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J. Perelman
"Entertaining physics". Book 1.
Chapter 10. Sound and hearing

HOW TO FIND AN ECHO?

No one had seen,
And to hear all heard,
Without a body, and it lives,
Without language screams.

N. Nekrasov

Among the stories of the American humorist Mark TWAIN is ridiculous fiction about the misadventures of a collector who had the idea to create a collection... what do you think? Echo! Crank tirelessly bought up all the land, where he played multiple or anything other than a wonderful echo.

“First of all, he bought the echo in the state of George, which was repeated four times, then six times in Maryland, then a 13-fold in Maine. The next purchase was 9-fold echo in Kansas, hereinafter - 12-fold in Tennessee, cheaply bought, because I was in need of repair: part of the cliff collapsed. He thought he could repair completion; but the architect who took on this task has never been more streeval echo and therefore ruined it in the end -- after processing it could fit only for shelter deaf-mutes...”

This, of course, a joke; however, a remarkable multiple echoes exist in various, mainly mountain areas of the globe, and some long since gained world fame.

List a few famous echo. In the Woodstock castle in England echo clearly repeats 17 syllables.

The ruins of the castle of Derenburg near Halberstadt gave 27-complex echo, which, however, ceased since then, as one wall was blown up. Rocks scattered in the form of a circle near Adersbach in Czechoslovakia, I repeat, in a certain place, triple 7 syllables; but a few steps from this point even the sound of the shot gives no echo. Very multiple echo was observed in one (now defunct) castle near Milan: a shot made from the window wing, repeated by the echoes of 40 - 50 times, and loud word 30 times.

Not so easy to find a place where you can clearly hear the echo and once. We in the Union, however, find a similar place relatively easily. There are a lot of plains, surrounded by forests, many clearings in forests; is loud scream on this meadow to the forest wall came more or less distinct echo.

In the mountains echo is more diverse than on the plains, but occurs much less frequently. To hear the echo in the mountain areas harder than forest-fringed plains.

You now understand why this happens. Echo is nothing like returning sound waves reflected from any obstacles; and when light is reflected, the angle of incidence of the sound beam equals the angle of reflection. (Sound beam - direction, along which ran the sound waves.)


Echo missing.

Now imagine that you are at the foot of the mountains, and the obstacle, which should reflect the sound, is placed above you, for example in AB.

It is easy to see that the sound waves that propagate along the lines CA, Cb, Cc, they would not reach your ear, and will dissipate in space in areas AA, bb, cc.

Another thing, if you fit at the level of obstacles or even slightly above it.


A distinct echo.

The sound coming down the areas Sa, Сbwill get back to you in broken lines SaaS or СbbС, reflected from the soil once or twice. The hollow between the two points further contributes to clarity echo, acting as a concave mirror. On the contrary, if the soil between the points With and In convex, the echo will be weak and even reaches your ear: this surface diffuses the rays of sound as a convex mirror.

Pronouncement echo on rough terrain demands a certain skill. Even finding a suitable place, need to know how the echo of the call. First of all, should not be placed too close to the obstacle: it is necessary that the sound was quite a long way, otherwise the echo will come back too early and will merge with the sound. Knowing that sound travels 340 meters per second, it is easy to understand that, polstivshis at a distance of 85 m from the obstacle, we must hear the echo through half a second after the sound.

Although the echo bear “just the sound of your response in the empty air,” but not all the sounds it responds equally clearly. Echo is not the same, “roars whether the beast in the forest deep, blows Lee horn, rattles if the thunder sings whether the virgin over the hill”. The faster, travestie sound, the echo path. It is best to call the echo clapping of hands. The sound of the human voice for this less suitable, especially the voice of men; high tone women's and children's voices give a more distinct echo.

Entertaining physics J. Perelman

 




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