Piezoelectric pickup are mainly used on acoustic instruments and develop their energy through mechanical friction.
The string is pressed over a piezo element (usually made of titanium and barium crystals) and transfers
a flux of energy on the upper and lower faces of the element.
If the two face are electrically linked together, a flux of current which can be translated in sound,
A piezo electric pickup usually delivers a very high impedance and requires to be buffered to achieve the finest tone results. By lowering the impedance the whole signal of the piezo pickup will gain in quality and performance and could also be mixed to magnetics ones.
Piezo pickup deliver a very wide frequency response capable to cover the whole audible spectrum and to capture the finest
details of an acoustical instrument.
The sound of a piezoelectric pickup will be strongly influenced by the type of strings used especially on the electric bass. As the piezo element does not forcedly require the string to be built in magnetic material like steel, a bass equipped with a piezo bridge can be equipped with steel, nickel, bronze and also nylon strings and still deliver a fine tone.
The external winding of the string will also affect the piezo electric pickup tone. Flat wound strings will deliver more of an upright bass tone, nylon strings will perfectly fit an unplugged combo while stainless steel round wound strings will deliver a very bright sound.
All those differences become even more evident on the fretless bass.