18mm Piezo Element
This Piezoelectric element can turn any mechanical energy into electrical energy (AC). For this reason, you can use it as a knock or tap sensor. You can also implement it in any of your Arduino projects by connecting it to the analog inputs of the microcontroller (see link for more details). You can feed the sensor AC voltage as well. This (coupled with the right frequency) can turn the element into a buzzer or speaker.
You won’t need to worry about soldering any leads as they come with pre-soldered red and black wires.
Note that this sensor in particular has a diameter of 18mm.