The Soft Drone of Destruction - Acoustic Mirrors and the Aerial Campaign against Great Britain during WW1

Count Ferdinand von Zeppelin flew his first airship in 1898, but it took several years before they could be used for travel or air combat. Airships were used in combat for the first time in 1911 and 1912 by the Italians in their campaigns against the Ottoman Empire and various Libyan tribes. Meanwhile in Germany, Zeppelin and Schütte-Lanz airships became part of the Kaiser’s navy and an airship division was formed, although the German Army also had a limited number of airships.

Kaiser Wilhelm II was initially reluctant to use airships as a strategic weapon, but as the Western Front passed into stalemate and the French began bombing targets in Germany, reluctance turned in to reconsideration. It was hoped that attacks on the previously unreachable civilian population as well as production would undermine moral and lead to demands for peace from the population. This had been the case during the Franco-Prussian War of 1870-71, when the Prussian Army laid siege to Paris. However, Kaiser Wilhelm urged the airship crews not to attack the palaces of his cousins in London.

The first attacks on Great Britain were launched on January 19, 1915, and according to the Deputy Chief of the Naval Staff, Rear-Admiral Paul Behncke, a squadron of airships “successfully dropped numerous bombs in misty weather and rain. The ships were fired on, but returned undamaged." Five civilians were killed and 16 wounded. In all, 43 sorties were launched on the British Isles during 1915-1916, and more raids were to follow as the pace of the German strategic bombing campaign increased.

 A 1917 painting of a German airship crew

The British armed forces were initially not really sure how to defend England against the airship scourge. The threat of airborne attacks had been present since the outbreak of war, and there was something of a Zeppelin scare amongst the population. The airships were also difficult to engage for the frustrated defenders, since they flew at an altitude of up to 20,000 feet, and they had the ability to ascend much faster than early aircraft.  

However, the German naval communications had been compromised early in the war, and communication monitoring services could find out that a airship was heading towards England by listening for the phrase ”only HVB on board.” HVB (Handelschiffsverkehrbuch, the German procedures used for signaling merchant marine vessels) was an unclassified document and the only signaling book that was allowed on board while flying over enemy territory. This enabled the defenders to know that German airships were on their way, but not where the airships might be showing up. The defenses were initially weak. Available aircraft were few, old, and split between the Royal Flying Corps (RFC) and the Royal Naval Air Service (RNAS). The pilots had little or no training in night flying, and there were limited means of coordinating the few available resources. This led to the RFC advocating for a combination of search lights, anti-aircraft artillery, ground observers, wireless equipment, direction finders and aircraft being the best antidote to the German bomb raids.

One early warning system that was used both during and after the First World War was the acoustic mirror. These sound mirrors were typically concave round or oval concrete dish-like structures surrounded by a thick wall or frame surrounding the concave. These walls were usually protected by flanking walls to protect the reflection from noise interference and also to support the structure.  Experiments with acoustic mirrors started in 1915.

 Acoustic mirror with the "collector head" in front of it

The reflector was often a smooth bowl some 14 or 15 feet in diameter, inclined approximately 11 degrees to the vertical. The rear wall could be just short of 18 feet, while the flanking walls would be almost 12 feet tall. The reflected sound was detected by a “collector head”, i.e. a microphone placed on a metal pole in front of the dish and then transmitted to the headphones of the operator who sat in front of the microphone. By physically maneuvering the microphone to the angle that had the strongest noise reflection from the engines of the approaching airship or aircraft, the operator would be able to get a bearing in the direction of the raider. At least four acoustic mirrors were placed in positions in the vicinity of Folkestone and around the Thames Estuary.

The acoustic mirror at Namey Hill in Sunderland

 The acoustic mirrors amplified the sound of engines by as much as 20 dB, and the range was typically between 8 and 15 miles. The effective range was depending on both winds and background noise, and while sound pollution was limited during the First World War, the maximum range could only be attained when there was no or very little wind. Given that an airship typically cruised at around 50 miles per hour, the acoustic mirror would give between 15 and nine minutes of early warning in addition to the time it would take for the airship to fly from the location of the mirror to the target.

The acoustic mirror at Kilnsea

 The airships were joined by bomber aircraft on May 25, 1917, and German bombers conducted raids on Great Britain in general and London in particular until May 19, 1918, after which the raids were deemed to be too costly. By the end of 1916 17,341 officers and men were engaged in anti-aircraft defense of the British Isles, including twelve RFC squadrons with 110 aircraft, and the numbers were increased over the course of 1917 and 1918. Fifty-two airship raids and another 52 bomber raids were mounted on Great Britain during the First World War, killing 1,414 people and wounding another 3,416. Seventeen airships and 24 bombers were shot down, and many more were lost to accidents and errors. The effects of the German strategic air campaign where mainly psychological, although the resources tied to home defense were not inconsiderable. The air raids also led to the commissioning of the Smuts report written by General Jan Smuts which outlined a defense plan for Great Britain. This plan would eventually mature into the RAF air defense system of the Second World War that would be instrumental to the British victory in the Battle of Britain. It should also be mentioned that the last year of the First World War saw No. 44 Squadron deployed as one of the home defense units. This squadron was commanded by Arthur Harris, who eventually would take strategic bombing to an entirely different level.

German airships were indeed often referred to as "Baby-Killers" in British propaganda

The experiments with acoustic mirrors continued during the 1920s and 1930s, but the advent of radar and the increased aircraft speeds made acoustic mirrors obsolete. Today several lichen-encrusted acoustic mirrors can be found in the British countryside. Electromagnetic waves have since replaced their acoustic counterparts, but the acoustic technology that contributed to early warning against bomb raids almost a century ago is being re-invigorated for more artistic purposes: http://www.soundmirrors.org/