OK I'll take a swing at it. I think we need some more info about your rig to really get to the bottom of it. So for now let me explain my own set-up and offer a few suggestions. I use two Aura shakers model AST-1B-4 mounted to the frames of two separte chairs in my theater room. I bought them from Parts Express a couple years ago when they offered them in a kit with two mono amplifiers and some wiring. The amps look like a small car stereo amp with a few filter switches and level controls. They use an outboard power supply on an umbilical cable. They don't look like much of an amp but they do the trick just fine. My point being I don't think you need a very sophisticated amp or alot of power for that matter. As you can tell I am driving each shaker with it's own amp, if you connect more than one per amp then power may be more of a consideration. While a dedicated channel of amplification for each driver is the preferred way to go, Aura suggests series/parallel hook-ups are OK. This brings me back to your set-up. You say you have 3 drivers and one sub amp (mono I assume). Are all three connected in series? Since these units are 4 ohms that would probably be the best configuration as their impedances would add up to 12 ohms (an easy load for any amp). All three in parallel would yield a total impedance of only 1.3 ohms. That could spell danger to the amp. A combination of series and parallel would be OK impedance wise but it would give uneven power distribution to the 3 units. For now let's assume you have all three drivers connected in series. Here's my thoughts on what may be happening:
1) Impedance variations between the drivers themselves due to normal manufacturing tolerances or defect (partially shorted voice coil, etc). I have seen this cause level differences when hooking up full range speakers in series.
2) Differences in the way the drivers are mounted. (Could use more info on your set-up here too.) Since the shakers operate on an electrical/mechanical resonance centered around 40 Hz, if they are mounted in significantly different ways or to different materials their effect could be perceived quite differently as well. In fact, it can even alter the effective impedance of the driver by changing the back EMF. My shakers are mounted exactly the same on my two chairs...fastened to a pine 1x6 screwed and glued to the main side frame members. It bridges across the lumbar area of the back.
3) Polarity. If one of the drivers is connected with reverse polarity and they have a common mechanical connection you will get some cancellation. For a series connection of three drivers make sure they are connected + to - like batteries in a flashlight.
I hope this was of some help and wasn't too technical. By the way, Parts Express has the smaller, 25 Watt version, like mine on a closeout sale for $29.80/pair:
http://www.partsexpress.com/pe/pshowdetl.cfm?&DID=7&Partnumber=299-027 If you do add more drivers, try to end up with an even number. It makes the series/parallel configuration easier to accomplish. Good Luck!
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Tekdredger
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