Quote Originally Posted by curtis View Post
I can't explain it like Dave, but servo control is not a filter in the same sense as an EQ. It tracks the position of the woofer vs the output of the amplifier. It corrects in real time rather than preset filters....so it can handle anomalies such as the environment, memory, etc. On top of what the servo does, you can apply traditional EQ to it...as evidence of the parametric EQ on the amp.

and as sludge said, the Rythmiks use a sensing coil, not an accelerometer.
Curtis is correct. By design, Rythmik woofers have an additional specialized coil in the motor assembly. This coil generates voltage due to its movement within the magnetic gap of the woofer (which is in unison with the cone itself). From this voltage source (which is fed back to the plate amp) - acceleration and woofer position can be determined and instantly compensated for in real time.

There are a few reasons why you don't see many manufacturers going this route:

1. It is very complex. You are not going to see this from other ID manufacturers or in the DIY crowd as it really does take a highly educated and experienced engineer to properly design. This is not something that can be modeled by software and it throws typical woofer design (which rely on Thiele Small parameters) into the garbage. With Servo, T/S params are practically meaningless as they are compensated for by the servo functionality. This is what Brian mentions in his post - T/S params for any woofer will change over time as the compliance of the suspension system changes.

2. Rythmik's method of servo is protected by patent. The other method of servo is by use of accelerometer mounted to the moving mass (typically on the cone itself or the former). This is patented by Velodyne.

One of the reasons, amongst many, that I am so fond of Rythmik's subs is that I have a lot of experience with servo and its many benefits. I was privileged to be an engineer for a company that was at the forefront of subwoofer design, this was back in the late 80's early 90's when subs were just starting to become mainstream. Back then we had only one real competitor - and that just happened to be Velodyne. I spent a lot of time with the late Dr. Lester Field, a part owner of the company I worked for and a brilliant PHD'd engineer. He spent a lot of time measuring and evaluating Velodyne subs and I was there to assist him. We blew up a lot of amps back in those days (and a few $4k B&K mic capsules), but it is a very fond memory for me as Doc (as I liked to call him) was one of my major influences, a true mentor.

More importantly, even way back then - we realized the rather dramatic improvements servo brought to subwoofers. I assisted Doc in his evaluation of developing our own method of servo, but sadly, it never came to fruition due to his passing.

A decade later, as we (Ascend) started to develop our own line of subwoofers - I was introduced to a little known DIY company called Rythmik. Brian and I had many discussions back then and I knew he was the real deal, exceeding my own knowledge of subwoofer design (which was extensive). After he sent me a fully assembled F12 - (all while I continually evaluated many ID offerings such as HSU and SVS) - I was both disappointed and hooked. Disappointed because I knew right then that I could not outdo this design in terms of sound quality (and sound quality over quantity was my goal) yet at the same time instantly hooked because this sub measured better and sounded better than anything else before it. I had data stored from close to a hundred different subwoofers from so many years of work in this field. In fact, it was the only ID sub I had measured that actually met its published specifications...

I instantly scrapped nearly two years of work on our subwoofer line - instead choosing to focus on assisting Brian in bringing these incredible servo subs to market.