Echolocation

How echolocation works

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:

$\mathrm{distance}\mathrm{travelled}=\mathrm{speed}\mathrm{of}\mathrm{sound}\times \mathrm{time}\mathrm{travelled}$,

or, in symbolic form,

$s=v_{\mathrm{sound}}t$,

where$s$is the distance the sound has travelled in metres,$v_{\mathrm{sound}}$is the speed of sound through the medium that the sound is travelling through in metres per second, and$t$is the time between producing the sound and hearing its echo in seconds. The distance$d$to the object that the sound reflected off is given by$d=s/2$, because the sound will have travelled twice that distance once you hear the echo:

$\mathrm{distance}\mathrm{to}\mathrm{object}=\frac{1}{2}\times \mathrm{distance}\mathrm{travelled}\mathrm{by}\mathrm{sound}$.

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

Extra information from echolocation

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).

Uses of echolocation

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$20\mathrm{kHz}$, 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.

Uses of echolocation by animals

These 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

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.

Echolocation sound

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).

Echolocation types

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.

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.

Examples of echolocation

For these examples, assume that the speed of sound is about$1480\mathrm{m}/\mathrm{s}$through water and about$343\mathrm{m}/\mathrm{s}$through air.