Reflection of Sound - Sound, Class 9, Science PDF Download

​REFLECTION OF SOUND

When a sound wave travelling in a medium bounces back to the same medium after striking the second medium, reflection of sound wave is said to take place. The reflection of sound wave is similar to the bouncing back of a rubber ball after striking a wall or the surface of a floor.

Just like light, sound is reflected by the solid and liquid surfaces. The reflection of sound obeys the laws of reflection.

The laws of reflection of sound are as follows :

(i) Incident angle = Reflected angle and (ii), The incident direction of sound, reflected direction of sound and the normal to the point of incidence all lie in the same plane.

Echo

If we clap our hands while standing at some distance from a high and huge wall or a hill, we hear the clapping of our hands again after some short interval of time. The sound of clap heard by us is known as echo. Echo is produced due to the reflection of sound.

Thus, echo is a repetition of sound due to the reflection of original sound by a large and hard obstacle.

Conditions for the production of an echo

1. Time gap between the original sound and the reflected sound

We can hear the two sounds separately if the time gap between these two sounds is more than 1/10 s or 0·1 s. The time interval equal to 0·1 s is known as persistance of hearing. This means, the impression of any sound heard by us remains for 0·1 s in our brain. If any other sound enters our ears before 0·1 s, then the second sound will not be heard by us. Thus, the echo will be heard if the original sound reflected by an obstacle reaches our ears after 0·1 s.

2. Distance between the source of sound and obstacle

Minimum distance between the observer and the obstacle for echo to be heard

Let

Distance between the observer and the obstacle = d

Speed of sound (in the medium) = v

Time after which echo is heard = t

Then, t =  or d = 

We know

Speed of soune; in air at 25°C = 343 m s^-1

For an echo to be heard distinctly,

t  0.1 s

Then d   or d 17.2 m

Thus, the minimum distance (in air at 25°C) between the observer and the obstacle for the echo to be heard clearly should be 17.2 m.

The speed of sound increases with a rise in temperature. Therefore, the minimum distance in air between the observer and the obstacle for an echo to be heard clearly at temperatures higher than 25°C is more than 17.2m. In rooms having walls less than 17.2 m away from each other, no echo can be heard.

3. Nature of the obstacle : For the formation of an echo, the reflecting surface or the obstacle .must be rigid such as a building, hill or a cliff.

4. Size of the obstacle : Echoes can be produced if the size of the obstacle reflecting the sound is quite large.

Reverberation

The repeated reflection that results in the persistence of sound in a large hall is called reverberation.

Excessive reverberation in any auditorium/hall is not desirable because the sound becomes blurred and distorted. The reverberation can be minimised/reduced by covering the ceiling and walls with sound absorbing materials such as, fiber-board, rough plaster, draperies, perforated carboard sheets etc.

Uses of multiple Reflection of sound

1. Megaphone : Megaphone is a device used to address public meetings. It is a orn-shaped. When we speak through megaphone, sound waves are reflected by the megaphone. These reflected sound waves are directed towards the people (or audience) without much spreading.

2. Hearing Aid : Hearing aid is used by a person who is hard of hearing. The sound waves falling on hearing aid are concentrated into a narrow beam of sound waves by reflection. This narrow beam of sound waves is made to fall on the diaphragm of the ear. Thus, diaphragm of the ear vibrates with large amplitude. Hence, the hearing power of the person is improved.

3. Sound boards : Sound boards are curved surfaces (concave) which are used in a big hall to direct the sound waves towards the people sitting in a hall. The speaker is (i.e. source of sound) placed at the focus of the sound board as shown in figure.

Sound waves from the speaker are reflected by die sound board and these reflected waves are directed towards the people (or audience).

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. Stethoscope : Stethoscope is a device used by doctors to listen the sound produced by heart and lungs. The sound produced by heart beat and lungs of a patient reaches the ears of a doctor due to multiple reflection of sound.

5. Ceilings of concert halls are curved : The ceilings of concert halls and auditoriums are made curved. This is done so that the sound reaches all the parts of the hall after reflecting from the ceiling as shown in figure. Moreover, these ceilings are made up of sound absorbing materials to reduce the reverberation.