Digital signal processing is very promising in audio applications, offering significant advantages over traditional analog processing.
Direct Servo and DSP are complementary technologies. Direct servo maintains a stable frequency response regardless of voice coil temperature/resistance at the same time providing distortion reduction using close loop feedback system. Both of these are difficult to implement with DSP alone. On the other hand, DSP can be used for room acoustic and frequency response contouring, which is outside of the scope of servo technology.
(parametric or DSP-based)
The design of Direct Servo subwoofers is such that it will produce an ideal flat frequency response when measuring close to the driver (referred to as near field measurement). This flat near-field frequency response becomes a reference. If one measures any deviation of that at the listening position, it is caused by either the crossover (if the deviation is around the crossover points) or the room modes or effects. If it is the latter, it is best addressed with room treatment such as bass-traps or room EQ, either based on parametric or DSP technologies.
Because the Direct Servo is relatively flat in the near field measurements, we recommend the following:
1) no boost at all below 30 Hz and only use the extension filter on the plate amp to adjust the frequency response below 30 Hz range, and
2) minimal boost (no more than 3 dB) above 30 Hz and let the room EQ do work on the suppressing of peaks in frequency response caused by room effects, instead of boosting the nulls.
This approach is superior to attempting to correct both the near field and room response together. It simply is a more difficult problem to address when those two problems are combined. The potential problem of this approach is it can over-boost the low end beyond the capability of the driver and cause it to overload the driver prematurely. Advice from professional audio community is towards using cut-only EQ. However, without a flat near-field frequency response, it is hard to achieve this goal. Some may argue the servo technology also applies some form of EQ. The key difference is EQ in servo sub is to only achieve flat near-filed frequency response, if a particular has a 3 dB notch at 20 Hz, our guideline does not recommend additional boost on room EQ. When boosting, the system can quickly go into overload prematurely.