Identifying the direction from which a sound comes, even with just one ear, is possible due to several factors that our auditory system takes into account. Here are some cues used to determine sound localization:
Intensity: When a sound source is closer to one ear than the other, it generally appears louder in the ear closest to the source. By comparing the intensity of the sound in each ear, our brain can infer the direction of the sound.
Timing: Sound waves take slightly different times to reach each ear depending on the direction of the source. Our brain processes these timing differences, known as interaural time differences (ITDs), to determine the location of the sound. This works best for low-frequency sounds because the wavelength is longer, allowing for more noticeable time differences.
Phase differences: In addition to the timing differences, the phase of sound waves can also vary between the ears. This is called interaural phase difference (IPD) and is most relevant for higher-frequency sounds.
Head-related transfer function (HRTF): The shape of our head, ears, and torso affects how sounds are filtered before reaching our ears. Our brain has learned to recognize these subtle filtering cues and uses them to localize sound. This is why sounds may appear different when they come from different directions.
Spectral cues: Depending on the direction of the sound, certain frequencies may be emphasized or attenuated by the head and outer ears. These spectral cues provide additional information that helps us determine sound localization.
By combining these cues and using our experience and learned patterns, our brain can estimate the direction of sound sources, even with just one ear. However, it's important to note that localization accuracy is generally better with both ears, as the brain can utilize the cues from both sides for a more precise determination of sound direction.