Rythmik Direct Servo - velocity and current sensing.
I only want to focus on the improvements of servo since it still takes a well-engineered driver. Servo is like the icing on a cake or whatever else you want it to be. It took me a while to write this since I wanted to make it simple enough to understand. Hopefully you understand some of the T/S parameters of drivers. If anybody notices that I mistyped something, please let me know. I tried to be as objective as possible.
The sensing coil reacts directly to velocity. It also sits in the same magnetic gap. It is able to react almost instantaneously. The latency is very short since and it does not need to convert measurements before it can correct. How quick the servo can respond is important in each of the points. The sensing coil weighs about 1 gram and is about 6 mils thick (6*2 for each side of coil is 12 mils total).
I am referring to sealed subs in general throughout this post to make it simpler besides the FV15HP link haha.
A) Coherence - The system does not suffer from amplitude modulation distortion as long as the amplifier has power on tap (aka power compression). The servo also creates a very linear response. Peaks and dips in the response cause extra ringing.
Under 25 Hz for the green line and the orange line are with amplifier clipping. Otherwise the response curve stays the same. Check both graphs in post 3.http://www.hometheatershack.com/foru...html#post45117
Here is another example measured by Josh Ricci. Under 30 Hz is port compression (ported sub). I linked to the two port mode so it is easier to focus on the lack of thermal compression due to less port compression. Scroll down to the long term power compression sweeps and chart.http://www.avsforum.com/avs-vb/showt...3#post20602943
B) The servo provides extra control to the cone. All servos reduce the Qts of the system but these use different methods (sensing coil, accelerometer, motor based). There are now three Q values. Every sub has a physical Q value. This is the Qtc of a driver in the box (The Qtc of a driver in a sealed enclosure is higher than the drivers Qts. The larger the box ,the smaller the Qtc of the enclosure. For Qtc to equal Qts, the enclosure would have to be near infinite in size). Non-servo subs system Q is the same as the physical Q. The Direct Servo reduces the system Q by about 3x. Those subs with EQ can also change the third Q or frequency response Q. After equalization, the Qtc of 0.5 theoretically produces no ringing. Here is a quick summary from Brian posted below.1) The physical Q value. Physical Q value determines the energy transfer ratio between our amps and speaker. For instance, even though 0 voice coil resistance can eliminate a lot of distortion sources, it causes an undesirable stall effect that the frequency response follows a -6db/oct starting from 100hz all the way to 5hz which is not very good at all. The best physical Q value is around 0.7 to 0.9 to ensure max energy transfer in the pass band.
2) The system Q value determined by the equivalent circuit should be as low as possible as it is proportional to "equivalent" Rvc. Currently it is 0.3 for 12" driver and 0.25 for 15" driver. Lower equivalent Rvc increases damping force. When you push the cone, you can feel a stronger resistive force pushing you back. This characteristic is best observed in free air because once you are in a box and you really need to push fast enough to emulate the frequencies above 10hz (otherwise, you will be just excercising the spring force of a sealed box). In a spring-mass-friction system, resistive force is the only one that dissipates energy (this is also how we are different accelerometer-based approach, which implements a very large spring force in the equivalent circuit, and spring force does not dissipate energy). It also means the cone would transfer less and dissipate more of the standing wave inside the enclosure. Moreover, if the system is accidentally push out of their linear operation range (such as clipping) or deviated from their linear path due to nonlearity of the system, it can recover 3x faster than nonservo as soon as it is back to linear range. The result is a sense of fast bass under almost all condition.
3) Final time domain/frequency domain Q value. Although system Q should be as low as possible as we push below 0.25, the final frequency response from the sub stay above 0.5. Anything below 0.5 can sound very lean. For this reason, we have provided three settings for everyone to try (0.5, 0.707, 1.0) and most listeners like 0.5.
To demonstrate the effectiveness of a lower system Q (more cone control and damping), Brian made a large enclosure. Inside the enclosure, he placed another driver. This inside driver is the active one and played the signal. The second driver faced the external environment as a typical sealed sub. The second driver was a reactor. The active driver inside played a sweep while SPL was recorded outside the reactor driver. This was done with the reactor driver without servo and also with it. This test shows how much of the bass energy from the back wave moves through the cone. The graph at the bottom of the link shows the increased damping of the servo sub. The number is the difference between the non-servo driver and the servo driver.http://www.rythmikaudio.com/re-radiation.html
I uploaded this video to the Official Rythmik thread to show the improvement in real life. After flipping the switch to the servo, it feels as if the cone is sitting in a viscous liquid. There is some rocking of the driver when the servo is on. It was hard to do this video by myself with one hand pushing, another hand flipping the switch, and trying to hold the magnet of the driver with my feet. NOTICE - DON'T TRY THIS WITH AN ACCELEROMETER SUB. I DON'T WANT TO BE RESPONSIBLE IF ANYTHING HAPPENS.
C) Non-linear distortion. The Bl profile of a driver shows the motor force of the driver depending on the displacement of the cone from the center position. One equation for calculating motor force at the weakest point is Bl^2/Re. The servo reduces the Re by 3x. So, to solve for the 15 driver, we have (15.8^2)/(3/3) = 250 rounded up. Rythmik has a driver coil and also a sensing coil (weighs about 1 gram). The sensing coils Bl profile replaces the driver's profile (shown in the link). It is linear without reducing efficiency. A linear Bl curve helps reduce non-linear distortion.http://www.avsforum.com/avs-vb/showt...9#post15541849
Here is a link to show the reduction of primarily spider and surround distortion. Scroll down to distortion reduction unless the whole article interests you.http://www.rythmikaudio.com/memory.html
D) Reduction of Memory Effect - This is tied to point 1 (Coherence). It is more complex but is talked about in the link above and the two links below.http://www.avsforum.com/avs-vb/showp...&postcount=156http://www.avsforum.com/avs-vb/showp...&postcount=170
A Quick Analogy: remember analogies are never completely correct.
Say we have a professional weight lifter who specializes in bench press and an Olympic gymnast who specializes in the still rings. Both weigh the same and are the best in their sport for producing upper body force. We now test them in the bench press. The weight lifter will most likely be able to put up more weight than the gymnast. Now we have both of these people perform a more complex test such as a planche on the rings. The weight lifter has the motor force but does contain the neurological coordination and feedback to stay true to the signal. Even if we went to a simpler movement on the rings, the weight lifter may come close but would still be shaking and producing unwanted movement. A gymnast also sometimes loses control (amp clipping) but is able to react faster to move back into position. Hope this helps if the above was too complicated. I wrote this real quickly after proofreading.