Long before radar became standard military technology, armies around the world had to rely on something far more basic to find enemy aircraft in the sky: sound. The solution was a range of large, strange-looking devices built around giant horns or microphones designed to capture the engine noise of approaching planes and airships. These acoustic location systems were in active military use from the middle of World War I through the early years of World War II, giving ground crews their only real warning of an aerial threat before the aircraft came into view.
The basic design connected large horn-shaped receivers to an operator’s ears through stethoscope-style earphones. The horns gathered and amplified sound from the sky, and trained listeners would try to identify the direction and distance of aircraft engines. Operators rotated and adjusted the devices until the sound was strongest, which gave them a rough bearing on the target. The systems were entirely passive — they emitted nothing and simply listened, which made them useful for nighttime and poor-weather operations when visual spotting was impossible.
By 1917, portable versions had already appeared on the battlefield. German artillery units fielded wearable sound-locating apparatus that individual soldiers could carry and operate, allowing acoustic detection to move with front-line forces rather than being fixed to a single position. These personal units were far less powerful than the large horn arrays, but they gave field commanders a mobile option for tracking aircraft movement during operations.
One of the earliest well-documented uses of acoustic location came in 1916, when Commander Alfred Rawlinson of the Royal Naval Volunteer Reserve deployed the equipment while commanding a mobile anti-aircraft battery along England’s east coast. His specific problem was locating German Zeppelins during bad weather and heavy cloud cover, conditions that made visual spotting useless. The sound locators gave his crews at least some ability to track the airships’ approach and direct anti-aircraft fire even when the sky was completely obscured.
After the war, militaries continued developing and testing acoustic location equipment. The United States Army experimented with multi-horn configurations at facilities like Bolling Field in Washington, D.C., where paired horn systems were evaluated for accuracy and range. These postwar tests helped refine the geometry of the devices — the wider the spacing between receiving horns, the more precisely operators could triangulate a target’s position in the sky.
German manufacturers were among the most active producers of acoustic locating equipment. The Goerz company produced early listening devices featuring curved receiving shells designed to focus incoming sound waves toward a central point. These shells functioned on the same principle as a cupped hand held behind the ear — a simple amplification technique scaled up to an industrial level.
The U.S. Army developed its own horn-based locators, some of which reached considerable size. Four or more large horns were often mounted together on a rotating platform, allowing a single crew to scan the sky efficiently. The combined input from multiple horns helped operators cross-reference signals and narrow down the direction of an incoming aircraft more accurately than a single-horn device allowed.
Germany’s Askania company produced the Doppelt Richtungshörer, a dual-direction listener designed to give operators both horizontal and vertical bearing information simultaneously. By capturing sound on two planes at once, the device let crews estimate not just the compass direction of an aircraft but also its altitude — a critical piece of data for directing anti-aircraft guns or searchlights.
France fielded its own acoustic detection equipment through manufacturers like Barbier, Bénard et Turenne, a firm better known for producing lighthouse optics. Their sound-locating devices reflected the same precision engineering philosophy applied to their optical work. French military planners integrated these systems into their broader air defense network during the interwar period.
The Netherlands also built acoustic locators, producing equipment starting in 1934 for both domestic use and deployment with the Netherlands Army in the East Indies. The Dutch designs were engineered for practical field use by the Engineers Regiment. Alongside these larger systems, Dutch engineers also explored miniaturized listening devices throughout the 1930s, trading some detection range for the ability to move equipment quickly between positions.
Japan built some of the most visually striking acoustic locators of the era. By 1936, the Imperial Japanese military was using massive trumpet-shaped electric listening devices — often referred to as ‘war tubas’ in Western press coverage — that dwarfed a standing soldier. These enormous units funneled engine noise into electric microphone systems, which amplified the signal and fed it to operators seated at control stations. They were imposing pieces of equipment, and photographs of Japanese soldiers operating them attracted considerable attention in the international press.
By the late 1930s, acoustic detection had spread across every major military power, but the technology had serious limitations. Aircraft were getting faster. A plane detected by sound was often already directly overhead or past the listener by the time the operator could react. The time delay between an aircraft’s actual position and the sound reaching the horns grew worse as engine speeds increased. Operators had to calculate where a plane would be — not where it was — which made targeting increasingly difficult.
Britain kept pace with developments on the continent. Four-horn arrays were in use throughout the 1930s, giving anti-aircraft crews across England a ground-level early warning capability. These systems worked alongside searchlight batteries, with sound locators providing an initial bearing that searchlight crews could use to start scanning before the aircraft came close enough to be caught in the beam visually.
Germany formalized this searchlight-support role with the Ringtrichterrichtungshoerer, or RRH. This acoustic locator was specifically designed for use with anti-aircraft searchlight batteries, giving crews an initial aiming point for night operations before radar could confirm the target. The RRH used a ring of small funnel-shaped receivers arranged in a circle, which operators adjusted until all receivers picked up sound equally — the point at which the device was aimed directly at the source.
Some acoustic devices took on unusual forms. The Shout-O-Phone, photographed in 1940, represented one of the more unconventional takes on sound amplification for military purposes. Meanwhile, the Soviet Union continued fielding acoustic locators well into the war years, equipping crews with ZT-4 sound locators that remained in service even as radar became available. Soviet forces operated in conditions — particularly on the Eastern Front — where maintaining every available detection method made practical sense.
Acoustic location had one fundamental problem that no amount of engineering could fully solve: the speed of sound. Sound travels at roughly 767 miles per hour at sea level. By the 1930s and into the war years, military aircraft were approaching or exceeding 300 miles per hour. That meant the sound reaching a listener’s ears always lagged behind the plane’s true position by several seconds — enough time for a fast aircraft to move hundreds of meters. Radar, which uses radio waves traveling at the speed of light, eliminated this delay entirely. Once radar networks were established in Britain and then across other nations, acoustic systems became obsolete almost immediately.
