Do magnetic-driven speakers provide better "quality" (more frequencies) than piezo speakers?
Do they consume power even when not in use?
Piezoelectric speakers do not have a very flat frequency response, which makes them less than ideal for reproducing the audible spectrum faithfully. However, they can be made very physically thin, and are typically good at making certain sounds very loudly with very little power. They also tend to be louder or more suited for high (including supersonic) frequencies.
Sometimes, when design constraints are more important than audio fidelity, an engineer may select a piezoelectric speaker over a dynamic loudspeaker (the kind that use a voice coil and a permanent magnet). For example, thin tablets or phones often use piezoelectric speakers because there's no room for the necessary magnets and coils.
Neither speaker technology uses power when not in use (depending on your definition of "not in use"). The crystal in a piezoelectric speaker requires power to oscillate, and no power to just sit idle.
In a piezoelectric tweeter:
"An audio signal is applied to the crystal, which responds by flexing in proportion to the voltage applied across the crystal's surfaces, thus converting electrical energy into mechanical."
Therefore, if no voltage is applied, the crystal does not oscillate, and no sound is produced.
In a dynamic loudspeaker, with no current through the voice coil, there is no magnetic flux and thus no opposition or attraction to the permanent magnet. The speaker will sit idle at whatever position the diaphragm and suspension are designed to at rest.
Within whatever technology used to drive a speaker, there are many variables which will influence quality. There are excellent piezoelectric drivers and poor dynamic drivers. What you select depends on the application, budget, and design constraints.
If you need a single frequency, considerable volume, and low power consumption, a piezoelectric speaker is probably ideal (buzzers, warning devices, alarms). If you require audio fidelity (more accurate reproduction of frequencies across a range), a dynamic loudspeaker or other technology is probably a better choice.
I agree your points. In addition, if PC = "tablet PC", the piezoelectric speaker will be a better choice than coil speaker. My experience is coil speaker performs worse as the speaker body size shrunk. It sometimes come to the "dead cross" with the performance of piezoelectric speaker in the case above.
The fundamental operation between coil and piezoelectric speakers make the coil speaker with better bass(inductive driving), but poorer energy transformation(from electric--> magnetic force-->mechanical force) and also there will be a certain phase shift from the original signal. However, the coil speaker is a science over 100 years, a lot of genius solved most of the problem.
On another hand, the piezo is "somehow too young", the advantage of zero phase shift and higher energy transfermation, may / may not be attractive for people to spend time and money to overcome the problem.
Some of the technology takes time and chance to fly(take off). Guess no one who could predict precisely?