What is an echo as it happens. What is an echo? Preparing and conducting research
An echo occurs when sound waves propagating outward from a source (called incident waves) collide with a solid obstacle, such as a mountainside. Sound waves are reflected from such obstacles at an angle equal to the angle of their incidence.
The key factor for the echo to occur is the distance between the obstacle and the sound source. When the obstacle is nearby, the reflected waves travel back quickly enough and mix with the original waves without creating an echo. If the obstacle is at least 15 meters away, the reflected waves return after the incident waves have scattered. As a result, people will hear the repeated sound as if it were coming from the side of the obstacle. Acoustic engineers must design auditoriums and concert halls with echoes in mind, adding sound-absorbing elements and eliminating excessively reflective surfaces.
Reflection rule
In this experiment, low-frequency waves from the sound generator pass through glass tube A, are reflected from the mirror and enter tube B. The experiment proves that the angle of reflection of the wave is equal to the angle of its incidence.
During the day - faster
Sound travels more rapidly in warm air near the ground (picture below text) and slows down when it reaches the colder upper atmosphere. Such a change in temperature leads to refraction (deflection) of the wave upward.
Slower at night
Low nighttime air temperatures near the surface of the earth slow down the passage of sound (figure below the text). In the warmer overlying layers, the speed of sound increases.
Sound carried along with the wind
The wind speed at significant altitudes is much higher than near the ground. When sound waves travel from a ground source, they travel with the wind. The listener to the wind will hear only a faint, subtle sound; the downwind listener will hear the bell at a very great distance.
Echo. What do we know about this interesting physical phenomenon? Anyone who remembers well the school course of physics will surely answer that an echo is a physical phenomenon, the essence of which is the acceptance by the observer of a wave that is reflected from some obstacle. However, the echo is by no means as easy as it seems. This article will provide some interesting facts about this truly amazing physical phenomenon that you may not have known. So, let's begin.
What is an echo?
As mentioned above, an echo is a sound wave reflected from an obstacle (however, it can also be electromagnetic, but you, of course, will not hear such an echo). The reflected sound waves return to the observer (the source of the noise), who can sometimes hear them much later. It is this sound reflected from obstacles that is called an echo. 
About the origin of the word echo
This word has a rather interesting history. In Russian it came from the German word echo. This word got into the German language, like many other words in Western European languages, from Latin - ēсhō ... And Latin took over this word from Greek - ἠχώ , which meant "echo".
Conditions for the existence of an echo
Several conditions are required for the echo to appear. Have you ever wondered why the echo is not heard in an apartment or store, but at the same time it is extremely easy to hear it in the mountains? The fact is that the human ear hears an echo only when the reflected sound sounds separately from the spoken one, and does not "lay down" on it. To create such an effect, it is necessary that the time elapsed between the influence of the sound itself and the reflected wave on the ear must pass at least 0.06 seconds. In a normal environment (for example, in an apartment) this will not happen due to the short distance and various objects that also absorb sound. 
Sometimes the echo is suppressed
There is a term "echo cancellation". It is used in telephony. The echo cancellation process is the removal of unnecessary echo in a communication that degrades the quality of the communication. Echo cancellation is needed not only to improve sound quality, but also to increase the bandwidth of the communication channel.
There is a room where there is no echo at all. It's called an anechoic chamber. Anechoic chambers are of two types. Each type serves to "muffle" one type of echo or another. Simply put, in such a chamber, sound (or radio waves) simply do not bounce off the walls. The first is the acoustic type. It, as the name implies, serves to suppress the usual sound echo. The second, respectively, radio frequency and is necessary to suppress the reflection of radio waves. 
Light echo is an astronomical term. This phenomenon occurs when there is a sharp flash of light (for example, in the outbreak of new stars). With such a flash, light is reflected from objects and reaches the observer much later. 
World echo
World echo, also known as "long delay echo", is a special effect associated with radio waves. This special type of echo is sound, sometimes occurring in the short wavelength range, that returns some time after the signal is transmitted. This unusual and difficult to explain phenomenon was in 1927 by the Scandinavian Jorgen Hals.
Ancient Greek myth about the nature of echo
The ancient Greeks explained many natural phenomena with myths. The echo was no exception. The myth of the birth of the echo goes something like this: once the jealous wife of Zeus, Hera, punished the beautiful nymph Echo, forbidding her to answer questions - Echo could only repeat the last words addressed to her. Echo saw the beautiful Narcissus walking through the woods. He, hearing the rustle, exclaimed:
- -Who is there?
- -Here! Echo shouted back.
- -Come here!
- -Here! - Echo answered happily, running towards Narcissus, but he pushed her away, because he believed that only he himself was worthy of his love. So the beautiful nymph now hides in the forests and mountains, occasionally repeating the words of the travelers.
About echolocation
Everyone knows that bats and dolphins use echolocation to orient themselves in space. However, few people can answer the question "how does it all work?" And it works like this. The mouse primarily emits ultrasound. Then she catches the echo of that very sound emitted to her, reflected from objects. The bat has the ability to recognize ultra-short intervals that go from emitting a sound signal to returning an echo. Thus, the mouse determines the distance between trees or other objects, and also "sees" how far this or that insect is from it. Surprisingly, the bat perfectly distinguishes the echo from a static (immovable) object from a moving object.
Echolocation was discovered in dolphins more than half a century ago. Dolphins, like bats, use ultrasound, mainly frequencies 80-100 Hz... The signals emitted from dolphins are incredibly powerful: for example, they can "see" a school of fish at a distance of more than one kilometer! 
Small interesting facts
- If the distance from the noise source is to the nearest obstacle (wall or rock), no echo is generated.
- The famous German river Rhine is full of surprises. For example, there is a place where the echo repeats 20 times
- In the city of Verdun in France, there are two towers. If you shout while standing between them, then you will hear the echo from your voice up to 11 times.
- The ear of Dionysus is a true echo record holder. This is a grotto in Syracuse, in shape that really resembles a human ear. But that is not what makes it interesting. Due to its shape, the mainsail makes the echo incredibly strong. A throw of a stone or a simple clap will echo out of the darkness with real thunder
Sometimes it happens that you walk through the woods with your friends, scatter in different directions and begin to have fun with each other.
Suddenly ... what is it?
You hear that someone is pronouncing your own words, only muffled, quietly, even a little sadly. Echo!
Everyone really likes the echo, it’s funny to listen to it, and you start shouting to the whole forest: "Hey! .." - and call back for a long time.
But what is this - an echo? Why does it happen?
You shouted - and the air vibrated, because every sounding body vibrates: the strings of the violin, harp, piano vibrate, your vocal cords vibrate when you speak. The sounding body vibrates, and a wave spreads from it in all directions through the air, and when it reaches your ear, you hear a sound.
But here the sound wave hits some kind of obstacle, like a sea wave to the shore, and comes back, and for the second time you hear your voice, but only quiet, because the wave gradually weakens.
The echo you hear is far from always and not everywhere. This requires certain conditions: the obstacle that the sound wave stumbles upon must be at a sufficient distance so that the wave does not have time to return in one tenth of a second, because our ear can perceive the same sound wave no less than through such an interval time.
This is what an echo is. That's why it happens.
The person understood the nature of the echo, understood its mechanics. And on the basis of the laws of reflection of a sound wave, man created a wonderful device - an echo sounder.
This device, installed on board the ship, sends a sound wave into the depths of the sea. The sound spreads in the water, reaches the bottom and returns back, it is picked up by the device again. Knowing the speed of sound propagation in water and tracing how much time elapsed between the sending and receiving of sound, scientists determine the depth of the sea in this place.
And if you send the sound not into the depths of the sea, that is, not vertically, but horizontally, then you can determine how far the ship is from the coast, or during fog, find out if there are any obstacles ahead that the ship risks to stumble upon: Is there a ship coming towards, or is an ice mountain-iceberg floating? The sound wave bumps into an obstacle and comes back, it is picked up by a device called a sonar, and it reports the obstacle to the captain.
More than once a white dolphin during strong storms saved ships, seeing them off among dangerous reefs and underwater rocks. The sailors knew him well, loved him and declared his life inviolable. They called the dolphin "the white pilot", and the pilots are specialists who navigate ships along the well-studied waterway, along the fairway to ports.
This Bulgarian story tells about the life of various marine animals and about natural locators, thanks to which they swim freely in the depths of the sea, without fear of ripping their belly open on treacherous reefs, fleeing from enemies. The locator is a wonderful safety device. It is not only in marine animals.
The bat also has a natural locator.
For a very long time the behavior of these small animals, which fly freely in the dark, never bumping into any obstacles, always cleverly avoiding them, was a mystery to scientists. And on the fly, they still manage to destroy mosquitoes and very tiny mosquitoes. Meanwhile, the eyes of bats do not differ in visual acuity; on the contrary, they see very poorly.
What is the matter here?
Only recently, about thirty years ago, scientists have solved this secret. It turns out that bats have their own locator. They emit sounds that we cannot hear, our ear does not pick them up; these sounds bump into an obstacle, come back, and the mice catch them with their huge ears. So their eyes, in general, are unnecessary: \u200b\u200bears replace their eyes, they seem to illuminate the world around them with powerful sound headlights.
There is a wonderful invention created by man on the principle of reflection of a wave, only not sound, but radio waves.
Radio waves also have the ability to bounce off objects that cross their path. And just before World War II, scientists created a device capable of detecting enemy aircraft from afar, even before they appeared overhead. This device is called radar, otherwise - radar.
The radar can detect both enemy aircraft in the sky and enemy ships at sea, and determines both the distance to them and the direction.
Radars are needed not only during war, but also in peacetime. They are great helpers. Meteorologists use them to determine the speed and direction of winds at high altitudes, and detect the accumulation of thunderclouds. Astronomers, sending a radio wave to our closest satellite, the Moon, were able to determine very accurately the distance to it. These are just two examples, and there are many of them.
Echo! The nature of this phenomenon has long been explained. But in ancient times it seemed mysterious and wonderful. And the ancient Greeks invented a poetic legend about the forest echo.
... Once upon a time there was a beautiful nymph named Echo in the forest. She freely frolicked, sang and danced like her friends - the goddesses of meadows, streams, springs ... But poor little nymph Echo angered the formidable, powerful goddess Hera, and Hera punished her by forbidding her to talk. Nymph Echo could only now repeat other people's words.
That is why we sometimes hear the sad voice of the nymph Echo in the forest. And Pushkin, captivated by the poetry of the forest echo, created wonderful poems about him:
Whether the beast roars in the deaf forest, Does the horn blow, does the thunder rumble, Does the maiden over the hill sing - For every sound Your response in the empty air You suddenly give birth.
Even if you have never been to the mountains, you still probably know what an echo is and have met it more than once. Echo can lie in wait for us anywhere in the arch of a house, in an empty apartment, in the forest.
What is an echo and how do you hear it?
An echo is a reflection of sound. Echoes take place in physics in ninth grade, so everyone probably knows how it occurs. Sound is reflected, sometimes even several times, from various surfaces and returns to us. The question arises, why do we hear echoes not always, but in some cases? Why can't we hear echoes in small rooms, for example?
The fact is that, firstly, things and furniture in the premises dampen the reflected sounds, absorbing the echo. Secondly, for our brain to distinguish the reflected signal separately from the sent one, in the form of an echo, it is necessary that the difference between them be at least six hundredths of a second.
It can be easily calculated, given that the speed of sound is about 340 m / s, that at a distance of three meters from the wall to the reflected sound will come back after about two hundredths of a second. Such time is not enough for the brain; it will not perceive these two sounds separately.
And in large rooms, where the signal is not extinguished by a large amount of furniture, and the distance to the walls is great, the sound may take more than six hundredths of a second to return to us reflected. In this case, we will hear an echo.
Where is the best echo to be heard?
High in the mountains, where there is no furniture, and the sound is easily reflected from the rocks, and the distance between the rocks is great, you can hear the echo of your scream more than once. Reflecting from rocks at different distances, the sound arrives with a great delay, so we hear a repeating echo.
Much the same happens in the forest, where sound is reflected from tree trunks. True, in the forest, sound is absorbed by foliage, grass and earth, and in the mountains there is often nothing to absorb sound, and therefore a loud cry can easily cause a collapse.
The vibrations of the sound wave are transmitted to the rocks, and rocks and snow masses that are weakly adhering to the slopes can easily break down from the resulting vibration. Rolling, they knock down new stones and snow along the way, an avalanche arises. Therefore, you should always remember in the mountains about the danger of a collapse and not shout again unnecessarily.
The principle of operation of a horn is based on the use of echo. The horn is an expanding round tube. A person speaks at the narrow end, the sound of his voice is reflected several times from the walls of the speaker and exits through the wide end in one direction, without scattering in all directions. Thus, its power is increased in a given direction, and the sound can spread over a greater distance.