A speaker is used for handling high power sound frequencies and is able to reproduce exactly what's been fed at the input.
However a piezo transducer may not be as efficient as a speaker with power and output quality but there are a few of the features which make these devices outstanding.
A piezo transducer is specifically suited for generating very high pitched sound outputs, which a speaker might not be able to do.
Moreover a piezo transducer is cheap, very compact and sleek and does not require complicated circuits for operating.
So basically these are used for producing high pitched notes applicable in musical horns, warning devices etc.
Understanding and Using a Piezo Electric Transducer
A piezo transducer is round in shape with a metallic base, the 27mm diameter piezo transducers are more popular.
About 3mm from the outer periphery, the inner piezo material is coated on the metal base of a piezo.
This material is quite vulnerable especially while soldering wires on them.
Basically, these are two contact and three contact type. The metal base is used as the ground terminal and the inner poezo material coating becomes the positive terminal.
For the three contact type, the inner piezo material consists a small discretely separated piezo section which become the third contact and behaves mostly as the feedback element.
The above three contact piezo may also be used in two wire transducer application where the third small feedback contact is not used.
The external frequency from a piezo driver is applied across the metal base and the inner piezo material, the piezo then starts vibrating at the applied frequency level, generating a high pitched sound.
However this sound can be very insignificant and low in volume unless the piezo is fixed over a special plastic housing with a center hole.
The size of the hole matters and should not be more than 8mm in diameter or less than 6mm in diameter.
The plastic housing should be such that the piezo is stuck with a adhesive over a raise platform just a couple of mm above the base of the housing which consists of the above explained hole.
The raised portion should be just 2 mm wide, barely supporting the circumference edge of the piezo.
The whole sticking (installation) procedure has been explained in this simple buzzer circuit article.