Aviation reconnaissance before the radar was born

Before radar, how do people detect the appearance of aircraft in the sky, in addition to observing with the naked eye?

American sound locator in 1943.

Today it is easier to detect the movement of airplanes thanks to the help of radar stations. However, before the radar technology was available, how did people detect the movement of aircraft.

During the First World War, scientists developed a means to detect the movements of very funny aircraft. The method used is sound locators . The main factor of this audio locator is human ears.

The sound locator looks like an extended ear, designed with many different shapes but has the same operating principle. Sound locator uses antennas shaped like horn speakers.

These horns receive sound waves that travel through the air with a frequency that the human ear is not aware of. The received sound waves pass through an audio amplifier to the frequency that the human ear can hear.

The operators of this audio locator will analyze the sounds they hear and determine whether the moving object is an airplane or a vehicle, etc.

Thanks to the use of these sound locators, one can detect the movement of aircraft or vehicles at a certain distance.

The following is a picture of some sound locators used in World War I:

Sound locator with two stethoscope used in America in 1921,
This machine looks like a giant ear.

Sound locator with 4 stethoscope of Czech Republic in 1920.

The device locates 2-tone sounds of Czech Republic in 1920.

The British 4-pipe sound locator was used in 1938.

British 4-pipe sound locator in 1930.

Japan's sound locator in 1930, looks like a giant trumpet.

French sound locator in 1930, looks like a radar station, horn speakers
Able to rotate, help locate sound from different directions.

German sound locator in 1940, this machine came closer to radar technology.