Aircraft detection before radar, 1917-1940

Acoustic space was used for passive identification of aircraft by picking up engine noise from mid-WW1 to early WW2.

Passive acoustic space involves detecting a sound or vibration created by the object being detected, which is then analyzed to determine the location of the object.

Horns give both acoustic gain and directionality; The increased inter-horn spacing increases the observer's ability to localize the direction of a sound compared to human ears.

Acoustic techniques had the advantage that they could 'see' around corners and hills due to the refraction of sound. The technology became obsolete before and during WW2 with the introduction of radar, which was far more effective.

The first use of this type of equipment was claimed by the commander of the Royal Naval Volunteer Reserve, Alfred Rawlinson, who was commanding a mobile anti-aircraft battery off the east coast of England in the autumn of 1916.

He needed a means to detect zeppelins during cloudy conditions and improved an instrument from a pair of gramophone horns mounted on a rotating pole.

Many of these pieces of equipment were capable of delivering a fairly precise fix on approaching airplanes, allowing guns to be directed at them while out of sight.

Although no hits were obtained by this method, Rawlinson claimed that on one occasion a zeppelin was forced to set off its bombs.

Air-defense devices typically have large horns or microphones that are attached to the operators' ears using tubing, much like a large stethoscope.

Most of the anti-aircraft sound-ranging work was done by the British. He developed an extensive network of sound mirrors that were used from World War I through World War II.

Sound mirrors typically work by using a moving microphone to find the angle that maximizes the amplitude of the sound received, which is also the bearing angle to the target.

Two sound mirrors in different positions will produce two different bearings, which allows the use of a triangle to determine the position of the sound source.

As World War II drew to a close, radar began to become a reliable alternative to aircraft sound location. Britain never publicly acknowledged that it was using radar well into the war, and instead gave publicity to acoustic space in the United States.

It has been suggested that the Germans were wary of the possibility of acoustic space, and that the engines of their heavy bombers were synchronized in the hope that rather than synchronized (as was common practice, to reduce vibrations) Was that it would make it more difficult to detect.

For typical aircraft speeds at the time, the sound space gave only a few minutes of warning. Acoustic location stations were left in operation as a backup for radar, as exemplified during the Battle of Britain. After World War II, the sound range played no further role in anti-aircraft operations.