Sound localization refers to the ability of the human auditory system to determine the location of a sound source in space. This is done by analyzing the differences in the arrival time, intensity, and spectral content of the sound waves that reach the two ears. The human ear is able to localize sounds both horizontally (azimuth) and vertically (elevation) in the auditory space.
The brain processes the incoming sound signals from both ears to calculate the interaural time difference (ITD) and interaural level difference (ILD), which are used to determine the location of the sound source. Interaural time difference refers to the difference in the time it takes for a sound wave to reach each ear, while interaural level difference refers to the difference in the level of the sound wave that reaches each ear.
The auditory system uses both ITD and ILD as complementary cues that work together to allow for accurate sound localization in the horizontal plane, aka stereo field. For example, sounds coming from straight ahead might have similar ITDs at both ears but different ILDs, while sounds coming from the side might have similar ILDs at both ears but different ITDs.
It’s also worth noting that the relative importance of ITD and ILD can vary depending on the frequency of the sound. At low frequencies, ITD is the dominant cue for sound localization, while at high frequencies, ILD becomes more important. Research has suggested that the crossover frequency between ILD and ITD cues for human sound localization is around 1.5 kHz to 2.5 kHz, with ITD cues being more useful below this frequency range and ILD cues being more useful above this range.
In addition to ITD and ILD, the auditory system also uses spectral cues, such as the shape of the outer ear and the filtering effects of the head and torso, to determine the location of sounds in the vertical plane and also to identify backside audio events.
The temporal characteristics of an audio event, such as its onset and duration, can have an impact on sound localization as well. Generally speaking, sounds with a more distinct onset, such as a drum hit, are easier to localize than sounds with a more sustained signal, such as white noise. This is because the onset of a sound provides a more salient cue for the auditory system to use in determining the location of the sound source, especially in regards to ITD.
In the case of a drum hit, the sharp onset creates a more pronounced difference in the arrival time and intensity of the sound at the two ears, which makes it easier for the auditory system to use ITD and ILD cues to locate the sound source. In contrast, with a more sustained signal like white noise, the auditory system may have to rely more on spectral cues and reverberation in the environment to determine the location of the sound source.
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