Seeing with your Ears – Echolocation – Part I

Robert Traynor
August 14, 2012

This week at Hearing International we will begin a series on Echolocation.  There have been reports of humans using echolocation as part of mobility training for the blind.  Seeing with your ears?  Is that possible? As audiologists we all know lot about hearing and hearing rehabilitations, amplification and other techniques, but using the auditory system to see may be a bit foreign to us.   If it is possible, how does this work?  Bats do it, dolphins do it, but could humans?  Is it really used by the visually impaired, or this just a myth.

Donald Griffin (August 3, 1915 – November 7, 2003) was the zoologist who discovered how bats navigate. He also coined the term “echolocation” to describe how bats are able to find their way when flying in the dark. In addition to his discovery of bat echolocation, his work helped in the development of radar and sonar.

Let’s review how echolocation works with bats.  Echolocation is the use of sound waves and echoes to determine where objects are in space. Bats use echolocation to navigate and find food in the dark. To echolocate, bats send out sound waves from their mouth or nose. When the sound waves hit an object they produce echoes. The echoes bounce off the object and return to the bat’s ears. Bats listen to the echoes to figure out where the object is, how big it is, and its shape. Using echolocation, bats can detect objects as thin as a human hair in complete darkness.

Fenton of the University of Western Ontario reports that our view of the natural history of bats is inextricably linked to echolocation, even though not all bats echolocate and not all bats that do use echolocation in the same way. The availability of commercially produced, inexpensive bat detectors is one reason for the explosion in data linking echolocation with the biology of bats. Among microchiropteran bats (i.e., the sub-order of bats that use echolocation),  the species can be divided based on type of echolocation behavior: 1) species using high intensity versus low intensity calls; 2) species producing calls at low versus high duty cycle.

Furthermore, microchiropterans use a wide range of frequencies in echolocation, from around 10 kHz to over 200 kHz. Simmons et al (2008)  have found that bats with small cochleas do not use echolocation while those that use echolocation have larger cochleas, an interesting point for audiologists.  Limited range and information leakage are two major disadvantages of echolocation. It is becoming increasingly obvious that echolocation calls can simultaneously serve a communication role in bats.

Tejarachi (2008) discusses how dolphins use sound to detect the size, shape, and speed of objects hundreds of yards away. Fascinating and complex, the dolphin’s natural sonar, called echolocation, is so precise it can determine the difference between a golf ball and a ping-pong ball based solely on density. Although humans have researched these intelligent marine mammals for decades, much of their acoustical world remains a mystery.

One of the keys to dolphin echolocation is water’s superb conduction of sound. Sound waves travel 4.5 times faster in water than they do in the air. Dolphins use this to their advantage, in ways that would make a superhero envious. Using nasal sacs in their heads, dolphins send out rapid clicks that pass through their bulbous forehead, or “melon.” The sound is focused, then beamed out in front of the dolphin. The sound wave speeds through the water, bounces off the object under investigation, and is reflected back to the dolphin. Fat-filled cavities in the dolphin’s lower jaw receive this information and auditory nerves conduct it to the middle ear and brain, where an acoustic picture is created.

Scientists say that dolphins may also use clicking to communicate with one another. Although dolphins do not possess vocal cords, they still “speak” using sounds such as whistles, squeaks, and trills. A mother dolphin may whistle to her newborn for days, apparently to imprint a signature whistle upon her baby that will enable it to recognize her. It is believed that dolphins use whistles to identify one another and possibly for other functions, such as communicating strategic alerts while hunting in a group, but scientists have yet to crack the code. Many doubt, however, that dolphins have a formal language akin to that of humans.  There are some that have used these concepts among humans…….

Next week we will investigate the principles of echolocation and their use among visually impaired humans…..

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