In acoustics, localization is the recognition of the direction from which a sound comes in three-dimensional space and the recognition of the distance of the sound source. Localization is based on directional hearing with both ears (binaural) and distance hearing, which is also possible by hearing with one ear (monaural). Localization is a passive process in which only the received sound is localized by the ears, without the involvement of other sensory organs.
What is localization?
Localization is a passive process in which exclusively the received sound is localized by the ears, without involvement of other sense organs. In medicine, the term localization is used by several specialties with different conceptual content. For example, the term is used in neurology for the assignment of motor and mental functions to specific brain areas. In most cases, localization is understood as the ability to hear direction and distance without the involvement of other senses. Recognition of the direction from which sound comes in three-dimensional space usually requires bilateral (binaural) hearing because, among other things, the brain uses the slight travel time differences of the sound between the two ears for direction recognition. The shape of the auricles also plays a role. In principle, distance hearing also works with only one ear (monaural), because distance hearing can only take place indirectly. The brain evaluates certain qualities of the sound such as loudness, frequency spectrum and sound reflections, compares them with empirical values and “estimates” the distance of the sound source from them. Direct distance hearing is not possible, as this would only be possible in combination with directional hearing and would require a significantly greater distance between the left and right ear for sound sources that are further away. The unconscious comparison of the parameters of the received sound with empirical values plays the greatest role in distance hearing of a sound source.
Function and task
The localization of a sound source by auditory impression alone, without the involvement of other senses such as vision, is of great importance to humans. The localization ability is used to localize sound sources according to the classification of dangerous or not dangerous, in order to derive an action decision from the classification and from the localization. The special feature is that localization is possible even with limited vision or with complete loss of vision. For example, localization and the additional estimation of a vehicle’s speed via auditory perception provides a decision-making aid for crossing a busy road without danger – even with severely impaired vision. Furthermore, the localization of a sound source allows in some cases also a navigational rough orientation. In a wooded area with no foresight and no other means of orientation, the localization of a sound source, especially the determination of the direction from which the sound is coming, can provide a means of orientation. Directional hearing usually requires bilateral (binaural) hearing. In the case of laterally located sound sources, the brain can “calculate” the position of the sound source from the differences in propagation time between the left and right ear, which amount to only a few milliseconds, and from the level differences resulting from shadowing effects of the head. If sound sources have to be localized centrally in front of or behind the body or above, binaural hearing does not provide clear results for physical reasons. Here the outer ear with the special shape of the auricles and the auditory canal plays a special role. Resonances, sound reflections and slight frequency distortions at the auricles can be evaluated by the brain in such a way that the localization of the sound source is possible, for example, from the front or the back. Simple verification is possible by turning the head so that the sound source is on the side, as localization then achieves the highest accuracy.
Diseases and complaints
Unrestricted directional and distance hearing are the prerequisites for being able to clearly localize a sound source. This means that limitations in localization ability are usually associated with hearing loss in one or both ears. If a unilateral hearing impairment is present, directional hearing is particularly impaired.However, it is astonishing that even in the case of a total loss of hearing on one side, directional hearing is not completely lost, since the loss of hearing in one ear can be compensated to a small extent via the effects of the auricle. A central hearing loss that affects both ears equally may be a conductive or sensorineural hearing loss. The latter also includes a hearing loss in which the problems are either in the conversion of the physical sound vibrations into nerve impulses in the cochlea, or there are limitations in the neural transmission and/or processing of the signals in the auditory centers in the CNS. This means that the ability to localize is also impaired because not enough or incorrectly processed auditory signals arrive at the auditory centers or the incoming signals cannot be processed correctly. The impaired ability may be temporary or permanent. For example, neurotoxic poisons cause temporarily impaired localization ability. This also includes excessive alcohol consumption or other drug use. Directional hearing requires a particularly sensitive auditory system, so that any central auditory disorder directly affects directional hearing and thus localization ability. Tinnitus and other central hearing disorders also have a diminishing effect on directional hearing. Often, the onset of hearing loss is not recognized until there is symptomatic dysfunction in directional hearing.
