Human echolocation

Human echolocation is the ability of humans to sense objects in their environment by hearing echos off those objects. This ability is used by some blind people to navigate within their environment. They actively create sounds, such as by tapping their canes or by making clicking noises with their mouths. Human echolocation is similar in principle to active sonar and to the animal echolocation employed by some animals, including bats and dolphins.

By interpreting the sound waves reflected by nearby objects, a person trained to navigate by echolocation can identify the location and sometimes size of nearby objects, and use this information to steer around obstacles and travel from place to place. However, since humans make sounds with much lower frequencies and slower rates, human echolocation can only picture comparatively much larger objects than other echolocating animals.

Background
Human echolocation has been known and formally studied since at least the 1950s. Before it was known to be based on localization of echos, human echolocation was sometimes described as facial vision.   The field of human and animal echolocation was surveyed in book form as early as 1959.

Some blind people have described the phenomenon not as a learned method of navigation, but as an inherent and intuitive extra sense. For example, a blind person could walk past a line of trees and feel a "pressure" at their side as they passed each tree. The cause of this would be the echo of the sound of their footsteps, however they may not be consciously aware of this mechanism, only that the phenomenon exists and can often be relied upon to detect obstacles.

Mechanics
Vision and hearing are close cousins in that they both can process reflected waves of energy. Vision processes light waves as they travel from their source, bounce off surfaces throughout the environment and enter the eyes. Similarly, the auditory system processes sound waves as they travel from their source, bounce off surfaces and enter the ears. Both systems can extract a great deal of information about the environment by interpreting the complex patterns of reflected energy that they receive. In the case of sound, these waves of reflected energy are called "echoes."

Echoes and other sounds can convey spatial information that is comparable in many respects to that conveyed by light. With echoes, a blind traveler can perceive very complex, detailed, and specific information from distances far beyond the reach of the longest cane or arm. Echoes make information available about the nature and arrangement of objects and environmental features such as overhangs, walls, doorways and recesses, poles, ascending curbs and steps, planter boxes, pedestrians, fire hydrants, parked or moving vehicles, trees and other foliage, and much more. Echoes can give detailed information about location (where objects are), dimension (how big they are and their general shape), and density (how solid it is). Location is generally broken down into distance from the observer, and direction (left/right, front/back, high/low). Dimension refers to the object's height (tall or short) and breadth (wide or narrow).

By understanding the interrelationships of these qualities, much can be perceived about the nature of an object or multiple objects. For example, an object that is tall and narrow may be recognized quickly as a pole. An object that is tall and narrow near the bottom while broad near the top would be a tree. Something that is tall and very broad registers as a wall or building. While something that is broad and tall in the middle, while being shorter at either end may be identified as a parked car. Whereas an object that is low and broad may be a planter, retaining wall, or curb. And finally, something that starts out close and very low, but recedes into the distance as it gets higher is a set of steps. Density refers to the solidity of the object (solid/sparse, hard/soft). Awareness of density adds richness and complexity to one's available information. For instance, an object that is low and solid may be recognized as a table, while something low and sparse sounds like a bush; but an object that is tall and broad, and very sparse is probably a fence.

James Holman
One of the earliest documented cases of echolocation was "the blind traveler" James Holman (1786-1857) who used the sound of a tapping cane to become one of the 19th century's greatest world travelers.

Daniel Kish
Echolocation has been further developed and popularized by Daniel Kish. He is completely blind and taught himself to derive information about his surroundings by clicking his tongue and listening to the echo; using this technique, he is able to ride a bike and hike in unknown wilderness. He has developed a click-emitting device and trains other blind people in the use of echolocation.

Ben Underwood
Diagnosed with retinal cancer at the age of two, American Ben Underwood had his eyes removed at the age of three.

When Ben came round from the surgery to remove both eyes when he was child he was crying saying "I can't see you anymore mommy, I can't see you"; and his mother Aquanetta Gordon said "You can smell me, you can hear me and you have got your ears, nose and mouth."

He discovered echolocation at the age of five. He does not use a cane but is able to detect not only the position, size and frequently composition of objects near him, but sometimes their shape, by making clicking sounds with his tongue about once every half a second. He now uses it to accomplish such feats as running, basketball, rollerblading, foosball and skateboarding.