# Reflection, Transmission and Echo

Let us suppose you are standing at the height of a mountain and yell Hello!!! Within a second, you can hear your voice back echoing in the manner Hello!! Hello!! Hello!! This is what exactly echo is. It reflects back to you. Let us study this in detail.

## Reflection of Sound

Why do we always need a large area to see reflections? When the sounds travel through a certain medium, it strikes the surface of other medium and bounces back to its original medium that is the initial medium. This is what reflection of sound is. The only requirement for the reflection of the sound wave is that the reflecting surface must be bigger than the wavelength of the wave. Reflection of the sound wave is seen in echoes, megaphone, ear trumpet etc.

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#### Laws of Reflection of Sound

1.  An angle of incidence is equal to the angle of reflection.
2.  Incident wave reflected wave and normal at the point of incidence, all lie in the same plane.

Reflection of Sound can be applied in:

• Sound producing devices use a conical shape like megaphones to increase the intensity of sound waves in particular directions.
• In stethoscope, sound undergoes multiple reflections in the wires of the stethoscope.
• Curved ceilings of halls use soundboards to evenly distribute the sound across the hall.

### Transmission of Sound

Every sound is produced by the vibration. Vibration is nothing but the back and forth movement of the object. How does the sound energy travel? Do you wonder how that works? Sound energy travels in the form of sound waves.  Sound waves travel in all directions. In solids, the particles are very close to the other one. Sound energy moves as one particle hit another particle. Here sound travels at a very high speed. In liquids, the particles are at a distance, so it takes more time to travel. In gas, the particles are spread out. Here the speed of the sound energy is the slowest.

## Echo

Every one must have heard of the word echo. Echo is basically a sound heard after the sound is reflected back when the actual sound has stopped. Thus the echo is the result of reflection. If we want to hear an echo, we cannot do it in a small room, we need a large area. When we shout in a large or vacant space, the sound is reflected back.

Suppose you are standing in front of a high wall and clap your hand, you can hear the same sound of the clap back again. This is an echo. An echo is heard when the distance between the person producing sound and the rigid obstacle is long enough to allow the reflected sound to reach the person least 0.1 seconds after the original sound is heard. Otherwise, the echo is not distinguished even on reaching the ears. This value can be calculated and comes out to be 17.2 m for air and 70 m for water.Whenever echo is to be heard the time between two objects or body or points should be 0.1 sec.

## Question For You

Q. Working of radars in oceans is based on the principle of

1. Geo positional statement
2. Reflection of sound
3. Locus of sound waves
4. None

Answer: B. Radars in oceans employ sound waves to detect the location o the surrounding object.

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