Originally Posted by kluken
That I knew, but I was looking for the Rythmik fans/owners to tell me if I was missing something or have they seen the F15HP deliver really goo deep tight lows. It was less about the comparison and more about vetting the claims of 14Hz out of a sealed sub and only being down 2db. Everything I have read is that the roll off of sealed subs starts around 30Hz, so I am struggling to see how 14Hz is only down 2db.
First the disclaimer. Standard spec of frequency response is +3/-3db range. So the -2db is based on that spec, which is -5db from the top of the FR curve. But it is still better than almost all sealed subs Data-bass site has tested. For instance, one of the tested sub is down -16db at 14hz. Please note a lot of sealed subs without using this +3/-3db spec cannot claim that they have 20hz extension.
So now back to the answer to your question. As soul burner has pointed out, it is because of servo. That is the shorter version of the answer. The longer version of the answer is how servo 1) alters the frequency response, and 2) addresses the challenge of having low extension and still sound good (not boomy).
Servo controlled subs compare the servo feedback signal to the power amp input signal and then output more power if it finds the woofer does not move fast enough, or output reverse throttle when it finds woofer moves too fast. So it is not just one direction control. This help the subwoofer to actively reject external interference acted upon the woofer. For instance, the box spring is one of such external force. So is the internal standing wave of the enclosure. Servo will have much less value if it cannot be analyzed formally. Without that, servo will be used as a "patching", or trial-and-error, or an adhoc technique. In fact, in early days, servo design was done that way. One of the former Genesis engineers told me that was how Arnold did their servo design in their early servo subwoofers, trying a different resistor value or capacitor value at a time and see if the output was better.
So after formal analysis of servo control design, I have found it behaves like changing the T/S of the driver. The frequency response is just like real world sealed subwoofer (with large diameter driver and large motor) from top to bottom. This is very different from the result of using EQ to achieve extension becasue the amount of EQ will be enormous at the bottom end and the designer needs to do a cut-off point decision. That is one of the reason you have seen other nonservo sealed subwoofers cannot go -5db down at 14hz. It will be just too much EQ. Another very important capability of servo is that we can "clone" the frequency response. That is why you will see our sealed subs all have very similar extension curve regardless driver diameters. That is by design.
Second part of how servo can do better is related to this comment "best bass is no bass". You can say this is sarcasm. Making low extension actually expose the sound quality weakness of low bass. In time domain, reproducing a 10hz signal takes 4 times as long as that of a 40hz. If there is a ringing, the ringing at 10hz will last 4 times longer too. That means there are 4x more reason that the sound can be bad. So instead of exposing the subwoofers to that problem, many designers just cut back the extension to avoid problems. We allow our subwoofers to go that low because we are able to control the impulse response to almost no ringing. In our frequency response plot, we publish both amplitude and phase response curves. No other manufacturers do that. What we want to show is our sealed subwoofers have the minimal phase shift among all sealed subwoofers. For instance, our F15HP as about 100 degrees phase shift from the microphone. Now in reality is F15HP has even lower phase shift because we are able to calibrate microphone with our servo signal (yah, we can claim no other manufacturer can do that) and found our measurement grade microphone, which is flat down to sub 10hz, has 18 degrees phase shift at 20hz and 45 degrees phase shift at 10hz. An ideal sealed subwoofer has 2nd order roll-off, which has 90 degrees phase shift at the corner frequency. So our sealed subwoofers are not far from that ideal case. In addition, when the subwoofer is -14db down, you can expect the phase shift to 180 degrees or even more.
Some may challenge that while we have low extension, how much output can the subwoofer do, sort of hinting the extension is not useful at all. Real life sound does not have a lot of sub-sonic components. For instance, I have been observatory telescope dome. When I watch movies with similar scenes, what I want to hear is the reverberation is as realistically believable when the door is opened or when the dome is turn. That does not need a lot of subsonic energy. But it is important to align the subsonic signal (which is the reverberation) with the rest of the sound so that it sounds well integrated and believable. But any subwoofer with limited extension makes the dome sound smaller than what it is. Same thing is motorcycle throttle/exhaust noise. There are just too many examples. And lastly if the movies have too many artificial LFE signals that makes the sound boomy, we have rumble filter and other controls to cut back the extension.