Question

The minimum distance between the source of the sound and the reflecting surface necessary to cause echo is:

& A.1.7m \\\ & B.17m \\\ & C.7m \\\ & D.70m \\\ \end{aligned}$$

Answer 1

Since the velocity of dry air is approx $343m/s$ at $25^{\circ}$C, and persists for $0.1s$ in mind. So, to hear a distinct echo, the difference between two sounds must be $0.1s$. So, we assume an obstacle at ‘d’ distance away from the observer, which reflects sound. Sound should travel to the obstacle and back to the listener after reflection in $0.1s$ for echo to be heard.

Complete answer:
The word is derived from the Greek word ‘echos’ which means sound. Echo is the reflection of sound that arrives back to the listener after getting reflected by a reflecting body, like the bottom of a well, by a building, or by the walls of an enclosed room and an empty room. The delay is directly proportional to the distance of the reflecting surface from the source and the listener. This is due to the discontinuity in the propagation medium.
The human ear can distinguish the echo from the origin of the sound only if the delay is greater than $\dfrac{1}{10}$ seconds.

Since the velocity of dry air is approx. $343m/s$ at $25^{\circ}$C, then, the total distance covered by the sound wave is $34.4m$ the total distance is the distance covered by the actual wave and the reflected wave.

Since the reflected wave covers the same as the actual wave,there, there must be a minimum of $17m$ distance between the source of the sound and the reflecting surface necessary to cause echo.

So, the correct answer is “Option B”.

Note:
The echo is measured in terms of decibels (dB). $dB=\dfrac{1}{10} bel$, named after Alexander Graham Bell. It is a quantity which expresses the ratio of one value of a power or field quantity to another, on a logarithmic scale. Where the logarithmic quantity is called the power level or field level, respectively. Echoes are of two types : desirable and undesirable. Some animals use echo for location sensing and navigation, such as cetaceans (dolphins and whales) and bats.