When the Big Bad Wolf donned grandmothery garb so as to surprise Little Red Riding Hood, he assured her that the big ears were “all the better to hear you with.” Essentially, the Big Bad Wolf was explaining the basic operating principle behind most of the world’s acoustic location devices.
Originally, acoustic location was used for ship detection in fog conditions but from mid-World-War-One to the early years of World War Two the devices were often used for aircraft detection. They were all rendered obsolete by the introduction of radar, but for a time they served a useful purpose in national defense.
If not effective they were at least distinctive. At the Brussels Inventor’s Fair of 1960, Frenchman Jean Ausgher exhibited his wearable acoustic navigation device. It was to be used by small ships in case of radar failure. The distance between the horns increased the observer’s ability to localize the direction of a sound. Unfortunately, in this case the horns weren’t far enough apart. With Ausgher’s device you would hear an oncoming vessel about the time it was to collide with you.
Operation of most large acoustic detectors usually required the use of three crewmen and four horns. One man’s task was to operate and adjust the elevation of the device for maximum reading, another adjusted for the greatest direction bearing, and a third reported the settings to a central location. Using several results from multiple detectors, the target’s location could be triangulated, and the information was then passed on to anti-aircraft defenses. The whole process was done in a surprisingly short period of time.
No detector was better than the German Ringtrichterrichtungshoerer (RRH). The detector was used mainly in anti-aircraft searchlight batteries for the detection of British night bomber formations. The RRH could detect targets at a distance of twelve kilometers, and depending upon weather conditions and operator skill, it could help detect the size of the aircraft formation. It had a directional accuracy of 2 degrees. The device had a crew of three with the dial reader in the middle. The rolled up material over the operator’s heads could be unfurled to provide cover in bad weather.
The Japanese “war tuba” is a name sometimes applied to Imperial Japanese military acoustic locators due to their visual resemblance to a musical tuba. The name derived from a misidentification, probably in jest, of a historical photo from the 1930s featuring the Japanese emperor Hirohito inspecting the acoustic locators with anti-aircraft guns in the background. It was used around major military targets and Tokyo.
The British and Americans also had small acoustic detectors of limited effectiveness. However, the British did build a series of huge stationary concrete “acoustic mirrors.” Inside these structures, a trained listener would use a stethoscope to detect the distance and direction of incoming aircraft as far as 20 miles away. A few of these remain in Britain today, and are now being preserved for their historical significance.
Another remarkable machine was a French acoustic locator based on a hexagonal layout. Each of the four assemblies carried thirty-six smaller, hexagonally-shaped horns. This layout was presumably used to increase the directional gain of the equipment. Because the detector was so large and out in the open, the type was abandoned after being repeatedly bombed by the enemy.
Acoustic detectors are still used today by television crews to pick up the sounds of players and coaches on the field during televised sporting events, where use of conventional microphones would be too intrusive. They are also used as novelty items — “whisper dishes” — in science museums to allow patrons to whisper across long distances.