Stereophonic sound recording, as we know it today, comes from the patent of Alan Blumlein. He developed what he called a “binaural sound” in 1931. It started when he and his wife viewed a “talkie”(an early sound film) at a local cinema. Blumlein noted that the sound used a single speaker. He then declared to his wife that he knew how to make the sound follow the actors across the screen. Despite Blumlein’s initial description of stereo sound, today we associate the term binaural with the actual binaural sound effect, not stereo recording or sound systems. The wide use of the term as a music industry buzzword during the 1950s added to this confusion.
The binaural recording and its proper reproduction
A binaural recording is a type of recording technique that incorporates two microphones. They are arranged to create a 3-D sound resembling the experience of a listener positioned in the same room.
That binaural sound we can achieve by using a technique called “dummy head recording”. Here, we need to outfit each ear of a mannequin head with a single microphone. This intends to mimic how our ears receive sound. As it comes, it bounces off the head, gets filtered by our ear canal, reaches one ear and then the other, etc. These small deviations are known as HRTF`s, which is short for head-related transfer functions.
The binaural sound can be replicated using headphones after it has been recorded in this way. It doesn’t translate well with mono playback, nor on stereo speakers. In the case of the latter, the arrangement’s characteristic acoustics distort channel separation through natural crosstalk. This separation is essential to achieving the binaural effect. An approximation may be obtained if the stereo system uses crosstalk cancellation equipment, but this tends to be very costly. Thus, headphones have usually ideal configurations for the reproduction of binaural recordings, and several designers have produced high-end headphones specifically for it.
Alternatively, instead of a mannequin head, we could use a simpler separating element between the mics during recording. We can’t keep all cues needed for a true binaural sound that way but they can translate better on loudspeakers.
Issues with binaural sound
A common issue regarding binaural sound is timbral coloration. Mics, room, dummy head, and basically every part of the signal chain can cause this. Our ears have a very complex way to interpret sound as it comes in. The smallest coloration might mean insufficient or hampered HRTF data, which will affect the true binaural experience on the listener. The HRTF’s are also unique to everybody, so the end-user might simply not respond well to the collected data, adding to the difficulties of the reproduction.