--please correct me if I'm mistaken--> Piezo appears to focus more on the higher resolution side of the industry whereas Seiko and Spectra-Polaris tend more towards the robust, high production capabilities required in a manufacturing environment.
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The primary print head technologies used in ultra-wide format digital outdoor printing comes down to a few print head manufacturers world wide including (but not retricted to) Piezo, Seiko and Spectra as the primary competitors and their print heads are used in most machines from a variety of manufacturers.
His current solo electronic music project is called Piezo.
Getting a Bigger Sound is a book Bart Hopkin wrote with Robert Cain and Jason Lollar about amplification of sound sources with several types of pickups ranging from piezo disc pickups to common pickups often used in electric guitars.