At what frequency is the buzzer switched on

Piezo buzzer coil or resistance oscillator

The diagram on the left was posted by me in one of my summer articles, and this is where I will try to possibly give a better explanation of how the system works.

Please note that the piezo element does not generate any significant amount of sound at low frequency and voltage. The coil has about 500 to 1000 turns, which may create many 100 volts across the piezo. I practically didn't measure it, but since I could sometimes see strong arcs in the piezo element tearing apart the central white part of the piezo, I was able to conclude that the voltage from the coil was significantly high depending on the number of turns.

How does the circuit generate a high frequency?

This is likely to happen in the following ways:

When the power supply is first turned on, the transistor conducts the coil and shorts it to ground so that the coil can store an amount of voltage equal to its number of turns. This also short-circuits the piezo connections, which triggers a reverse voltage pulse at the base of the transistor which turns it off, which prompts the inductor to repel the energy stored on the piezo. Once the transistor turns off, however, the negative feedback from the piezo disappears, forcing the transistor to feed back, and this process creates rapid oscillations across the coil which in turn causes the high frequency high voltage pulses on the piezo and induces the required sharp beep.