Chapter 7: SOUND
Reflection of sound waves and echoes
1. Distinction between light and sound waves:
LIGHT WAVES SOUND WAVES
1.These are electromagnetic waves. 1.These are mechanical waves.
2.They can travel in vacuum. They require a material medium for
propagation. They cannot travel in vacuum.
3.The speed of light wave is very high. The speed of sound wave is low.
4.The speed of light waves (visible) is very The wavelength of sound waves is in the
small, of the order of 10-6 m. range of 10-2 m to 10m.
5.These waves are transverse. 5.In air, these waves are longitudinal.
2. Reflection of sound waves: The return of a striking a surface (such as a wall, metal sheet,
plywood, etc.), back in the same medium is called the reflection of sound wave. The only
requirement for the reflection of the sound wave is that the size of the reflecting surface
must be bigger than the wavelength of the sound wave. The phenomenon of reflection of
light is utilized in making the megaphone, soundboard, ear trumpet, etc. Reflection of sound
waves obeys the laws of reflection.
3. Echo
(i) Define echo.
(ii) What do you mean by persistence of hearing?
(iii) Calculate the minimum distance between the source of sound and reflecting surface
for echo to be heard.
(iv) State three conditions necessary for echo to be heard.
(v) What is reverberation?
ANSWERS:
(i) The sound heard from a distant obstacle (such as a cliff, a hillside, wall of a building,
edge of a forest, etc.) after the original sound has ceased, is called an echo.
(ii) The sensation of a sound persists in our ears for about 0.1 second after the exciting
stimulus ceases to act. This is known as persistence of hearing.
(iii) If ‘d’ is the distance of the observer from the obstacle and ‘v’ is the speed of sound in
the medium, then the total distance travelled by the sound to reach the obstacle and
then to come back is 2d. The time taken to hear the echo (or reflected sound) is:
D=total distance travelled/speed of sound=2d/v
D=v t/2
By putting t=0.1 s and v=340 ms-1 in air at ordinary temperature we get d= 340 × 0.1/2
=17m
(iv) Three conditions necessary for an echo to be heard are:
The minimum distance between the source and the reflector must be 17m in air.
, The size of the reflector must be large enough as compared to the wavelength of
the sound wave.
The intensity of the sound should be such that the reflected sound reaching the
ear should be sufficiently loud to be audible.
(v) If there are repeated reflectors at the reflecting surface, the sound gets prolonged and
the phenomenon is called reverberation. This can be experienced in high toms like Taj
mahal, Sikandra, etc.
4. Uses of echo:
(i) The process by which bats and dolphins locate the presence of obstacles ahead of
them by producing ultrasonic sound and receiving their echo is called sound ranging
(ii) The process by which we can calculate the depth of the sea or locate the position of
a submarine, a sunken ship, or a shoal of fish by producing ultrasonic sound and
receiving their echo is called echo-depth-sounding.
(iii) Sonar: Sound navigation and ranging (makes uses of ultrasonic waves)
(iv) RADAR: Radio detection and ranging (makes use of radio and micro waves)
(v) Medical use: Ultrasonography, echocardiography, etc.
5. Why are ultrasonic waves used for sound ranging and echo depth sounding?
Ans) It is because:
They can travel un deviated through a long distance.
They can be confined to a narrow beam.
They are not easily absorbed in a medium.
Natural, damped, forced, vibration, resonance
Q 1. Free vibrations/ Natural vibrations
(i) What do you mean by natural vibrations or free vibration?
(ii) Give example.
(iii) Draw displacement time graph for free or natural vibrations.
Q 2. Damped vibrations
(i) What are damped vibrations?
(ii) Give example.
(iii) Draw displacement-time graph for damped vibrations.
Q 3. Forced vibrations
(i) What are forced vibration?
(ii) Give example.
Q 4. Resonance
(i) What is resonance?
(ii) What are resonant vibrations?
Reflection of sound waves and echoes
1. Distinction between light and sound waves:
LIGHT WAVES SOUND WAVES
1.These are electromagnetic waves. 1.These are mechanical waves.
2.They can travel in vacuum. They require a material medium for
propagation. They cannot travel in vacuum.
3.The speed of light wave is very high. The speed of sound wave is low.
4.The speed of light waves (visible) is very The wavelength of sound waves is in the
small, of the order of 10-6 m. range of 10-2 m to 10m.
5.These waves are transverse. 5.In air, these waves are longitudinal.
2. Reflection of sound waves: The return of a striking a surface (such as a wall, metal sheet,
plywood, etc.), back in the same medium is called the reflection of sound wave. The only
requirement for the reflection of the sound wave is that the size of the reflecting surface
must be bigger than the wavelength of the sound wave. The phenomenon of reflection of
light is utilized in making the megaphone, soundboard, ear trumpet, etc. Reflection of sound
waves obeys the laws of reflection.
3. Echo
(i) Define echo.
(ii) What do you mean by persistence of hearing?
(iii) Calculate the minimum distance between the source of sound and reflecting surface
for echo to be heard.
(iv) State three conditions necessary for echo to be heard.
(v) What is reverberation?
ANSWERS:
(i) The sound heard from a distant obstacle (such as a cliff, a hillside, wall of a building,
edge of a forest, etc.) after the original sound has ceased, is called an echo.
(ii) The sensation of a sound persists in our ears for about 0.1 second after the exciting
stimulus ceases to act. This is known as persistence of hearing.
(iii) If ‘d’ is the distance of the observer from the obstacle and ‘v’ is the speed of sound in
the medium, then the total distance travelled by the sound to reach the obstacle and
then to come back is 2d. The time taken to hear the echo (or reflected sound) is:
D=total distance travelled/speed of sound=2d/v
D=v t/2
By putting t=0.1 s and v=340 ms-1 in air at ordinary temperature we get d= 340 × 0.1/2
=17m
(iv) Three conditions necessary for an echo to be heard are:
The minimum distance between the source and the reflector must be 17m in air.
, The size of the reflector must be large enough as compared to the wavelength of
the sound wave.
The intensity of the sound should be such that the reflected sound reaching the
ear should be sufficiently loud to be audible.
(v) If there are repeated reflectors at the reflecting surface, the sound gets prolonged and
the phenomenon is called reverberation. This can be experienced in high toms like Taj
mahal, Sikandra, etc.
4. Uses of echo:
(i) The process by which bats and dolphins locate the presence of obstacles ahead of
them by producing ultrasonic sound and receiving their echo is called sound ranging
(ii) The process by which we can calculate the depth of the sea or locate the position of
a submarine, a sunken ship, or a shoal of fish by producing ultrasonic sound and
receiving their echo is called echo-depth-sounding.
(iii) Sonar: Sound navigation and ranging (makes uses of ultrasonic waves)
(iv) RADAR: Radio detection and ranging (makes use of radio and micro waves)
(v) Medical use: Ultrasonography, echocardiography, etc.
5. Why are ultrasonic waves used for sound ranging and echo depth sounding?
Ans) It is because:
They can travel un deviated through a long distance.
They can be confined to a narrow beam.
They are not easily absorbed in a medium.
Natural, damped, forced, vibration, resonance
Q 1. Free vibrations/ Natural vibrations
(i) What do you mean by natural vibrations or free vibration?
(ii) Give example.
(iii) Draw displacement time graph for free or natural vibrations.
Q 2. Damped vibrations
(i) What are damped vibrations?
(ii) Give example.
(iii) Draw displacement-time graph for damped vibrations.
Q 3. Forced vibrations
(i) What are forced vibration?
(ii) Give example.
Q 4. Resonance
(i) What is resonance?
(ii) What are resonant vibrations?
