Edumarz

# SOUND

Reflection of Sound

Bertleja S, Academic Content Writer for Physics at Edumarz.

Bouncing back of sound waves from the surface is called Reflection of Sound.

• Similar to reflection of light.

• Follows laws of reflections.

LAWS OF REFLECTION OF SOUND:

• The angle of incidence = the angle of reflection

• The incident sound wave, the reflected sound wave and the normal at the point of incidence lie on the same plane.

Fig 1: Laws of reflection of sound waves

APPLICATIONS:

Echo

• A repetition of sound produced by the reflection of sound waves from a wall, mountain, or other obstructing surface is called an echo.

• The sound persists even after the source of the sound has stopped vibrating.

• The time interval between the original sound from the source and the reflected sound should be at least 0.1s to effectively hear a distant echo.

• The speed of sound in air at 20°C is 344 ms-1

Velocity = Distance Travelled

Time taken

=  2d

t

2d = 344 × 0.1

d = 17.2 m

• The minimum distance between the obstacle and the source of sound is 17.2 metre.

Multiple Reflection of Sound:

Multiple reflection of sound is the reflection of light back and forth through reflecting surfaces several times.

Applications of Multiple Reflections of Sound Waves:

1. Reverberation

2. Stethoscope

3. SONAR

4. Megaphones

Reverberation:

• The persistence of sound that is caused by multiple reflections is called reverberation.

• The duration for which the sound persists is called reverberation time.

• In an auditorium or big hall, high reverberation will greatly affect the quality of sound heard. This can be avoided by covering the walls and ceilings with sound-absorbent materials

• The ceilings of a big hall or auditorium are curved. This will minimise the spreading of sound waves in all directions and also enhances the uniform distribution of sound throughout the hall so that a person sitting at any position in the hall can hear the sound clearly.

Fig 2: Curved ceiling of a conference hall

Stethoscope:

• Principle: Multiple Reflections

• Multiple reflections take place in the rubber tube that connects the chest piece and the ear piece.

• The sound of heart beats or any sound produced by internal organs reaches the ear piece through the rubber tube after multiple reflections.

Fig 3: Stethoscope and multiple reflection of signal in a rubber tube

SONAR ( SOund Navigation And Ranging) :

• SONAR technique uses the reflections of sound waves in water to locate the position or motion of objects.

• Similarly, bats and dolphins use the SONAR principle to locate objects in the darkness.

Megaphones:

• Megaphones, horns and musical instruments like trumpets are designed to send the sound from source after multiple reflections towards a particular direction without allowing the spreading of sound in all directions.

Fig 4: Multiple Reflection of Sound in Megaphone

Note:

• Echoes may be heard more than once due to multiple reflections.

• The rolling of thunder is caused by the multiple reflections of the sound of lightning having reflecting surfaces such as clouds and the earth’s surface.