phase differences) and interaural level differences play a role for the hearing of many animals. but the influences on localization of these effects are dependent on head sizes, ear distances, the ear positions and the orientation of the ears. smaller animals like insects use different techniques as the separation of the ears are too small. lateral information (left, ahead, right) if the ears are located at the side of the head, similar lateral localization cues as for the human auditory system can be used. this means: evaluation of interaural time differences (interaural phase differences) for lower frequencies and evaluation of interaural level differences for higher frequencies. the evaluation of interaural phase differences is useful, as long as it gives unambiguous results. this is the case, as long as ear distance is smaller than half the length (maximal one wavelength) of the sound waves. for animals with a larger head than humans the evaluation range for interaural phase differences is shifted towards lower frequencies, for animals with a smaller head, this range is shifted towards higher frequencies. the lowest frequency which can be localized depends on the ear distance. animals with a greater ear distance can localize lower frequencies than humans can. for animals with a smaller ear distance the lowest localizable frequency is higher than for humans. if the ears are located at the side of the head, interaural level differences appear for higher frequencies and can be evaluated for localization tasks. for animals with ears at the top of the head, no shadowing by the head will appear and therefore there will be much less interaural level differences, which could be evaluated. many of these animals can move their ears, and these ear movements can be used as a lateral localization cue. odontocetes dolphins (and other odontocetes) rely on echolocation to aid in detecting, identifying, localizing, and capturing prey. dolphin sonar signals are well suited for localizing multiple, small targets in a three-dimensional aquatic environment by utilizing highly directional (3 db beamwidth of about 10 deg), broadband (3 db bandwidth typically of about 40 khz; peak frequencies between 40 khz and 120 khz), short duration clicks (about 40 μs). dolphins can localize sounds both passively and actively (echolocation) with a resolution of about 1 deg. cross-modal matching (between vision and echolocation) suggests dolphins perceive the spatial structure of complex objects interrogated through echolocation, a feat that likely requires spatially resolving individual object features and integration into a holistic representation of object shape. although dolphins are sensitive to small, binaural intensity and time differences, mounting evidence suggests dolphins employ position-dependent spectral cues derived from well-developed head-related transfer functions, for sound localization in both the horizontal and vertical planes. a very small temporal integration time (264 μs) allows localization of multiple targets at varying distances. localization adaptations include pronounced asymmetry of the skull, nasal sacks, and specialized lipid structures in the forehead and jaws, as well as acoustically isolated middle and inner ears. in the median plane (front, above, back, below) for many mammals there are also pronounced structures in the pinna near the entry of the ear canal. as a consequence, direction-dependent resonances can appear, which could be used as an additional localization cue, similar to the localization in the median plane in the human auditory system. there are additional localization cues which are also used by animals. head tilting for sound localization in the median plane (elevation of the sound) also two detectors can be used, which are positioned at different heights. in animals, however, rough elevation information is gained simply by tilting the head, provided that the sound lasts long enough to complete the movement. this explains the innate behavior of cocking the head to one side when trying to localize a sound precisely. to get instantaneous localization in more than two dimensions from time-difference or amplitude-difference cues requires more than two detectors. localization with coupled ears (flies) the tiny parasitic fly ormia ochracea has become a model organism in sound localization experiments because of its unique ear. the animal is too small for the time difference of sound arriving at the two ears to be calculated in the usual way, yet it can determine the direction of sound sources with exquisite precision. the tympanic membranes of opposite ears are directly connected mechanically, allowing resolution of sub-microsecond time differences and requiring a new neural coding strategy. ho showed that the coupled-eardrum system in frogs can produce increased interaural vibration disparities when only small arrival time and sound level differences were available to the animal's head. efforts to build directional microphones based on the coupled-eardrum structure are underway. bi-coordinate sound localization (owls) most owls are nocturnal or crepuscular birds of prey. because they hunt at night, they must rely on non-visual senses. experiments by roger payne have shown that owls are sensitive to the sounds made by their prey, not the heat or the smell. in fact, the sound cues are both necessary and sufficient for localization of mice from a distant location where they are perched. for this to work, the owls must be able to accurately localize both the azimuth and the elevation of the sound source. history the term 'binaural' literally signifies 'to hear with two ears', and was introduced in 1859 to signify the practice of listening to the same sound through both ears, or to two discrete sounds, one through each ear. it was not until 1916 that carl stumpf (1848–1936), a german philosopher and psychologist, distinguished between dichotic listening, which refers to the stimulation of each ear with a different stimulus, and diotic listening, the simultaneous stimulation of both ears with the same stimulus. later, it would become apparent that binaural hearing, whether dichotic or diotic, is the means by which sound localization occurs. scientific consideration of binaural hearing began before the phenomenon was so named, with speculations published in 1792 by william charles wells (1757–1817) based on his research into binocular vision. giovanni battista venturi (1746–1822) conducted and described experiments in which people tried to localize a sound using both ears, or one ear blocked with a finger. this work was not followed up on, and was only recovered after others had worked out how human sound localization works. lord rayleigh (1842–1919) would do these same experiments and come to the results, without knowing venturi had first done them, almost seventy-five years later. charles wheatstone (1802–1875) did work on optics and color mixing, and also explored hearing. he invented a device he called a "microphone" that involved a metal plate over each ear, each connected to metal rods; he used this device to amplify sound. he also did experiments holding tuning forks to both ears at the same time, or separately, trying to work out how sense of hearing works, that he published in 1827. ernst heinrich weber (1795–1878) and august seebeck (1805–1849) and william charles wells also attempted to compare and contrast what would become known as binaural hearing with the principles of binocular integration generally. understanding how the differences in sound signals between two ears contributes to auditory processing in such a way as to enable sound localization and direction was considerably advanced after the invention of the stethophone by somerville scott alison in 1859, who coined the term 'binaural'. alison based the stethophone on the stethoscope, which had been invented by rené théophile hyacinthe laennec (1781–1826); the stethophone had two separate "pickups", allowing the user to hear and compare sounds derived from two discrete locations.