Question

What should be the minimum distance between the reflecting surface and the source of sound to hear an echo?

• A. 10 m
• B. 15 m
• C. 17 m

Answer 1

The correct option is C

17 m

Given:

Speed of sound, $v=340m/s(weknow)$

Finding the minimum distance:

Let the distance between the source and the reflecting surface be $d$.

We know the minimum time required to hear an echo is, $t=\frac{1}{10}s$.

And to hear an echo sound has to travel a distance $=2d$

We know that, $speed=\frac{\mathit{d}\mathit{i}\mathit{s}\mathit{t}\mathit{a}\mathit{n}\mathit{c}\mathit{e}}{\mathit{t}\mathit{i}\mathit{m}\mathit{e}}$

$\therefore distance=speed\times time$

Because time t should be greater than 1/10 s, so distance travelled by the sound to reach the ear,

$$\begin{gathered} \Rightarrow 2d\ge v\times t \\ \Rightarrow 2d\ge 340\times \frac{1}{10} \\ \Rightarrow d\ge 17m \end{gathered}$$

Hence the minimum distance to hear an echo is 17 m.