In the ancient war environment, the Chinese invented various types of buzzers, which provided important tools for reconnaissance of the enemy. In the ancient book "Mozi Preparing Points", it is recorded that the Mohists used buzzers to detect enemy situations during the Warring States Period in China. At that time, all the vassal states in China used solid city walls as their own defenses. Therefore, since the attacking enemy could not break through the city wall, they had to dig tunnels. In order to protect their country and detect whether the attacking enemy had dug tunnels, people at that time invented the buzzer. This kind of buzzer is a pottery urn with a capacity of about 80 liters, and the mouth of the urn is covered with leather. This is the oldest simple buzzer in the world but it contains deep scientific principles. When using it, people put the buzzer into a deep pit not far from the city wall, and choose a person with sensitive hearing to lie on the mouth of the urn and listen quietly. If an enemy is digging a tunnel, the urn will make a "buzzing" sound, and based on the size, thickness, length and other phenomena of the sound emitted from the urn, the enemy's position and distance can be judged, so that an early and powerful counterattack can be taken measure.