is as simple as those few resonant frequencies lingering that aggravates my tinnitus, but I'm not so sure that is the only thing going on here. I don't really care, as I have found the speakers that work for me.

Getting back to subs versus a dsp sub. The servo sub can minimize the standing waves in the cabinet coloring the sound, where dsp does nothing. The servo sub can address unit to unit variation, because nothing can be manufactured exactly the same from one time to the next, there are tolerances on components in the driver, cabinet, even the port. When there is anything about how the signal is being reproduced that differs from what is supposed to be done, it continuously and instantaneously fixes the problem. Another downside of a DSP sub is the flipping back and forth from analog to digital and then back to analog. Something is lost in the signal from each conversion, since that process involves approximation rather than a closed form exact solution. But, a servo sub can avoid the conversion altogether, keeping the signal more accurate.

To my ears switching from my previous dsp sub (designed by Hsu) to a Rythmik servo sub was quite a step up. The dsp sub didn't have the transient response to match my Sierra 2s, where I needed to choose a crossover of 50 to 60Hz to not have the 50 to 80Hz region lose detail. With music this isn't a problem, but with movies there can be huge swings in the output in that region where I'd be worried about damaging the Sierra 2 asking it to play those frequencies at high levels. With a Rythmik sub, I can have that 80Hz crossover and not lose the detail. I'm sure that there are DSP subs that can do better in that 50 to 80Hz region than my previous dsp sub could do, but subwoofers are expensive to return if you hook them up in your room and find that they don't have an improvement in the upper bass region. Several others on this forum have experienced this same phenomenon, so I figured the easiest solution to try out Rythmik's servo subs, and it fixed the problem.