What year was stereo sound patented? 
Stereophonic sound recording, which attempts to provide a more natural listening experience by reproducing the spatial locations of sound sources in the horizontal plane, was the natural extension to monophonic recording, and attracted various alternative engineering attempts. The ultimately dominant "45/45" stereophonic record system was invented by Alan Blumlein of EMI in 1931 and patented the same year. EMI cut the first stereo test discs using the system in 1933 (see Bell Labs Stereo Experiments of 1933) although the system was not exploited commercially until much later.