Originally Posted by Gracepreacher
Cheryl, thank you so much for your explanations! That makes complete sense.
Next question, I just read about "group delay" induced from cascading crossovers in another forum, and I do have a slight sense of the timing being off on the bass shakers. Could this either be from doubling of crossovers, or maybe the fact that the AVR is set up with a subwoofer speaker delay setting measured from my MLP to the sub, but the shakers should theoretically be set to zero (because they are right under me)? Has anyone else had issue with timing and solved them? Maybe a compromise by setting the distance in the AVR to half/average between the two? All the more reason to try my sub nearfield!
Group delay refers to the relative time delays of the various components in a signal.
Group delay is important but usually only when multiple drivers (or drivers and resonators like ports or passive radiators) in a single system simultaneously produce the same frequency. Out of phase components might add or cancel in a complicated fashion, altering transients, inducing audible tonal coloration, or smearing the perceived location of the sound.
The amount of delay that a crossover or lowpass induces is only a cycle or so of waveform, just a couple milliseconds at most and almost certainly imperceptible from a shaker, especially since there are no out-of-phase components that could be substantially interacting with the output from the shaker and it is unlikely that your tactile senses could even discern frequency-dependent phase-induced cancellation anyway.
I suspect that what might be occurring is that the shaker, chair frame, seat suspension, and your body form a damped resonator that needs multiple cycles' worth of energy from the amplifier to build up to full shake.
A speaker driver is a low mass cone with a relatively powerful motor driving it and it reaches full output very quickly in response to a 'steady-state' signal such as a tone burst test. In contrast, a shaker/chair is a high mass system with a relatively weak motor driving it. A shaker likely relies upon gradual buildup of resonating energy in the seat frame to compensate for the lack of driver power.
The experience could be a strong function of the shaker, the chair, even the size and shape and density of the person sitting in it might noticeably affect the experience.
It is also possible that the lower frequencies in the sound track are delayed for better realism, or out of sloppiness, who knows?
For more realism, turn up the system volume so you can feel the slam from the subwoofer. Add more/heavier shakers to your chair (yes, the mass of the shaker is an important factor since it is the 'anchor' that the motor pushes against). Make sure that you orient all the shakers on a single chair so they all drive toward your body in unison and get the polarity right when you connect them.
I doubt that nearfield placement of the subwoofer will do much since the room EQ will automatically reduce its output to compensate anyway. Nearfield placement will tend to make the room resonant modes less prominent in the localized sound near the 'sweet spot' but should not affect the actual balance between sub and satellites much. It might also drastically impair the sound in the rest of the room too so before you start embedding electrical conduit in a concrete slab floor try it out first to make sure you like what you hear.
Do not expect a shaker to create a convincingly realistic illusion of trembling Earth. Maybe shakers can enhance realism if done well, but to me, shakers are a gimmick to excite kids on amusement park rides (the only place I have actually felt a shaker). IMO the infrasonic experience of home theater should not come from an amusement park gimmick but rather from a high quality subwoofer driving realistic infrasonic levels and shaking the whole room, not just the chairs.
There is one person on this forum in particular (popalock) who has built two separate subwoofer systems to accomplish exactly that - one has 16x18" subwoofer drivers and the other has 2x24" subwoofer drivers (currently under construction). They can produce powerful output down into the 10Hz range where it can be felt but not heard. People who have experienced the 16x18" subwoofers claim the experience they generate is physically palpable like no other sound system and personally I believe them.
A shaker is just not going to feel like reality IMHO.