How Do Bats Avoid Obstacles in the Dark?
– The Way They "See" With Bouncing Sound
In pitch-black caves and night skies, bats race around at high speed without hitting a branch or a power line. In that darkness they can also catch a tiny flying insect with precision. Eyes are almost useless there, so what is the bat relying on to fly?
At dusk, have you ever seen small shadows zigzagging over a park or a riverbank? Many of those are bats. In conditions where a person can barely see anything, bats fly freely. They don't hit power lines, branches or other bats.
In fact, bats keep making high-pitched sounds that people can't hear, from the mouth or nose, as they fly. They listen for the "reflected sound" that bounces back when those sounds hit things around them. This is thought to let them "see" their surroundings with sound.
This way of sensing surroundings by sound reflection is called echolocation.
Most bat calls are thought to be sounds of such a high frequency (ultrasound) that the human ear cannot pick them up.
The bat is thought to work out how far away an object is from the time the sound takes to bounce back and reach its ears.
Let's look at this "distance from the time delay of sound" step by step.
Sound Travels Only at a Fixed Speed
Sound travels through air at a fixed speed of roughly 340 metres per second. That is far slower than light. Even over everyday distances, we can notice the difference in travel time. The pause between a firework flashing and its bang reaching you is also thought to come from this slowness of sound.
Bats are thought to use this fact, that the speed of sound is fixed, to measure distance. Sound bouncing back from a nearby object reaches the ears quickly. Sound from a distant object arrives a little later. The length of this "delay" is the clue to distance.
They are simply telling apart the tiny lengths of time it takes for sound to return.
Why Use Sounds Too High for People to Hear?
Most of the sounds bats make are thought to be ultrasound, with frequencies far above the range of the human ear. High-pitched sounds have short wavelengths, so they reflect clearly even from small things such as tiny insects or thin twigs. With a low sound (a long wavelength), small objects don't reflect it, and it tends to pass straight by.
To spot and catch a small flying insect accurately, a high-frequency sound that can reflect from fine objects is thought to be an advantage.
Telling Moving Prey Apart by Changes in the Sound
Bats are thought to read not only distance but also whether the prey is moving, from the sound. The clue is the same effect that makes an ambulance siren sound higher as it approaches and lower as it moves away. This is called the Doppler effect.
When a bat calls while closing in on an insect, the frequency of the returning echo is slightly higher than the original sound. From the size of this change, the bat is thought to read how fast it and the prey are closing on each other.
Finding distance and position from reflected sound is used in many technologies, including fish finders that locate the seabed and shoals of fish from a ship, submarine sonar, and car obstacle sensors. Inspired by bats, researchers are also developing devices that let people with visual impairments sense where objects are by sound.
Things You Can Check Yourself
- Find a large place where sound reflects well, such as a gym, a tunnel or the front of a big wall
- Stand a few metres from the wall and clap your hands once, sharply
- Listen closely for a slightly weaker "echo" bouncing back from the wall just after the clap
- Move away from the wall and closer again, and check how the time before you hear the echo seems to change
Even the human ear can sometimes sense the tiny delay before a sound bounces back. A bat tells such delays apart far more finely and accurately.
Summary
Bats are thought to make ultrasound that people can't hear and to judge the distance to things around them from the time it takes the echo to return. Because sound travels through air at a roughly fixed speed, the length of time is a direct clue to distance. From the change in the echo's frequency (the Doppler effect), they are also thought to read how fast their prey is moving.
Even in darkness where eyes are no help, bats can see a "map made of time delays".
For another animal that senses the unseen without sound, see our article on sharks, which feel the faint electric signals given off by prey through their skin.
For Those Who Want to Know More – Terms, Numbers and Links to TextbooksFrom middle-school science to topics under research, each part is labelled with its level
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school "Basic Physics"
- High school+High-school "Physics", or material treated as advanced or a sidebar in textbooks
- UniversityUniversity-level specialist subjects (neuroscience, biology) not taught in high school
- ResearchTopics researchers are studying now, not yet taught even at university as settled knowledge
Middle schoolTerms: Words Around Echolocation
- Ultrasound: sound of a high frequency, above the range the human ear can hear (roughly 20 Hz to 20,000 Hz).
- Echolocation: a way of finding where things are by listening to the reflections (echoes) of sounds you make yourself.
- Doppler effect: the change in the pitch you hear when the source of a sound and the listener move toward or away from each other.
High schoolChecking With a Formula: Distance From the Time Delay
Let's calculate the distance between a bat and an object from the speed of sound and the time it takes the sound to return.
Distance to object = speed of sound × round-trip time ÷ 2
| Speed of sound | Said to be about 340 metres per second in air |
| Round-trip time | The time from sending the sound until the echo returns |
| ÷2 | The sound covers the distance to the object twice, on the way out and on the way back |
| Round-trip distance (m) | 1.7 × 2 ＝ 3.4 |
| Time taken (s) ＝ round-trip distance ÷ speed of sound | 3.4 ÷ 340 ＝ 0.01 |
| Convert seconds to milliseconds | 0.01 × 1000 ＝ 10 |
| Result | About 10 milliseconds (10/1000 of a second) |
To pick out a branch just 1.7 m away, a bat has to tell apart a very short time delay of 10 milliseconds (1/100 of a second). In actual flight, echoes keep coming back from objects at many different distances, so the bat's ears and brain are thought to process these time delays extremely fast.
* The speed of sound changes with temperature, and 340 m/s is only an approximate figure.
High school+Echoes From Approaching Prey Have a Higher Frequency
With the Doppler formula we can work out how the speed at which the bat closes on its prey relates to how much higher the echo's frequency becomes. Suppose a bat approaches its prey at 5 m/s while sending out 50,000 Hz ultrasound.
| Frequency of the sound reaching the prey (Hz) | 50000 × 340 ÷ 335 ≒ 50746 |
| Frequency of the echo returning to the bat (Hz) | 50746 × 345 ÷ 340 ≒ 51492 |
| Difference from the original sound (Hz) | 51492 − 50000 ＝ 1492 |
Because the Doppler effect acts on both the outward and return trips, the calculation shows that the echo comes back about 1,500 Hz higher than the original sound. From the size of this frequency change, the bat is thought to read how fast it and the prey are closing.
UniversityHow Does the Brain Handle Countless Echoes?
In neuroscience, it has been reported that the bat's auditory nervous system has nerve cells specialised for detecting the tiny delay between sending a sound and the echo's return. These cells are thought to let the bat instantly separate and process overlapping echoes that come back almost at once from several objects.
ResearchWhat Is Still Unclear
- How a bat picks out just the echoes it needs in a complicated setting, such as a forest thick with leaves where reflections tangle together, is still being studied in neuroscience.
- There are reports that when many bats fly in the same place at once, they seem to adjust the frequency and timing of their calls so their ultrasound doesn't interfere with each other. The details of this "jamming avoidance" are also still under study.
- Research is also under way on applications inspired by bat echolocation, such as sound-based aids for people with visual impairments and obstacle-detection technology for robots.
The abilities of bats flying in the dark open up a rich field, where the physics of sound meets neuroscience and where research is still going on.
Links to Textbooks (by Level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: properties of sound | Basic terms: ultrasound and echolocation |
| High school | Basic Physics: sound waves | Calculating distance from the speed of sound and time |
| High school+ | Physics: Doppler effect (advanced) | Calculating the frequency change of echoes from approaching prey |
| University | Neuroscience | How the auditory nervous system detects time delays |
| Research | Neuroscience and behavioural ecology (under study) | Picking out echoes in complex settings; avoiding interference among many bats |
- Explanations of bat echolocation in biology textbooks and reference materials.
- Explanations of the speed of sound and the Doppler effect in physics textbooks.
- Research reviews in neuroscience on time-delay detection in the bat auditory nervous system.
- Research reviews in behavioural ecology on how bats avoid ultrasound interference with each other.
- Explanations in acoustic engineering of obstacle-detection technology that applies echolocation.
* Figures such as the speed of sound and frequencies are commonly used approximate values. Actual values are said to vary with temperature, the individual animal and the situation.
* This article is a general science explainer. When observing wild bats, take care not to approach them needlessly or disturb their roosts.