Echo. What do we know about this interesting physical phenomenon? Anyone who remembers a school physics course well will probably answer that an echo is a physical phenomenon, the essence of which is that the observer receives a wave that is reflected from some obstacle. However, echo is not as simple as it seems. This article will provide several 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 some obstacle (however, it can also be electromagnetic, but you, of course, will not hear such an echo). Reflected sound waves return to the observer (the noise source), who can sometimes hear them much later. It is this sound reflected from obstacles that is called echo. 
About the origin of the word echo
This word has a rather interesting history. It came into Russian from the German word echo. This word came into the German language, like many other words in Western European languages, from Latin - ēсhō. And Latin adopted this word from Greek - ἠχώ , which meant “echo.”
Conditions for the existence of an echo
Several conditions are required for an echo to appear. Have you ever wondered why echo is not heard in an apartment or a store, but at the same time it is extremely easy to hear in the mountains? The fact is that the human ear hears an echo only when the reflected sound sounds separately from the spoken sound, and does not “layer” 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 be 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 called echo cancellation. It is used in telephony. The process of echo cancellation is the removal of echo that is unnecessary in a communication, which degrades the quality of communication. Echo cancellation is needed not only to improve sound quality, but also to increase the throughput of the communication channel.
There is a room where there is no echo at all. It's called an anechoic chamber. There are two types of anechoic chambers. Each type serves to “jam” one or another type of echo. Simply put, in such a chamber, sound (or radio waves) simply do not reflect from the walls. The first is the acoustic type. It, as the name implies, serves to suppress ordinary sound echo. The second, accordingly, is 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, during the outbreak of novae). 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 a sound, sometimes occurring in the short wavelength range, that returns some time after the signal has been transmitted. This unusual and difficult to explain phenomenon was discovered in 1927 by Scandinavian Jorgen Hals.
Ancient Greek myth about the nature of echoes
The ancient Greeks explained many natural phenomena with myths. Echo was no exception. The myth about the birth of echo goes something like this: one day, Zeus’ jealous wife Hera punished the beautiful nymph Echo by forbidding her to answer questions - Echo could only repeat the last words addressed to her. Echo saw the beautiful Narcissus walking through the forest. He heard the rustling and exclaimed:
- -Who's here?
- -Here! – Echo shouted back.
- -Come here!
- -Here! – Echo answered joyfully, running towards Narcissus, but he pushed her away, because he believed that only he himself was worthy of his love. So now the beautiful nymph hides in the forests and mountains, occasionally repeating the words of the travelers.
About echolocation
Everyone knows that bats and dolphins use echolocation to navigate in space. However, few people can answer the question “how does it all work?” And it works something like this. First of all, the mouse emits ultrasound. Then she catches the echo of the same sound emitted to her, reflected from objects. The bat has the ability to recognize the ultra-short intervals that pass from the emission of a sound signal to the return of the echo. In this way, the mouse determines the distance between trees or other objects, and also “sees” how far this or that insect is from it. What is surprising is that 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 gHz. The signals emitted by dolphins are incredibly powerful: for example, they can “see” a school of fish more than one kilometer away! 
Small interesting facts
- If the distance from the noise source to the nearest obstacle (wall or rock), then no echo is formed.
- 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, you will hear the echo of your voice up to 11 times.
- The ear of Dionysus is a real record holder in the field of echo. This is a grotto in Syracuse, shaped like a human ear. But that’s not what makes him interesting. Due to its shape, the grotto makes the echo incredibly strong. Throwing a stone or a simple clap will echo out of the darkness with real thunder
All of you are probably familiar with such a physical phenomenon as echo. The echo appears in a variety of places - in the mountains, in the forest, in empty rooms (in an apartment, concert hall, temple, well) and so on.
Echo manifests itself in such a way that it repeats sounds, voices, words. In a place where an echo exists, any spoken word will be repeated with a slight delay, and in some areas, for example, in the mountains and forests, sounds can be repeated not once, but many times.
In ancient times, the existence of echoes was explained by the presence of mimicking spirits or magic, but with the advent of the science of physics, this phenomenon was studied in detail, and even more, put at the service of man (echo sounders and other devices).
To understand what an echo is, you must first understand what sound is.
Sound concept
Sound is the propagation of waves in a gaseous medium (in this case), that is, in the air. Simply put, air is a gaseous medium.
By making a sound, we thereby produce waves in this environment, as if we produced waves by throwing a stone into the quiet surface of a pond. Exactly the same waves as in the case of water occur in the air. For the same reason, sound does not propagate in space, since there is no gas in space and there is simply nowhere to form waves.
The ear of a person or animal is configured in such a way as to capture air vibrations, that is, those same waves. A certain strength and sequence of air vibrations are captured by the ear, after which they enter the brain, where they are given a meaning that is understandable for perception - sounds, voices, noise, individual words, and so on.
How to explain to a child what an echo is
To explain to a child what an echo is, you can give as an example the same quiet pond where you threw a stone. From the center of the fall of the stone, waves begin to diverge in all directions. As soon as the waves reach any obstacle, they are reflected and begin to reverse. The magnitude and strength of the reflected waves are less, but they are still present.
The same thing happens with waves in the air. If sound is reflected from any surface (which does not absorb it, but from which the waves bounce like a ball), then the waves return back to their source, as a result of which we hear our own voice a few moments later.
The presence and strength of the echo also depends on the density and rarefaction of the air and many other factors - the distance of the sound source and the reflecting surface, the angle of the reflecting surface in relation to the sound waves, the interval between the direct and reflected waves.
Echo in nature
A person hears only a pronounced and strong echo due to the structure of his hearing organs, but some animals are able to hear it constantly, even if it is very slight. This phenomenon has been adopted by some species of animals, for example, bats. Bats make a sound that bounces off surfaces and comes back.
The ears of bats are extremely sensitive and can pick up the slightest changes in air vibrations, in particular, their own echo, so a bat, even developing enormous speed, never encounters obstacles in flight, since it knows exactly where and at what distance the obstacle that reflected the emitted sound is located. her sound.
Even if you have never been to the mountains, you still probably know what an echo is and have encountered it more than once. The 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 can you hear it?
An echo is a reflection of sound. Echoes are taught in physics in the ninth grade, so everyone probably knows how it arises. Sound is reflected, sometimes even several times, from various surfaces and returns to us. The question arises: why do we not always hear an echo, but in some cases? Why don't we hear echoes in small rooms, for example?
The fact is that, firstly, things and furniture in the premises dampen reflected sounds, absorbing the echo. Secondly, in order for our brain to distinguish the reflected signal separately from the sent one, in the form of an echo, the difference between them must be at least six hundredths of a second.
It can be easily calculated, given that the speed of sound is approximately 340 m/s, that at a distance of three meters from the wall, the reflected sound will reach back in about two hundredths of a second. This time is not enough for the brain; it will not perceive these two sounds separately.
And in large rooms, where the signal is not damped by a large amount of furniture, and the distance to the walls is large, 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 echo best 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 large, you can hear the echo of your cry more than once. Reflecting from rocks located at different distances, the sound arrives very late, so we hear a repeating echo.
Much the same thing happens in the forest, where sound is reflected from tree trunks. True, in a forest, sound is absorbed by leaves, grass and earth, but in the mountains there is often nothing to absorb sound, and therefore a loud scream can easily cause a collapse.
The vibrations of the sound wave are transmitted to the rocks, and stones and snow masses that are weakly held on the slopes can easily fall down from the resulting vibration. As they roll, they knock down new stones and snow along the way, causing an avalanche. Therefore, you should always remember in the mountains about the danger of a collapse and not shout unnecessarily.
The operating principle of the horn is based on the use of echo. The horn is an expanding round tube. A person speaks into the narrow end, the sound of his voice is reflected several times from the walls of the horn and comes out through the wide end in one direction, without scattering in all directions. Thus, its power is enhanced in a given direction, and the sound can travel a greater distance.
I, like many of you, am familiar with such a physical phenomenon as echo, and even in many ways the nature of this phenomenon was clear to me. But it also conceals a number of secrets. One has only to wonder why we don’t hear an echo in an apartment or in a huge concert hall? Is there any benefit from it and where is it used?
If sound waves caused by our own voice or any other sound source encounter an obstacle during their propagation (a wall, a steep cliff, a forest), then they are reflected and can reach the observer’s ear again, sometimes much later than during their direct propagation. This repeated sound, caused by the reflection of sound waves, is called an echo.
Since the speed of sound propagation in one second is approximately (depending on the temperature) 333 m, it is clear that if the wall reflecting the sound is located at this exact distance of 333 m from us, then we will hear a repetition of the sound we caused (a word or an entire phrase) in two seconds.
We hear an echo if the reflected sound is perceived separately from the spoken sound. To do this, it is necessary that there be 0.06 s between the impact of these sounds on the ear. But in an apartment the interval is much smaller due to the short distance. In addition, many objects (upholstered furniture, curtains) absorb sound.
Look at the fragment below to understand the nature of the echo.
From this it is clear, at the same time, that one can use the echo to determine the speed of sound itself, for which it is necessary to measure the distance and the corresponding period of time. This method of determining the speed of sound was actually used (see “Journal of the Russian Physico-Chemical Society”, 1895, H. Gezehus).
A wonderful echo, extremely clearly and loudly repeating the spoken words, is obtained, by the way, in the long grotto located in the Gatchina palace garden. A common echo phenomenon among Swiss and other mountains.
Multiple echoes, caused by several successive reflections of sound waves, are also common. Thus, in Potsdam, near Berlin, in one of the halls of the Sanssouci Palace, a loud single clapping of hands gives the impression of a long volley of rifle fire.
The echo in the Simonetta Palace near Milan is especially remarkable; a shot fired from the window of the main building is repeated up to 50 times, due to the reflection of sound from various outbuildings of the palace.
A special case of echo is the concentration of sound by reflecting it from concave curved surfaces. So, if a sound source is placed in one of the two focuses of the ellipsoidal vault, then the sound waves are collected in its other focus. This explains, for example, the famous “ear of Dionysus” in Syracuse - a grotto or recess in the wall, from which every word spoken by those imprisoned in it could be heard in some place remote from it. A church in Sicily had a similar acoustic property, where in a certain place one could hear whispered words in the confessional. Also famous in this regard are the Mormon Temple at Salt Lake in America and the grottoes in the Oliva Monastery Park near Danzig.
The phenomenon of reflection is also characteristic of ultrasonic waves (mechanical waves with a frequency above 20,000 Hz). From special piezoelectric emitters, ultrasound is directed in a narrow beam to the object under study, reflecting from which it allows you to see the picture in detail. Echolocation (a method for determining distances), ultrasonic flaw detection (detection of defects, cavities, cracks in cast metal products), and echo testing in medicine are based on reflection (see video below).
The reflection of infrasounds (mechanical waves with a frequency of up to 20 Hz) allows fishing vessels, using special devices, to quickly detect schools of fish.
Echolocation in nature
The bat emits ultrasounds and then picks up the echoes reflected from obstacles. Possessing the ability to estimate ultra-short periods of time from sending a sound signal to its return, it very accurately determines the distance to the insects it hunts, and confidently flies through dense forests without bumping into trees.
The natural sonars of bats are a source of fascination for researchers and, in particular, they are the envy of radar specialists. They have very high sensitivity, allowing them to pick up extremely weak echoes from a mosquito flying at a distance of several meters. What's even more amazing is that a bat's sonar allows it to differentiate between an echo from a stationary obstacle and an echo from a moving object. The mouse itself is in motion. And at the same time, it not only easily distinguishes between stationary and moving objects, but is capable of perceiving a weak echo from a flying mosquito against the background of a many times stronger echo from the surface of the earth, trees, etc.Radar experts know how difficult it can sometimes be to distinguish between radio echoes from a low-flying aircraft and those from the ground. Naturally, they cannot help but be interested in the natural sonar of a bat.
Like bats, dolphins have excellent natural sonar. Echolocation of dolphins was discovered in the early 50s of the last century. Echolocation in dolphins, like in bats, is carried out at ultrasonic frequencies. Dolphins use mainly frequencies from 80-100 kHz. The power of location signals emitted by dolphins can be very high; They are known to be able to detect schools of fish at distances of up to a kilometer. The dolphin is able to perceive very weak echoes in very loud noise. For example, he perfectly “notices” a small fish that appears at a distance of 50 m.
Echo is the reflection of elastic mechanical waves - a phenomenon characteristic of both sound waves and ultrasonic waves, inaudible to humans. It is found in nature and helps animals navigate in space. The man took it into his service and learned to use this unusual physical phenomenon.
What is an echo and where does it come from?
There are two types of echo - linear and acoustic. Linear is created by electrical circuits connected to wire lines. Acoustic echo is a problem for all devices that read and reproduce sound. It occurs when sound is reflected, due to its entry from an external speaker into the microphone. In this case, the interlocutor hears not only the speech of the other subscriber, but also his own, returned to him as an acoustic echo.
Consequently, the causes of echo can also be divided into two groups. The first is subscriber equipment (your phone) and its accessories, the second is the equipment of your operator, or the company providing wired telephone services.
The first group of reasons. Since the speaker and microphone of your device are located in the same housing, under certain conditions an acoustic echo may occur. Sound from the speaker can get into the microphone for two reasons: the first is the lack of a “tightness” of the connection between the speaker and the hole for it in the housing, due to which the sound from this speaker spreads throughout the housing and enters the speaker. This phenomenon can be caused either by mechanical loosening of the housing during operation, or by its design; The second possible reason is the cover. After some time has passed since the start of use, cases tend to “wear out” somewhat, due to which a certain amount of free space is formed between the case and the phone body, which helps to shield sound from the speaker to the microphone.
In the second group of reasons, the following can be distinguished: the first is the transceiver of the base stations of your operator, near which you are located, the second is the wired telephone provider whose subscriber you are talking to. This happens due to the reflection of part of the signal when moving from the local line to the main line.
How to determine the cause of the echo effect in your mobile phone.
To effectively solve a problem, it is necessary to accurately determine the cause of its occurrence. Unfortunately, this is unlikely to be done with 100% accuracy, but the probability is still high.
So, if the echo is a constant phenomenon or, on the contrary, it is impossible to trace any pattern in its appearance, then the reason is most likely your device. To be sure of this, try turning down the speaker volume. In cases where the echo disappears after this, the cause of its occurrence was in the phone. Also try removing the cover. The echo disappeared - the case is the reason.
If the echo occurs in certain places, or when calling certain numbers, or occurs for a certain period of time, then your terminal should not be blamed. One of the second group of reasons is at work here. The BS is to blame if an echo occurs in some place, and it is not to blame if, when an echo appears, you contact a landline telephone. When you experience an echo, try a SIM card from another operator in the same place. The echo disappeared - the reason for the NDE, no - no.
How to deal with this.
If the problem is with your device, the solutions are as follows: reduce the volume of the speaker, if the problem is in the case, if this does not help, there is only one way out - replacing the phone. The reason is in the cover - the solution to such a problem, as you might guess, is to refuse to use the cover, or, if this option is not acceptable, to replace it with a new, denser one.
If the operator is to blame for your unbearable torment, then you need to call the subscriber service and report the place where you constantly feel deficiencies in the quality of communication. Your application will be accepted, but when the reason will be eliminated (and whether it will be eliminated) is unknown. The average user has only one way to influence the operator - call and call. Or, if everything is really bad, you can change the operator, after making sure that you will not have similar problems with the new one.
To combat the echo that occurs when calling a regular landline phone, there are special echo suppressors. These devices identify human speech coming from one end of the connection and suppress all signals coming in the opposite direction.
As you can see, the problem is not solved without costs in all cases, but it can still be solved. By the way, echoes can occur due to the fault of listening devices; here you need to read less books about spies.
