Have you ever heard of three-dimensional sound recordings? Well, it may come as a surprise to you, but these types of audio recordings have even been used commercially. This concept may not seem all that important to the average consumer. However, if you are seriously into audio engineering, you need to ask – What is Binaural Recording?
In my personal experience, I have not heard the subject of today’s article being discussed a lot among audio practitioners. I guess this is understandable, since 3D sounds are rarely used in conventional music production. I will briefly explain to you the principles behind this recording method, and how it affects listeners. Let us start learning!
Introduction – 3D? Sounds Cool!
Essentially, binaural recording is a sound recording method that uses two microphones, positioned in a way that creates a 3-D stereo sound experience for listeners, akin to being in the room with the performers. This effect is typically achieved by using a technique called “dummy head recording”, where a mannequin head is outfitted with a microphone in each ear.
Binaural audio recordings are only suitable for playback through headphones and will not translate correctly over stereo speakers. The concept of 3D or “internal” form of sound has also influenced many useful advances in audio technology such as stethoscopes creating “in-head” acoustics and IMAX movies being able to produce a three dimensional acoustic experience.
This is unlike stereo recordings, where the natural ear spacing of the head and ears is not considered. This factor naturally occurs as a person listens, generating their own ITDs (interaural time differences) and ILDs (interaural level differences). Since conventional stereo produces “loudspeaker-crosstalk” which interferes with binaural reproduction, headphones are used instead.
A pinna-less dummy head may be ideal for quasi-binaural recordings, that are intended for listening through conventional stereo systems. The basic principle behind true binaural sound, is that an audio recording and reproduction system chain, from microphone to listener’s brain, should contain only one set of pinnae (preferably the listener’s own) and one head-shadow.
Here are some other topics that we’ll be looking at:
- Recording techniques
- Playback requirements
- Frequency response
- Common uses
The most simple recording practice includes two microphones that are placed 18 cm apart facing away from each other. This technique will not produce a true binaural recording, but the distance and position roughly represents the position of an average human’s ear canals. More elaborate techniques however, requires specialized recording equipment.
A complete binaural recording unit consists of a dummy head with two high-fidelity microphones mounted inside, that has inset ear-shaped molds to capture all of the audio frequency attenuation that happens naturally as sound wraps around the human head and is “shaped” by the form of the outer and inner ear. The Neumann KU-81, and KU-100 are the most well known binaural recording sets.
You can also use microphones with a separating element between them (such as the Jecklin Disk microphone technique), in order to produce a stripped down version of binaural recordings. Do take note that by using this method, not all physical information required for exact localization of the sound sources are preserved, but it is more suitable for loudspeaker reproduction.
The effect of binaural recordings can be faithfully reproduced through headphones or a dipole stereo. Mono playback and stereo loudspeaker systems do not support binaural recordings, as the acoustics of these systems distorts the channel separation due to crosstalk (this can be remedied by carefully designing the listening environment to include expensive crosstalk cancellation equipment).
Even consumers who are just using a cheap set of headphones can enjoy binaural recordings. There are also several manufacturers that have produced units that are specially engineered for binaural playbacks. However, these in-ear-canal phones usually have poor externalization (inside-head localization) and even normal headphones suffer from this, especially if it completely blocks the ear from outside.
Through experiments, it has been found that open-ear headphones are more suitable, where the drivers are sitting in front of the pinnae with the ear canal being “opened” to the air. The general opinion is that when the ear canal is completely blocked, the radiation impedance seen from the eardrum to the outside will change, which negatively affects externalization.
During natural hearing, there tends to be a boost from 2kHz to 5kHz. Since headphones are very near to the ears, they actually nullify this natural frequency response by the head and ear. Hence, most headphones are designed to produce the 2kHz to 5kHz frequency boost, which reflects natural hearing. Even the so-called “high quality” headphones have this same boost of frequencies.
For binaural listening however, headphones require a flat frequency response to the ear-canal entrance (not eardrum), since the recording microphones are originally placed at the ear-canal entrance. Although it is theoretically possible to equalize headphones for a flat response at the entrance, an ideal solution is to use headphones designed without the boost in the first place.
Most people are not aware of the 2 kHz to 5 kHz boost that most headphones have, and this may have led to an inaccurate assessment of binaural recordings, since flat headphones are not common. If headphones have a flat frequency response, and used with headphone amplifiers that have equalizers, then there could be a more controlled listening experience, whether in binaural or stereo mode.
Binaural recordings are rarely used in commercial audio productions, due to the high cost of specialized equipment required to produce high quality recordings. Also, the types of sounds that can be recorded in the studio do not typically have a high market value. It is mainly used for live orchestral performances, and ambient “environmental” recordings of nature, city sounds, and other similar subject matters.
That’s all I have for you folks today. Are you ready to incorporate binaural recordings into your projects? Well, at the very least, I hope that you are now more familiar to the principles behind it.
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