The two 250lb loudspeakers are visible on the right-hand-side of the jeep.Credit:Drew Gardner/National Archive, Kew During the Normandy landings on June 6, 1944, Col Barlow’s sonic warfare vehicles were mounted on landing craft in the Pas de Calais area, 80 miles away from the real D-Day landing sites. They played recordings on 35-mm sound film and contributed to the delay of vital German reserves being sent to the Normandy beaches.Early tests involving a rather primitive ‘lash up’ of speakers fixed to chairs proved the concept, after which equipment was sourced from the US. The finished audio equipment, code named ‘Poplin’, consisted of two three-foot cube speakers, weighing 250 lbs each, and an amplifier mounted on a White Scout car which played the sounds of battle from a reel to reel tape player. The US army brought out a later version – ‘Blossom’ – which converted sound signals into electrical impulses to be carried on thin steel wire. A 30-minute recording could be generated from a mile and a half of the wire. Some of the White scout cars were adorned with paintings of Walt Disney characters as Col Barlow shared distant descendants with the American film producer from D’Isigny in France, and shared the name. The Poplin Sonic warfare unit mounted on the White scout car undergoing immersion tests, prior to the D-Day landings, believed to be in Ballantrae, Scotland.Credit:Drew Gardner Colonel Cecil Disney Barlow OBE.Credit:Drew Gardner Although tragically killed aged 39 on July 26, 1944, in the breakout from the Normandy beachhead, Col Barlow was noted for inventing ‘’an entirely new form of warfare which may well play an important part in future campaigns”. The citation for his OBE continues: “His own immediate military advancement may well have been prejudiced by the time he has devoted to this special subject but he has never permitted personal considerations to stand in the way of bringing his organisation to fruition.”Colonel Barlow is buried in Ranville war cemetery in Normandy. Britain’s ‘sonic warriors’ helped deceive the German army into thinking the D-Day landings would take place 80 miles away, previously unseen documents have revealed.Specially trained and equipped units played tapes of army activity, including tanks, trucks and even gunfire to fool the Germans into thinking they were facing a bigger force in a different location.Lord Mountbatten, head of Special Operations and responsible for new technical inventions, said sonic deception should be used to replicate a “feint landing to be carried out at some distance from the actual landing”.Previously unseen files at the National Archives at Kew had been covered by the 100-year secrecy rule and were originally meant to remain sealed until 2044.However, the Telegraph has obtained exclusive access to the files – stamped ‘MOST SECRET’ – and can reveal one of D-Day’s most extraordinary stories. The invention was so closely guarded that Col Barlow’s own family had no idea of his secret wartime role.Penelope Marland, his daughter, told the Telegraph: “I know that he was involved in something very secret but I was three when he died…and none of the relations told us anything.””It was his idea to make recordings to make it sound as if there was a big army about to attack, not in Normandy but pretending it was in Calais.” The idea of using sounds of war as a deception strategy was the brainchild of Colonel Cecil Disney Barlow of the Kings Shropshire Light Infantry. Having taken his idea to the War office in 1941 Col Barlow was tasked with developing his Light Scout Car units at a remote Scottish base on the Ayrshire coast at Ballantrae.The remote location was perfect to record the sounds of British Army Sherman and Churchill tanks, as well as other weapons and equipment, whilst not drawing attention from the locals. The tapes had to be made at night so as not to record unnecessary noises such as barking dogs.To this day local people have little idea of the work that was carried out at the Light Scout Car field park, as the area was known. Part of a top secret training document which depicts the deployment of the weapon system, from a file at the National records Office, Kew.Credit:Drew Gardner Want the best of The Telegraph direct to your email and WhatsApp? Sign up to our free twice-daily Front Page newsletter and new audio briefings.