Professional drivers vs non-professional drivers
I got a few questions from chucks0, maybe bear123 and others will be interested and therefore I think it is best I post here. I will post them in a mini series.
When I look at the drivers that Josh has tested on his databass site, I notice a trend: there is a competition of force-factor (which is BL^2/Re). I want to warn that those drivers are not suitable for low tune subwoofers. Some 15 years ago Steve C. has advocated LLT (large Low Tune) ported subs. His conclusion at that time is the drivers suitable for such alignment need to have higher Qts. If the Q value is too low, it actually constricts the SPL output at low frequencies bands. In fact, it can be explained in mathematics in terms of force factor, or specially BL value. Steve C. commented the impact is via Qts value, but in fact it is via BL value. LLT need a driver with moderate BL value, lower output from such driver in a ported subwoofers can be made up in enclosure size and large enclosures accommodate larger ports, so there is no catch. That is the physical part of the vented sub design. Now servo comes into picture. Servo can make the BL value of the drivers to "appear" (in what we call equivalent circuit) to be a much higher BL value driver. So why do we want that? Because high BL value actually sound "faster" by having better cone control (if the drivers do not suffer from other problem such as low Vas). In short, servo enable a sound quality that matches the high BL drivers, but without the shortcoming of max SPL loss from high BL value drivers. Of course the other advantage of servo is it has the most consistent frequency response across the bass frequency band. Josh publish the "compression" plots. He uses 90db as baseline and plot the variation of frequency response at each 5db increment. Then he also does this "repeat" to show after the most intense playback (in his test), if the first 90db frequency plot can be replicated in his 2nd sweep. If not, that means the sub has one form of memory effect (there are others), meaning the frequency response at current moment depends on what one has played some few seconds ago. That is not good, isn't it? In each one of the subs we submitted for test, the compression is zero except in frequency bands where amp running out of juice or at around port tuning where port compression can play a factor. Memory effect is one issue plaguing transducer. So thermal is one form of memory effect. Another memory effect is spider/surround hysteresis. It is there. How do we demonstrate that? Use a marker on the voice coil. Then one can observe the following: first pull the cone up and then release, then observe the rest position using the marker; second, push the cone down and then release. One will notice the resting positions in these two cases are different. It is not huge, in the range of 0.5-1 mm. But such hysteresis manifests itself at say midbass where it does not take large cone excursion to get high SPL, it will become very audible, in the form of muddy bass sound. Again servo can help reduce the effect of such hysteresis by reducing the distortion/memory effect caused by this hysteresis. A low Vas driver has a stiffer spider/surround which in turn requires a larger correction force.
[EDIT] The discussion of the above Qts or Q in terms of how they impact low end output is actually via the BL value. I have corrected accordingly.
Last edited by Rythmik; 05-28-2020 at 10:04 AM.