Binaural, in the broad sense, means “hearing with both ears,” that is, listening to sound with the left and right ears. In the audio field, the term binaural source is generally used for material recorded with equipment that has microphones built into the positions of the ears on a model of the human head, known as a dummy head.
A dummy head captures sound reflections and diffraction caused by the head, pinnae, and shoulders, together with differences in arrival time and level between the two ears. Played back over headphones, it therefore gives a three-dimensional impression and a sense of direction, as though one were listening at the very position of the dummy head. This strong sense of immersion allows binaural, too, to be regarded as a form of immersive audio.
Dummy-head recording, however, did not become widely used in music production. The sound field is difficult to adjust after recording, the reproducibility varies among individuals because of differences in ear shape. In addition, because information from the opposite ear is also mixed into each channel, the original three-dimensional effect is difficult to reproduce over loudspeakers.
In recent years, however, techniques for converting 3D audio sources such as Dolby Atmos and AURO-3D into binaural sources for headphones using HRTF (head-related transfer function) have advanced. This technique is known as binaural rendering. The three-dimensional sound experienced over headphones through Apple Music’s Spatial Audio and similar services also makes use of this technology.
