In December 1931, a 28-year-old engineer at EMI named Alan Dower Blumlein filed one of the most astonishing patents in the history of audio: UK Patent 394,325, "Improvements in and relating to Sound-transmission, Sound-recording and Sound-reproducing Systems." It was granted on June 14, 1933. In 70 dense pages and dozens of claims, Blumlein single-handedly invented stereo — and most of the techniques the recording industry still uses to capture, mix, and cut it.

The legend is that Blumlein, watching a film at the cinema with his wife, was irritated that an actor's voice came from a speaker on the opposite side of the screen from the actor's face. He told her he'd figured out how to fix it. Within months, he had.

What the patent actually covers. Blumlein didn't just describe "two channels of sound." He worked out the underlying psychoacoustics — that human directional hearing depends on both interaural intensity differences (which ear hears it louder) at high frequencies and interaural time differences at low frequencies — and then engineered a complete system around it:

• The Blumlein Pair: two coincident bidirectional (figure-of-eight) microphones angled at 90°, capturing a stereo image purely through intensity differences. Still a working studio technique 95 years later.
• The "sum and difference" matrix (M/S encoding): combining Left+Right and Left−Right signals so a stereo pair could be transmitted over limited bandwidth or made mono-compatible. This is the math behind FM stereo broadcast (invented 1961) and behind modern stereo data compression in MP3 and AAC.
• The 45/45 stereo groove: cutting two channels into a single record groove by modulating each wall at 45° to vertical. RCA "rediscovered" this in 1957 and made it the universal vinyl stereo standard. Every LP since plays back using Blumlein's geometry.
• Stereo film soundtracks, stereo headphone listening, and the use of delay lines to simulate room acoustics — all sketched in the same filing.

The demonstrations. Blumlein's team made experimental stereo recordings in 1933–34 at EMI's Abbey Road studios, including a famous shellac disc of Sir Thomas Beecham conducting Mozart's Jupiter Symphony and a short film, Trains at Hayes Station, with stereo sound that moves convincingly across the screen. Listen today and the imaging is indistinguishable from a modern recording.

Why it vanished for 25 years. The Depression killed it. EMI couldn't justify the cost of stereo cutting lathes, stereo cartridges, and a parallel pressing infrastructure when mono records were barely selling. The patent sat. Blumlein moved on to designing the 405-line Marconi-EMI television system (still the most influential pre-war TV design) and then to wartime radar — he invented the AI Mk. IV airborne radar and was developing H2S ground-mapping radar when his test plane crashed in 1942. He was 38. He had filed 128 patents in 17 years.

Modern relevance. Open any DAW — Pro Tools, Logic, Ableton — and the "M/S" plugin you use to widen a mix is Blumlein's matrix. The stereo cartridge in any turntable traces his 45/45 groove. The Blumlein pair is taught in every recording school. Spatial audio formats like Dolby Atmos and Apple's binaural rendering still build on his core insight that directional hearing is decodable, encodable, and reproducible with the right geometry. Even Bluetooth's "joint stereo" codec mode is the sum-and-difference trick, in DSP form.

A single 1931 patent, written by an engineer who would be dead in eleven years, still defines how billions of people hear recorded sound.