What is  binaural?

 

I very often use the word “binaural” on this website, and therefore this page is dedicated to explain this term.

But before we look at the details, I want to clearly define the term “binaural”.

Definition of “binaural”

First, the term “binaural” is used in psychology.

There it describes the localization of sound in space, but also especially the so-called “binaural beats”. This is an acoustic illusion that is perceived when both ears are exposed to sound with slightly different frequencies. Unlike beats, binaural beats are not created by superimposing sound waves in the ear, but in the brain.

They are used, in addition to the exploration of the sense of hearing, to stimulate brain waves to promote relaxation, sleep, meditation or concentration.

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Second, in the context of hearing aid fitting, “binaural” means when both ears receive a hearing aid.

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Third, whereto I am referring here, however, is exclusively to the tonal meaning.

In this context, “binaural” describes the binaural sound recording.
To this day, it is the only way, apart from Ambisonics and WFS Holofonie, to reproduce the sound field of the recording room in all three dimensions of space.

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Ethymology

The Latin prefix “bi” means two.
Aural, from the Latin “auris”, means: what is relative to the ear. 

Binaural listening therefore refers to listening with two ears, which corresponds to our natural listening.
This is why the so-called binaural technique approaches a three-dimensional reconstruction of the sound environment, as in real life.

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Psychoacoustic Phenomena

To be able to locate a sound in space, our brain analyzes three fundamental psychoacoustic phenomena: the differences in time and intensity between the sound waves that arrive on each of our ears, as well as the spectral cues.

Let’s take an example: if a sound source is in front of me on my right, the sound wave emitted will arrive first on my right ear and will arrive with a slight delay on my left ear.
The sound intensity will be louder on my right ear than on my left ear (my head acting as an acoustic obstacle) and we will perceive the sound object with a particular timbre.

Now, if this sound object is now located at the back on my right, the differences in time and intensity will be about the same as in the previous example, but the timbre will be modified.

This is the particularity of binaural compared to stereophonic sound recording: spectral cues. It is thanks to this that the binaural allows hearing in three dimensions and allows to differentiate the back from the front and the top from the bottom.

Interaural runtime difference
Interaural level difference
Soundwave reflections into the ear

Techniques to create binaural sound

There are two main methods for creating binaural sound:

  1. either directly at recording, it is called native binaural or binaural recording, or
  2. at post-production, it is binaural synthesis or binaural by signal processing.

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The native binaural is based on a simple principle: the goal is to reproduce as accurately as possible the psychoacoustic phenomena mentioned above.

To do this, simply place microphones as close as possible to our eardrums to take advantage of the diffraction, absorption and reflection phenomena that occur on our heads and earpieces.

The principle is therefore to place small microphones in the ears of an artificial head (you can see many pictures of the head “Fritz”, my homemade binaural head).

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Dummy head microphone in front of a drum set
Microphone capsule in a dummy head
Dummy head microphone mounted on stand

Signal-processed binaural uses software based on the wave equations of classical acoustics.
These algorythms attempt to describe acoustic phenomena with a mathematical approach.
All you have to do is position the sound source where you want it to be using software!

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