Reflection of sound is a very interesting property of sound, which has numerous applications in practical life. Let us understand some of them-
If you stand in front of a high building or a hillock, such that you are at a distance of more than 20m and shout loudly, you will notice that you can hear your own sound. It is because the sound is reflected from the building. This hearing of reflected sound is called echo.
The repetition of the sound, which is reflected from a high building or any such object is called an echo.
Why is an echo formed?
The human ear can hear two sounds separately only if they reach the ear after an interval of 1/10 of a second. This is a natural feature of any human ear. The average speed of sound is 332 m/s. Thus in 1/10 of a second the sound will travel a distance of 332 m/s X 1/10 s = 33.2 m. This means if a sound which starts from a point, on reflection from some hard object comes back to the same point, then the distance between the reflecting body and source of sound should be 33.2 m ÷ 2 = 16.6 or 17 m (approx). This hearing of a reflected body is called an echo.
It has been found that an echo can be heard only if-
- There is a high-rise building or hill.
- Minimum distance between source of sound and reflecting body is 17 m.
- The loudness of the sound should be sufficient so that it can be heard after reflection.
Concept of Sonar-
A device fitted in a ship to find the depth of sea is called sonar (SOund NAvigation and Ranging). It is based on the principal of reflection of sound. From the point A, ultrasonic vibrations are sent. These vibrations, after striking the sea bed, get reflected upward and are picked up at point B. The time in which reflected sound is received is recorded. Using the speed of sound in water, and the time in which the sound gets reflected back and received by the Sonar, the depth of the sea can be calculated.
Sonars are also used to find sunken ships, depths and distances of submarines, icebergs, etc.
Just as we use Sonar in the ocean, which is based on the reflection of sound, marine mammals such as dolphins make use of reflection of sound waves to navigate their way through the ocean. The sound they make travels through the water, hit objects around the animal and reflect back to the dolphin. The dolphin then uses the reflected sound to form a picture of their surroundings. This is known as echolocation.
Similarly, bats use the property of echolocation to find their way at night by sending out sounds which hit any objects in their way and reflect back to the bats. The bats are thus able to navigate their way without hitting any obstacle.
Some other applications of reflection of sound-
Other devices which work on the property of reflection of sound are hearing aids, medical ultrasounds- used to detect abnormalities in the body and echo-cardiograms (ECG)- used to detect blockages in the heart valves.