First of all, we must know that buzzer can be divided into two kinds of buzzer according to its structure: Piezoelectric, electromagnetic.

Its internal structure is not discussed in detail here! Maybe we have encountered such a thing:

Why buzzer around the people to buy as long as a suitable voltage buzzer to stop calling, but I only called, the reason is very simple! Because the buzzer according to whether it is with the signal source is divided into two types of active and passive! Note: active buzzer and passive buzzer. Active buzzer with internal oscillator, passive buzzer without internal oscillator. As used herein, "source" does not mean "power source" but "source of oscillation." Active buzzer directly connected to the rated power can be continuous sound; and passive need to add the power supply terminal with the level of constantly changing electrical signals can be driven sound. Here's a simple example. For passive, we can let BUZZ = 1 then delay for a short time and turn BUZZ off again (ie BUZZ = 0); in this way we can hear continuously changing sounds.