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11. Sound eLearn.Punjab
Measuring Speed of Sound by Echo Method
Do You Know?
Apparatus: Measuring tape, stopwatch, flat wall that can
produce a good echo.
Procedure:
1. Use the tape to measure a distance of 50 meters from
the wall.
2. Now clap your hands in front of the wall at a distance The speed of sound in air was first
of 50 meters and check if you can clearly hear an echo accurately measured in 1738 by members
from the wall. Make sure the echo is not coming from of the French Academy. Two cannons
any other wall in the area. The time taken by the sound were set up on two hills approximately
to travel 100 meters is the time difference between the 29 km apart. By measuring the time
clap and the echo. interval between the flash of a cannon
and the “boomâ€, the speed of sound
3. Now restart the clapping and start the stopwatch at was calculated. Two cannons were fired
the first clap. Count the number of claps, and stop the alternatively to minimize errors due to
clapping and the stopwatch when you hear the echo of the wind and to delayed reactions in the
observers. From their observations, they
deduced that sound travels at about 336
ms-1 at 0oC.
the 10th clap (say).
4. Now find the average time for 10 claps. After calculating the time interval t between claps and
using the formula S = vt, we can calculate the speed of the sound.
Example 11.3: Flash of lightning is seen 1.5 seconds earlier than the thunder. How far away is the
cloud in which the flash has occurred? (Speed of sound = 332 ms-1).
Solution: Given that, time t = 1.5 s, speed of sound v = 332 ms-1.
Therefore, distance of the cloud S = vt = 1.5 s × 332 m s-1 = 498 m
11.5 NOISE POLLUTION
We enjoy the programmes on radio or television by hearing sounds of different qualities. In musical
programmes, we hear sound produced by musical instruments such as flute, harmonium, violin,
drum etc. Sound of these instruments has pleasant effect on our ears. Such sounds which are
pleasant to our ears are called musical sounds.
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