Question

What is an echo? Mention the condition for the echo to be heard.

Answer 1

Hint: In this question, we first define echo as the reflected sound that arrives at the observer after some time. This happens when a sound is produced and it encounters an obstruction. Then we see the important conditions to hear an echo that are The minimum distance between the observer and the obstacle should be 17m in the air. The minimum time gap between the source and the echo should be 0.1 seconds.

Complete Step-by-Step solution:
When a source or an observer produces a sound it travels in space and it reflects back after encountering any obstacle. This reflected sound arrives at the observer after some time and the observer can hear it. This reflected sound is known as an echo. This can be seen in figure 1.

Figure 1

The most popular places where we generally hear an echo are at a hill station or mountains, by the bottom of a well, by the walls of an empty room or hall, etc.
The necessary conditions required to hear an echo are:
There needs to be a minimum distance between the observer and the obstacle and that is equal to 17 m in the air. This distance may vary from medium to medium depending upon the sound's speed in that particular medium.
The size of the obstruction should be large enough in comparison to the wavelength of the sound wave.
The intensity of sound after reflection should be sufficiently loud so that it is audible to the observer.
There is a need to have sufficient time delay between the original sound and the echo because our human ear will unable differentiate the echo from the original sound if the time gap is shorter than 1/10 of a second, so the echo should reach our ear after a minimum time of 0.1 seconds from the time the original sound was produced.

Note: For these types of questions we need to be well versed with the concept of sound that is how it is produced and travel through a medium. We need to know about some famous terms related to sound like echo, sonar technology, sonic boom, ultrasound and infrasound, music, noise, etc.