The US developed this existing noise-maker into the FXR, which was to be towed singly around 500 feet behind the ship. This entered production by July 1943. The single noise-maker however would work effectively only if the torpedo was equally sensitive to sounds from both the front and rear. If a torpedo passed directly over the noise-maker from the rear the ship itself would be directly ahead, if the torpedo had a reduced sensitivity to noise from behind, it might ignore the greater output from the noise-maker, and proceed to home on the ship.