A piezoelectric pickup is a device consisting of crystals, for example of barium titanate, capable of generating sound (electric field) as a result of a mechanical stress. In the case of an instrument, this is string pressure.
The string pressed on the piezoelectric element develops energy on the two upper and lower faces of the crystal.
If the two sides are electrically connected, a current is generated which can be transformed into an audio signal.
The piezoelectric pickup is normally coupled with electronics that not only transform the signal from high to low impedance for greater manageability and sound quality, but also often has the task of mixing the piezo signal to a normal magnetic pickup one.
The result is a timbre that covers the entire spectrum of audible frequencies and captures the smallest acoustic nuances of the instrument, adapting perfectly to reproduce the original sound of acoustic instruments.
On the electric bass, the overall sound will also be strongly influenced by the type of strings used. Different string types will make different sounds: from the more unplugged tones of the Nylon strings to the more double bass ones (flat round-wound) to a more standard sound (round-wound).
The influence of the string type becomes even more important in the fretless bass and acoustic bass where the use of different strings provides incredible versatility.