Binaural Sound Recording
Riley Dyen
Binaural recording is a method of recording sound that uses two microphones, arranged with the intent to create a 3D stereo sound sensation for the listener of actually being in the room with the performers or instruments. This effect is often created using a technique known as dummy head recording, wherein a mannequin head is fitted with a microphone in each ear. Binaural recording is intended for replay using headphones and will not translate properly over stereo speakers. This idea of a three-dimensional or "internal" form of sound has also translated into useful advancement of technology in many things such as stethoscopes creating "in-head" acoustics and IMAX movies being able to create a three-dimensional acoustic experience.
The term "binaural" has frequently been confused as a synonym for the word "stereo", due in part to systematic misuse in the mid-1950s by the recording industry, as a marketing buzzword. Conventional stereo recordings do not factor in natural ear spacing or "head shadow" of the head and ears, since these things happen naturally as a person listens, generating interaural time differences (ITDs) and interaural level differences (ILDs) specific to their listening position. Because loudspeaker-crosstalk with conventional stereo interferes with binaural reproduction (i.e. because the sound from each channel's speaker is heard by both ears rather than only by the ear on the corresponding side, as would be the case with headphones), either headphones are required, or crosstalk cancellation of signals intended for loudspeakers such as Ambiophonics is required. For listening using conventional speaker-stereo, or MP3 players, a pinna-less dummy head may be preferable for quasi-binaural recordings such as the sphere microphone or Ambiophone. As a general rule, for true binaural results, an audio recording and reproduction system chain, from the microphone to the listener's brain, should contain one and only one set of pinnae (preferably the listener's own), and one head-shadow.
The history of binaural recording goes back to 1881.[1] The first binaural unit, the thtrophone, was invented by Clment Ader.[1] It consisted of an array of carbon telephone microphones installed along the front edge of the Opera Garnier. The signal was sent to subscribers through the telephone system, and required that they wear a special headset, which had a tiny speaker for each ear.
The dummy head recording is associated with the use of the physical synthetic head called the Kunstkopf. The Kunstkopf would be placed in concert halls during the recording of a live orchestra or in the film industry actors could stand around the head whilst recording their dialogue. The dummy head could also be used to imprint positional information on prerecorded sound effects by playing sounds through a loudspeaker in a suitable orientation to the head. For example, thunder and birdsong sounds to be played above the dummy head.
In 1974 Virgin Records issued the first solo album by Tangerine Dream's leader Edgar Froese, titled Aqua. The brief sleeve notes inform listeners that side 2 of the disc (i.e. the tracks NGC 891 and Upland) were recorded using the artificial head system developed by Gunther Brunschen. Listeners were advised to optimize their listening by using stereo headphones for that side of the album.
Although Edgar was keen to continue to use and promote this system for subsequent recordings, it was abandoned, due to the fact that, although it worked well through headphones, the improved sound quality did not translate adequately through a hi-fi speaker system.
Binaural stayed in the background due to the expensive, specialized equipment required for quality recordings, and the requirement of headphones for proper reproduction. Particularly in pre-Walkman days, most consumers considered headphones an inconvenience, and were only interested in recordings that could be listened to on a home stereo system or in automobiles. Lastly, the types of things that can be recorded do not have a typically high market value. Studio recordings would have little to benefit from using a binaural set up, beyond natural cross-feed, as the spatial quality of the studio would not be very dynamic and interesting. Recordings that are of interest are live orchestral performances, and ambient "environmental" recordings of city sounds, nature, and other such subject matters.
During the 1990s, electronic devices which used digital signal processing (DSP) to reproduce HRTFs were made commercially available. These devices would allow the sound engineer to use dialled parameters to adjust the apparent direction of real time sounds. They were unusual and expensive, but would allow the sound engineer to alter special effects of prerecorded sounds quickly and conveniently. Through the manipulation of the parameters, sound engineers could take a monophonic recording of a passing car and make it sound as if it were passing behind them in real time. Recording with an actual dummy head for the same outcome would require a recording booth and a moving speaker, or an array of speakers as well as multiple panning or switching devices.
The modern era has seen a resurgence of interest in binaural, partially due to the widespread availability of headphones, cheaper methods of recording and the general increased commercial interest in 360 audio technology.
The rise of Dolby Atmos and other 360 audio film technology in relation to commercial entertainment has seen a rise in popularity of the use of binaural simulation. This is with the purpose of fully adapting the 360 soundtrack for headphones and earphones. Users can ostensibly watch 360 films and music with the immersive surround sound experience remaining intact despite using just the two headset speakers. Notably, any full 360 multi-channel soundtrack is automatically converted to simulated binaural audio when listened to with headphones.
In 2015, Singaporean singer-songwriter JJ Lin released his debut experimental album From M.E. to Myself, using dummy head recording. This is also the first album in pop music industry using this technology.[4]
With a simple recording method, two microphones are placed 18 cm (7") apart facing away from each other. This method will not create a real binaural recording. The distance and placement roughly approximate the position of an average human's ear canals, but that is not all that is needed. More elaborate techniques exist.
A typical binaural recording unit has two high-fidelity microphones mounted in a dummy head, inset in ear-shaped molds to fully capture all of the audio frequency adjustments (known as head-related transfer functions (HRTFs) in the psychoacoustic research community) that happen naturally as sound wraps around the human head and is "shaped" by the form of the outer and inner ear. The dummy head (also known as artificial head, Kunstkopf[7] or Head and Torso Simulator) is used to generate binaural recordings. The tracks are then listened to through headphones allowing for the listener to hear from the dummy's perspective. The dummy head is designed to record multiple sounds at the same time enabling it to be exceptional at recording music as well as in other industries where multiple sound sources are involved.
The dummy head is designed to replicate an average-sized human head and depending on the manufacturer may have a nose and mouth too. Each dummy head is equipped with pinnae and ear canals in which small microphones are placed, one in each ear.[8] The leading manufacturers in Dummy Head design are: Brel & Kjr, Head Acoustics GmBH, Knowles Electronics, and GRAS Sound & Vibration.[8]
The dummy head manufacturers design their products differently to one another catering for specific situations. The GRAS dummy's are flexible: they include head or torso with replaceable pinnae of different sizes and materials, set of different type ear-canal simulators, ear-drum simulators. Heads may include or not include mouth simulator. Brel & Kjr design includes soft moulded pinnae, nose, mouth and torso. Any dummy head or HATS can be used to record audio of the same nature but different types of them are specifically designed to accomplish different tasks. A new manufacturer to the market for recording binaural is 3Dio with the purpose of recording on a smaller scale. The 3Dio microphones are situated in the ears at an average head distance apart, however the model does not include the full head or torso.
The main focus of recording with a dummy head is to achieve a perfect binaural playback that is suited to all listeners. The problem arises that each human head has different shaped and sized features. Due to the diversity in HRTFs it is impossible to create a binaural effect compatible for everyone's ears. Therefore, the simulated dummy head recording algorithm uses average HRTFs to create a moderate binaural effect for everyone.[6]
Conventional music recording is produced for stereo playback which makes use of only Left and Right playback for speakers and headphones. The implementation of Dummy Head allows the recording artist to make use of three dimensional sound reproduction. This is because through playback via headphones the listener perceives sound as if they were in the position of the dummy. The recording is perceived through the pinnae of the dummy head.
The technique of binaural re-recording is simple but has not been well established. It follows the same principles of Worldizing,[10] a technique used by film sound designers in which sound is played over a loudspeaker in a real-world location and then re-recorded, taking along all the aspects and characteristics of the real-world environment with it.[11]
In binaural re-recording, a binaural microphone is used to record content being played over a multi-channel speaker set-up. The binaural head, or microphone, is therefore theoretically making a recording of how humans will hear multi-channel content. The soundtrack to a film, for example, will be recorded by the binaural microphone with all the environmental cues of the given location, as well as reverberations, including those commonly created by the human torso (assuming a HATS[13] model is used). This method, like certain binaural recordings made with a Neumann KU 100.[14]
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