Question

What is an echo? What factors are important to get a distinct echo?

Answer 1

To answer this question, we have to use the phenomenon of reflection of light. Also, we need to use the value of the minimum time gap between two distinct sounds for the human ear, which is about $0.1s$. From the value of the speed of sound, we can calculate the minimum distance between the source and the object for getting a distinct echo.

Formula used: The formula used to solve this question is
$v = \dfrac{d}{t}$, where $v$ is the speed, $d$ is the distance, and $t$ is the time.

Complete step by step answer
We know that the sound is a type of longitudinal wave, which is transmitted as a series of compressions and rarefactions of the particles of the medium in which it is travelling. When the sound is produced from a source, then it is spread all around and is therefore incident on all of the surrounding objects. After being incident on an object, it gets reflected and returns back to the source. The source thus hears the same sound which it had produced, and perceives it as the echo.
For the human ear, for distinguishing consecutive sounds, the minimum time difference between the two sounds must be equal to $0.1s$. We know that the speed of sound is equal to about $343m{s^{ - 1}}$. Let the minimum distance between the sound source and the object required to a distinct echo be $x$. When the sound is incident on to the object and returns to the receiver, it travels a total distance equal to $2x$. We know that the speed is related to the distance and the time as
$v = \dfrac{d}{t}$
$\Rightarrow d = vt$
Substituting $d = 2x$, $t = 0.1s$, and $v = 343m{s^{ - 1}}$ above we get
$2x = 343 \times 0.1$
$\Rightarrow x = \dfrac{{34.3}}{2}$
On solving we get
$x = 17.15m \approx 17m$
So the minimum distance between the source and the object for getting a distinct echo is equal to $17m$.
Therefore, the factors important to get a distinct echo are
-The minimum distance between the source of the sound and the reflecting object must be $17m$
-The reflecting object must be of a large size, relative to the wavelength of the sound.

Note
The echo is heard many times in daily life. The examples include the words a speaker hears many times when he is speaking on a microphone in a hall. Also, when we speak in a cave, our sound is repeated again and again.