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Have you ever heard that bats use echolocation to understand their surroundings? Since they are generally nocturnal with poor vision, they use sound in order to probe their environment, making them a deadly night-time predator. But how is this possible? In this article, we will discuss the meaning of echolocation, how it works, where it is used, and we will cover some examples.
The meaning of echolocation is already in the word itself.
Echolocation is the use of echoes, i.e. reflected sound, to locate objects.
The basic principles behind echolocation are the constant nature of the speed of sound through a given medium (another word for material) and the fact that sound waves partially get reflected off of interfaces (another word for boundary surfaces) between different mediums. The idea is to produce a sound travelling in a particular direction, wait until you hear its echo, and then calculate the distance the sound has travelled. This can be done using the equation of constant motion:
,
or, in symbolic form,
,
where
is the distance the sound has travelled in metres,
is the speed of sound through the medium that the sound is travelling through in metres per second, and
is the time between producing the sound and hearing its echo in seconds. The distance
to the object that the sound reflected off is given by
, because the sound will have travelled twice that distance once you hear the echo:
.
See the figure below for a schematic representation of echolocation.
A bat is producing the sound indicated with blue, and the echo through reflection off of the prey is indicated with red, Wikimedia Commons Public Domain
You are in an empty and quiet space, and there is a big wall facing you. You want to know how far away it is, so you clap your hands and record the time that passes until you hear an echo. You see that it took
. You conveniently know the speed of sound in air to be
, so you calculate the distance
to the wall as follows:
.
You conclude that the wall is
away from you.
This is not the only information that can be gathered from an echo. In addition, by employing knowledge of the Doppler effect, the frequency of the echo compared to the frequency of the original sound gives information about the velocity of the object. Furthermore, the loudness of the echo (the amplitude of the sound wave) can be used to gather information about the reflection of the sound wave, which is influenced heavily by the density of the object. Therefore, the density of the detected object can also be determined. Lastly, if you can determine the direction of objects (see below for how animals do this), then you can also determine the size of objects by seeing in which directions the object is present (bear in mind that you already know how far away the object is, so some simple trigonometry will give you a size).
Echolocation is used by animals and ships, but it is also used in medicine. One part of a ship's use of sonar is the use of echolocation. A ship can send an ultrasonic sound wave (i.e. of a frequency higher than
, outside the range of human hearing) through the water, and detect its echo. This way, it can map objects that are around the ship as well as the seabed, and it uses this information to navigate.
In medicine, an ultrasound scan can be used to look at an unborn baby. For this, we use equipment that emits ultrasound waves and can also detect them. The soundwaves will reflect off of tissue boundaries, so the equipment can form a 3-dimensional image of where all the tissue boundaries are. This way, it can construct a complete image of the baby inside the womb. This is also a form of echolocation. See the article on applications of ultrasound for more information on the use of echolocation by ships and in the medical field.
These Eurasian (or common) shrews use echolocation to detect their insect prey at night in dense undergrowth. The shrew's eyesight is very poor, so it relies heavily on echolocation to understand its surroundings, Wikimedia Commons CC BY-SA 3.0
Some animals that use echolocation are bats, some birds, whales, some shrews, and even some blind people. Using echolocation means that you don't have to rely on your eyesight to navigate. For all animals, this is useful during the night-time. For animals that live underwater, this is also useful during the daytime, because light does not travel very far through water, while sound does. Thus, whales are able to "see" a lot further using echolocation than using their eyes, which is highly beneficial in the vast oceans.
In general, the sounds produced by animals for echolocation are very loud so that they can perceive objects that are far away, and very high-frequency (mostly ultrasonic) because high pitches give information with better resolution (this can be compared to lightwave resolution and the use of scanning electron microscopes).
All echolocation is based on the same principle. One could choose to differentiate between biological and technological echolocation and call them different types. One could also choose to differentiate between what medium is used for the sound to travel through, which would categorise bats (air) and whales (water) into using different echolocation types. There is also a big difference in how animals determine the position of objects.
Dolphins have a special organ that concentrates their sound into a beam travelling in a particular direction. If an echo is heard, this must come from an object in that direction. Dolphins can 'scan' a whole animal by producing sound beams in multiple directions, and are able to tell different animal species apart via this method.
Echolocation used by a dolphin, where the organ above its jaws produces the concentrated beam of sound, Wikimedia Commons CC BY-SA 3.0
Alternatively, bats emit sound in all directions (they effectively just scream), but they use the fact that they have two ears to determine the direction that an echo is coming from, and the difference in timing and loudness that both ears perceive gives the bats the information they need. For example, if the right ear hears the echo before the left ear, then the echo must come from the right, so the object must be to the right. This is also how humans can interpret the direction of a sound.
For these examples, assume that the speed of sound is about
through water and about
through air.
A dolphin produces a short sound burst of
, and after
she hears an echo. How far away is the object that the dolphin just perceived?
Answer: The dolphin lives underwater, so sound will travel at the speed of sound in water,
. We calculate the distance
from the dolphin to the object by using the formula relating the travel distance
of the sound, the echo time
, and the speed of sound in the medium:
.
We conclude that the perceived object is
away from the dolphin.
A bat produces a screech and after
he hears an echo. His left ear picks up the sound earlier than his right ear. How far away is the perceived object from the bat and what direction is it in?
Answer: The sound from the echo is coming from the left, so the object is to the left of the bat. The bat is on land or flying in the air, so the sound will travel through the air. We calculate:
.
We conclude that the perceived object is
away from the bat.
is the speed of sound in the correct medium and
is the time between producing the sound and hearing the echo, then the distance
to the perceived object is given by 
.Echolocation is the use of echoes, i.e. reflected sound, to locate objects.
Echolocation works by producing a sound and measuring how much time passes before you hear an echo. This tells you how far away the object is.
Examples of echolocation are bats, whales and other animals using it to detect prey and predators, but also boats using it as part of their sonar to map the underwater world.
Some blind people have managed to develop a basic version of echolocation, so yes, this is possible with practice.
Echolocation cannot be divided into a standard list of types, but some difference between uses of echolocation are the sound medium, the production of the sound, the perception of the echo, and the pitch and intervals of the original sound.
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