Why does a piezo buzzer still make a noise when a constant signal is applied? As far as I know, if there is no oscillator, there can be no frequency (because "frequency" is the frequency of the oscillating signal being applied to the buzzer/speaker) and therefore, theoretically, there can be no sound. However, when a constant signal is applied, it still beeps. Why?

 

Some have have an oscillator built in and some don't.

 

A piezo buzzer has a built-in oscillator circuit.
A piezo transducer is just that and has no oscillator circuit.

 

A piezo buzzer has a built-in oscillator circuit.
A piezo transducer is just that and has no oscillator circuit.

Although the transducers are often called buzzers just to confuse.
This is the first I found on a search for 'piezo buzzer'. It is described as a buzzer and it says you need to supply it with a squarewave.
Caveat emptor.

 

Why does a piezo buzzer still make a noise when a constant signal is applied? As far as I know, if there is no oscillator, there can be no frequency (because "frequency" is the frequency of the oscillating signal being applied to the buzzer/speaker) and therefore, theoretically, there can be no sound. However, when a constant signal is applied, it still beeps. Why?

The buzzers have a small segment of metalisation on one side isolated to serve as a feedback signal - the oscillator can be as simple as a single transistor and a couple of resistors.

 

I see, thanks.