Originally Posted by dondino
I have dual empires and the opposing drivers eliminate any cabinet vibration. I have mine stacked and at very high volumes they don't vibrate at all. Also, I've done REW charts with them facing forward and to the side and there's no different in FR. You just need to keep a few inches clearance from the wall if the driver is facing the wall. It's design is similar to the seaton submersive.
I admit the opposing drivers is a good design. But in my point of view, the biggest problem in subwoofer is the cone itself. Everyone does a knock test on enclosure. Try that on cone. The cone is actually an opening to let people hear the sound inside. It is a difficult job -- the cone needs to generate sound and at the same time stop the sound from inside enclosure to escape to outside to be heard becuse sound is generated on both sides of cone. If the internal sound escapes to outside, it becomes a boxy sound. And there are various level of boxy sound. Sometimes a slightly boxy sound can sound exciting to people as it moves the mid bass forward, as often described as "in your face" type of sound. But actually it is a prominent or emphasized mid bass that masks details. After all, drum is based on a boxy sound inself. It uses the resonance inside drum shell to produce sound. This weekend, I went to Brazos Bend state park outside Houston. They have an observatory open to public and we had the chance to get in to hear the staff controlled the dome (open and turn). The sound it generated was a typical special effect sound we hear in movie -- moving a large heavy metallic object in a closed space. In real life, that sound comes and goes so fast and it is absolutely crisp even when the room is full of echo. Isn't that odd? Absolutely no boxy sound. Now the purpose of servo is to force the cone move according to the input signal and reject energy from inside to escape. This is advantage one.
The second advantage is no thermal memory effect. Recently I ran into a web page from Siegfried Linkwitz. http://linkwitzlab.com/frontiers_6.htm#Y
He did a comparison to demonstrate so called "amplitude modulation" (AM as in AM radio). He compare Neo3W with 25TFFC and conclude Neo3W is the worse performer in that regard. But something he may have overlooked includes: 1) impact of impedance to voice coil DCR ratio.
The frequency he picked --1.5khz exactly matches the impedance peak of 25TFFC and therefore gives unfair advantage to 25TFFC as AM is lowest at impedance peak when voice coil resistance is the smallest percentage of whole impedance. On the other hand Neo3W has a flat, also pure resistive impedance comprised mainly of VC resistance. If he had picked a different frequency point such as 5khz where both are at impedance minimal point, that would be a more fair comparison. 2) Efficiency normalization.
Neo3W has a lighter diaphragm and therefore more efficient. Putting 30V to both drivers actually makes Neo3W sound louder and therefore not an apple-to-apple comparison. These set aside, the figure labeled "Neo3W response at 30Vpp" is a good illustration of thermal memory effect, not on Neo3W, but on subwoofer. The so-called AM distortion is better described as memory effect. Often people call it compression. But compression is misleading as compression is static. But in reality, the compression goes up and down on the fly. Subwoofer voice coils are designed to withstand up to 250 degrees F increase for a good reason. It does get hot. But it does not stay hot. It is more like hot and cool and hot and cool cycle. What goes up must come down. As the voice coil heats up, the output goes down, on the fly. And when voice coil cools down (when signal becomes small), the opposite happens -- reverse compression (or expansion), and the output goes up, again on the fly. When this type of up and down happens, it is more like the machine plays Rubato for us. That is not good. I want to hear the Rubato from performer, not transducer. Servo minimizes thermal memory effect to almost unnoticeable. Ilkka is busy with his new job. I once discussed with him to modify his test procedure to demonstrate the memory effect in both servo and non-servo subs. He can do his normal 85db plot, a short break, then 90db, a short break, then next level, ...etc. But the moment he is done with the highest SPL plot, he can do a 2nd 85db plot and then compare if the 2nd 85db plot is same as the first 85db plot. I bet majority is not the same.
Lastly, servo is very scalable. The improvement can be heard over and over again, on 12", on 15", single driver, dual drivers, there are more to come.
Anyway, hope the above gives other side of story. I really want to be specific. When we say servo is better, by what measurement it is better? I hope the above can answer that too, instead of using servo as a buzz word.