The piezoelectric effect is where certain crystalline and ceramic materials
have the property of generating an electric current when pressure or a bending
force is applied.
This makes them sensitive to acoustic vibrations and they can produce a voltage
in response to sound.
Piezo mics (or transducers as they may be called – a transducer is any device
that converts one form of energy to another) are high impedance.
This means that they can produce voltage but very little current. To compensate
for this, a preamplifier has to be placed very close to the transducer.
This will usually be inside the body of the electro-acoustic guitar.
The preamp will run for ages on a 9 volt alkaline battery, but it is worth
remembering that if an electro-acoustic guitar, or other instrument with a piezo
transducer, sounds distorted, it is almost certainly the battery that needs
replacing, perhaps after a year or more of service.