A little about that kind of pickups.
This type of pickups is usually used in construction of the electro acoustic guitars.
The physical principle of working of this pickup based on the piezoelectricity. This is the effect of the polarization density emergence that was caused by a mechanical tension. Also there is a reverse piezoelectrical effect that is the mechanical deformations that are caused by an elecrtic field.
Actually the electro acoustic pickup works like a piezoelectrical microphone. In addition it can works with any types of strings, so that divice can be regarded as one-size-fits-all. That kind of pickups often is used to pickup the sound of the acoustic instruments like an acoustic guitar, a viola, a cello, etc. That’s because a piezoelectric pickup, like a michophone, is capable of picking all sounds that was produced into and out of our instrument and transform it into elecrtric signal.
Sources:
Radio technology in music 