Yamaha DTXpress/DTXplorer/DTXtreme

> 
> I'm just guessing, but maybe the parallel piezos create a kind of rim 
> to pad effect, or, more likely, are used for positional sensing with 
> certain voices, via the Roland TD-10's expansion card.

Ed,
The Roland dual zone mesh pads only have two piezos. According to td-8 and td-10 users on electronicdrums.com,  the only requirement 
for positional sensing is the mesh head piezo to be centered. 

Jun