I played around with accelerometers (Velodyne used that approach) and various other sensing methods before deciding upon a dual voice coil approach similar to what Rythmik does. I was in college at the time and it is not a very complex circuit; understanding the acoustic side was harder (and the subject of a grad class I took later). I still have my project in a nice little woodgrain (vinyl wrapped) metal box. It comprised an adjustable low-pass filter, high-pass filter (crossover), delay (all-pass) circuit, servo (LDI feedback) circuit, and a buffer to adjust the gain and bridge the amplifier driving the cone (several amps, but usually a Hafler DH-200 I had, and the speaker driver was a dual voice-coil Infinity IRS woofer and later a Radio Shack bass amp driver when a moving company destroyed my Infinity driver ).
Regarding "fast", as others said above to me it relates more to the ability of the sub to "start" and more importantly "stop" quickly. Back then a number of LF driver/amp combinations when "ring" for a while after the signal stopped, messing up the tail end (decay) of the transient. Most of the "fast" sound comes from frequencies well above the sub's range so what you "heard" was sort of a muffled woofing/pumping sound. There are various ways to solve that problem, as well as the problems of resonances and such, with servo being one way. Servo can also help compensate distortion and power compression (to a degree) and a little (analog or digital) EQ along with some safe operating area (SOA) circuits can often let a smaller sub play lower and louder than a comparable non-servo design.
FWIWFM - Don
Regarding "fast", as others said above to me it relates more to the ability of the sub to "start" and more importantly "stop" quickly. Back then a number of LF driver/amp combinations when "ring" for a while after the signal stopped, messing up the tail end (decay) of the transient. Most of the "fast" sound comes from frequencies well above the sub's range so what you "heard" was sort of a muffled woofing/pumping sound. There are various ways to solve that problem, as well as the problems of resonances and such, with servo being one way. Servo can also help compensate distortion and power compression (to a degree) and a little (analog or digital) EQ along with some safe operating area (SOA) circuits can often let a smaller sub play lower and louder than a comparable non-servo design.
FWIWFM - Don
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