CBSE Class 9 Answered

We can determine the sped of sound in air by the echo method . For this :-

1. The source of sound ( like a fire cracker) is kept at a distance say 'd' (more than 17 m ) from the reflecting surface ( say tall wall, cliff etc.)

2. Start the stop watch when the sound is produced by the source.

3. Stop the stopwatch as soon as the echo is heard and record the time 't'. This time interval 't' is the time in which echo reaches the place from where the sound was produced.

4. As soon as sound travels from from the source to the reflecting surface and back again, the total distance travelled in time 't' is '2d'.

5. The speed of the sound is calculated by the foll. formula :

speed = Total distance travelled / time interval

 v = 2d / t  ( m/s)

6. Repeat the expt. 3 to 4 times and calculate v₁ , v₂ ..so on and take the average of the results.

7. The obtained value of velocity of sound will be approximately 340 m/s.