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:
Reverberation
Stethoscope
SONAR
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.