does a piezoelectric crystal a buzzer and a same time a speaker?

Answer 1

Depends. Some may be usable as either. Some piezo devices may be encapsulated with the drivers in a little package so they can only be used as buzzers. Some may have a resonance in the audio range, which would limit their efficiency as a speaker/earphone. To drive them at or near their resonant frequency could cause them to crack and fail. Sometimes, those are the kind that are optimized to be used as buzzers only, like for alarm clocks or smoke alarms. Get hold of a catalog for an electronics parts distributor (Digi-Key is one) and look through the specs. You will find it educational.

24 JAN 07, 0214 hrs, GMT.