A binaural sound recording is a sound recording of sound signals with microphones that are intended to create a natural auditory impression with precise directional localization when played back only through headphones. When recording in artificial head stereophony, an artificial head is often used.
In the past, the term binaural was often equated with stereo.
In general, stereo sound recordings are mixed solely via loudspeaker systems during listening, hence the name loudspeaker stereophony. The characteristics used by humans for localization, such as the shapes of the head or auricles, are rightly not taken into account; because when listening naturally and when playing back through stereo speakers in a stereo triangle, the ear itself forms the ear signals. Binaural recordings are a type of stereo recordings with special recording technology, which are typically only played back correctly with headphones, hence the term headphone stereophony. Binaural recordings, which replace the natural ear signals prevented by headphone playback, are the best technical solution to realistically reproduce a spatial auditory impression.
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Fundamentals of Listening
A sound event only becomes an auditory event when the sound waves have penetrated the auditory, are present in the brain as a stimulus and have been processed by it into an auditory perception. It is also not possible to directly compare sound and auditory events, since the shape of the stimuli is changed in the middle and inner ear. This means that the ear distorts the signal to a certain extent. These stimuli are sensations that vary from person to person and depend on the frequency, duration and sound pressure level of the sound event. Sensations are not measurable, but can be statistically recorded psychoacoustically by listening studies.
Humans are able to assign their perceived auditory events to certain directions. Humans have perceptual mechanisms for the half horizontal plane and the median plane.
The in-the-head localization is an unpleasant effect that can occur especially during headphone playback, but also during speaker playback. The auditory events are then no longer localized outside the head, but in the head.
The brain compares the incoming ear signals to known interaural signal differences and known spectral structures. If the signals have interaural time and level differences that are unknown in this combination, or if the signals have a spectral characteristic that has never been perceived with one’s own outer ear, in-the-head localization may occur. In the case of loudspeakers, this can occur if the polarity is reversed and if they are placed at an angle of more than 90°.
Such localization can be simulated experimentally by inverting the amplitude values of one of the two channels in a stereo signal whose two channels are identical, and listening to the resulting signal with headphones.
Binaural Recording Technique
In the simplest binaural recording method, you need two microphones that point sideways away from each other and have a distance (microphone base) of about 17.5 cm, which is approximately the position of the ear canals of an average person. A sound-absorbing or reflective separator, such as a soccer ball or a metal plate, is placed between the microphones to simulate a head.
More elaborate techniques consist of exact head replicas using an artificial head. A typical binaural recording device has two omnidirectional condenser studio microphones inserted into the ear canal of the artificial head.
The first stereo artificial head with a replica of the human ear canal was built as early as 1933. Neumann’s KU-81 and KU-100 artificial heads are the most widely used binaural microphones today. In the amateur sector, there was the MKE 2002 manufactured by Sennheiser, which could either be worn on one’s own head or hung into the ears of a plastic head.
There are also alternatives, such as the ear stud with microphone, that work in a similar way.
