Question

What is an echo?

Answer 1

Hint: Echo is the term which is related to the sound. It is based on the concept of reflection of sound. It is the reflected sound that we hear after the original sound has diminished. So by using the above statement we can now easily define the term echo.

Complete step by step answer:
When we stand in front of some mountains or hills, which is at a certain distance away from us, and make some sharp noise we can hear back two distinct sounds of which one is the original sound made by us and another sound is heard almost at same time which is actually the reflection of the sound made by us. This reflected sound is known as echo.
Thus, echo can be defined as the sound heard after reflection from an object , which is at a certain distance away , after the original sound has ceased.
But is it so that we can hear echo whenever we make sound in front of any distant object? No. This is because echo can be heard only if the distance between the source of the sound and the rigid obstacle is such that the reflected sound can reach the source at least $0.1$ seconds after the original sound has ceased.
Let us calculate what the distance can be at ordinary temperature.
We know distance $d$ can be calculated from the ratio of time $t$ to velocity $v$ . Now , as the reflected sound has to travel two times so the total distance will be twice the original distance. So,
$2d = \dfrac{t}{v}$ ;
Now, taking $t = 0.1s$and $v = 340m{s^{ - 1}}$ :
$2d = \dfrac{{0.1}}{{340}}m$
$ \Rightarrow d = \dfrac{{0.1}}{{2 \times 340}}m = 17m$
Thus, at ordinary air temperature the distance between source and obstacle must be $17m$ .
Two other important conditions for hearing echo distinctly are:
1. The size of the reflector must be large enough compared to the wavelength of the sound wave.
2. The intensity of the emitted sound should be sufficient so that the reflected sound is audible.
Echo is now widely used in medical fields, sonar and echo depth sounding.

Note: The distance between the source of the sound and the rigid obstacle is such that the reflected sound can reach the source at least $0.1$ seconds after the original sound has ceased because in our ears, we have the sensation of the persisting sound for about $0.1$ seconds.