As You Call, So It Echoes

It was a hot sunny day.

Irishka was jumping rope, and Lenya was watching. Irishka jumped and jumped, and then she tripped - probably because it was so hot.

"You tripped, you tripped!" Lenya shouted. Irishka felt hurt, so she said:

"Come on, I'll show you something..."

The children ran under the archway that was in their building. They ran up, stopped. Irishka got ready and shouted:

"Ahhh!" - short like that: "Ahhh!"

And from above, someone called back:

"Ah!" - a little quieter, but in Irishka's voice.

"Who's that?" asked Lenya.

"Echo."

"Ahhh!" - Lenya shouted just as loudly and sharply.

"What are you yelling about here?"

The children turned around and saw Uncle Misha, an old man with a wide beard, like in the old days.

Uncle Misha was known in the courtyard as an avid fisherman. His voice was gruff, but his face was kind.

"It's not us yelling," said Lenya. "Echo is sitting up there. It's the one yelling."

"Ahhh!" Uncle Misha shouted, and from above, echo replied in Uncle Misha's voice:

"Ah!"

"Is that really an echo?" said Uncle Misha. "Well, tell Tanya to let you come fishing with me, and I'll show you a real echo!"

Tanya let them go. Not right away, of course...

It was cooler by the river than in the city, and so everyone's mood was more cheerful. Uncle Misha set up his fishing rods and was already getting ready to doze off, as all fishermen do.

"Where's the little old man?" asked Lenya.

Uncle Misha even flinched:

"What little old man?"

"Well, the one who's Echo."

"Oh, the echo," Uncle Misha remembered. "Ahhh!"

Uncle Misha shouted, but no one answered back, no matter how hard the children listened. Lenya was about to ask again about the Echo old man, when suddenly from far away someone answered in Uncle Misha's voice:

"Ah!"

"Why is he answering from over there?" Lenya wondered.

"Oh, you don't know anything," said Irishka. She took a stone and threw it into the water. The stone splashed, and from it, waves spread out in circles in all directions. The waves reached the shore and began bouncing back in the opposite direction.

You've probably already guessed that waves on water and sound in air are somewhat similar to each other.

Lenya understood this too.

"When I shout," he said, "the air starts to vibrate. This vibration spreads out in all directions. It runs to the shore, hits it, and comes back, right into my ear."

"That's right," said Uncle Misha. "And here you were talking about a little old man!"

"I was just joking about the old man because I didn't know," Lenya replied.
---

It's important to draw the child's attention to the fact that an echo - reflected sound - is not heard immediately, but after some time has passed. This happens because sound doesn't travel through air instantaneously, but at a speed of 340 m/sec. For comparison, you can say that passenger jet aircraft fly twice as slow as sound, while military jets fly faster than sound.

Here's another interesting fact: as we know, thunder is the sound from an electrical discharge - lightning. Why does lightning flash almost instantaneously, while thunder rumbles for a long time and not simultaneously with the lightning? The delay of thunder is explained by the fact that the speed of sound is a million times slower than the speed of light. We see light practically simultaneously with the electrical discharge (the speed of light is 300,000 km/sec), but sound needs several seconds to reach the observer. If you start counting seconds immediately after a lightning flash (zero, one, two, three, etc.), then by multiplying the time from the flash to the first thunder clap by the speed of sound, you can get the distance to the location of the electrical discharge.

Now it's easy to understand why thunder rumbles for so long. Lightning often stretches several kilometers in length. From those sections of lightning that were located closer to us, we'll hear the sound first of all. From the most distant sections of lightning, the sound will arrive several seconds later. Besides that, for some additional time we'll hear echo reflected from clouds and large earthbound objects (mountains, large forest areas, etc.).

Using the property of sound to travel at a relatively low speed, you can determine the distance to inaccessible objects with the help of an echo. To do this, you need to shout briefly but loudly and note the time between the shout and the echo. Since the sound traveled the distance from you to the object twice (there and back), this time needs to be divided by two and then multiplied by the speed of sound. For example, from the moment you shouted to the moment the echo returned, 3 seconds passed. Since the sound's path to the object is half the total path it traveled, we divide 3 seconds by two and get 1.5 seconds. Multiplying this time by the speed of sound (340 m/sec), we get the distance to the object - 510 m.

This principle forms the basis for the operation of echo sounders - special instruments for determining the depth of the sea. An echo sounder emits a strong, short sound signal, and then catches the echo reflected from the sea floor. By multiplying the speed of sound in water by the time between the signal and the moment the echo returns, and dividing this value in half, they determine the distance to the bottom. The depth of the sea sometimes exceeds 10 km, and it's impossible to measure such a depth with an ordinary lead line (a weight tied to a rope).