Monaural cues

Auditory localization is analogous to depth and scale perception for vision, which was covered in Section 6.1. Since humans have a pair of ears, localization cues can be divided into ones that use a single ear and others that require both ears. This is analogous to monocular and binocular cues for vision. A monaural cue relies on sounds reaching a single ear to constrain the set of possible sound sources. Several monaural cues are [371]:

1. The pinna is shaped asymmetrically so that incoming sound is distorted in a way that depends on the direction from which it arrives, especially the elevation. Although people are not consciously aware of this distortion, the auditory system uses it for localization.
2. The amplitude of a sound decreases quadratically with distance. If it is a familiar sound, then its distance can be estimated from the perceived amplitude. Familiarity affects the power of this cue in the same way that familiarity with an object allows depth and scale perception to be separated.
3. For distant sounds, a distortion of the frequency spectrum occurs because higher-frequency components attenuate more quickly than low-frequency components. For example, distant thunder is perceived as a deep rumble, but nearby thunder includes a higher-pitched popping sound.
4. Finally, a powerful monaural cue is provided by the reverberations entering the ear as the sounds bounce around; this is especially strong in a room. Even though the precedence effect prevents us perceiving these reverberations, the brain nevertheless uses the information for localization. This cue alone is called echolocation, which is used naturally by some animals, including bats. Some people can perform this by making clicking sounds or other sharp noises; this allows acoustic wayfinding for blind people.