Since the first aircraft managed to break away from the Earth and raise a person in the air, aviation began to develop rapidly. As you improve, the aircraft began to be adopted in various countries of the world. Aviation has shown its full force in the First World War, exposing bombardments to the strategic rear of the enemy: ports, plants, settlements. As a rule, such raids became a complete surprise: the human rumor was able to hear the hum of motors of the approaching aircraft too late to take preparatory measures. And before the construction of the world's first radar station, there was still far, not to mention the massive use of such systems in the current army.
In the meantime, scientists continued to study electromagnetic waves, engineers invented constructors, designed to at least improve the perception of sounds by human ear. Invoice, changing once a few hours, tried to catch a distant noise of motors. These means of sound surveillance were used until the Second World War, and then began to give way to the emerging radar stations.