It's a cool device that really does work, since the mind is tricked into thinking everything sounds bigger than it is because you can feel the vibrations.
I'm not sure if this is a real product or not, but Dave Mathews Band bassist Stefen Lessard is using an interesting twist on the idea called a Fonz Foot Wedge on the current DMB tour. This is the same idea as the bass shaker on the drum seat, but it's connected to a board that you can put your foot on to feel the vibration. In the case of Lessard, he's getting the vibrations from just the kick drum.
In this case, it's not so much to fooling the brain into thinking the kick is louder as it is to stay in the groove with the drummer. A trick of many studio bass players is to put their foot up against the kick drum to feel it, which helps to fall into the groove a little easier. The Foot Wedge seems to be just the ticket for this purpose.
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