Inflatable Sherman

“Aided by engineers from Bell Labs, a team from the 3132 went to Fort Knox to record sounds of armored and infantry units onto a series of sound effects records that they brought to Europe. For each deception, sounds could be ‘mixed’ to match the scenario they wanted the enemy to believe. This program was recorded on state-of-the-art wire recorders (the predecessor to the tape recorder), and then played back with powerful amplifiers and speakers mounted on halftracks. The sounds they played could be heard 15 miles (24 km) away.”

via Wikipedia