Important Notes of Science Class 9 CBSE
Sound
Production of Sound
Sound is produced due to the vibration of objects. Vibration is the rapid to and fro motion of an object. Vibrating objects are the source of all sounds Irregular, chaotic vibrations produce noise Regular, controlled vibration can produce music All sound is a combination of pure frequencies A stretched rubber band when plucked vibrates and produces sound.
Propagation of Sound
When an object vibrates, the particles around the medium vibrate. The particle in contact with the
vibrating object is first displaced from its equilibrium position
The disturbance produced by the vibrating body travels through the medium but the particles do not
move forward themselves.
A wave is a disturbance which moves through a medium by the vibration of the particles of the
medium. So sound is considered as a wave.Sound waves Require medium for transmission.Sound
waves are called mechanical waves. When a vibrating object moves forward, it pushes and
compresses the air in front of it forming a region of high pressure called compression (C). When the
vibrating object moves backward, it forms a region of low pressure called rarefaction (R).
A vibrating object producing a series of compressions (C) and rarefaction (R)
In these waves the particles move back and forth parallel to the direction of propagation of the
disturbance. Such waves are called longitudinal waves.
Compressions are the regions of high pressure and density where the particles are crowded and
are represented by the upper portion of the curve called crest.
Rarefactions are the regions of low pressure and density where the particles are spread out and
are represented by the lower portion of the curve called trough
Sound is a Mechanical Wave:
Sound requiers medium for its propagation it can travel through solid, liquid and gas but not vaccum. The waves which require a material medium for their propagation are called mechanical waves.
Sound is Longitudnal Wave
The wave in which the particles of the medium oscillate along the direction of propagation of the wave is called a longitudinal wave
Transverse wave
The wave in which the particles of the medium oscillate in a direction perpendicular to the direction of propagation of the wave is called a transverse wave.Light waves are transverse waves.They do not require medium for their propagation.Light can travel through vaccum.
Pulse: A wave of short duration which is confined to a small portion of a medium at any given time is known as a pulse.
Characteristics of a sound wave
Frequency of sound waveThe number of oscillations per unit time is called the frequency of the sound wave.
It is represented by the symbol ٧ (Greek letter nu). Its SI unit is hertz (Hz)
The pitch of a sound depends upon the frequency, higher the frequecy higher will be the pitch and vice versa.
Time Period:Time taken to complete one oscillation
Time period= 1/f
Amplitude of the Sound: The maximum displacement of particles of the medium from their mean positions during the propagation of a wave is known as amplitude of the wave.The amplitude of sound wave is the height of the crest or tough.It is represented by the letter A.
The SI unit is the same as that of density or pressure.
Velocity:The distance travelled by a wave in one second is called wave velocity.It depends upon the nature of the medium through which it passes.
v=frequency X Wavelength
Audible Frequency: The human ears can hear only those waves whose freqnecy lies between 20 Hz and 20,000HZ.
Infrasonic Waves:The waves having freqnecy less than 20 Hz are infrasonic
Ultrasonic Waves:The waves having frequency more than 20.000 Hz are ultrasonic waves
Reflection of Sound
Sound gets reflected at the surface of a solid or liquid and follows the laws of reflection.
i) The angle of incidence is equal to the angle of reflection.
ii) The incident ray, the reflected ray and normal at the point of incidence all lie in the
same plane.
Echo
If we shout or clap near a reflecting surface like tall building or a mountain, we hear the
same sound again. This sound which we hear is called echo. It is caused due to the reflection
of sound.
To hear an echo clearly, the time interval between the original sound and the echo
must be at least 0.1 s.
Since the speed of sound in air is 344 m/s, the distance travelled by sound in 0.I s = 344
m/s x 0.1 s = 34.4 m
So to hear an echo clearly, the minimum distance of the reflecting surface should be
half this distance that is 17.2 m.
Reverberation
Echoes may be heard more than once due to repeated or multiple reflections of sound
from several reflecting surfaces. This causes persistence of sound called reverberation.
In big halls or auditoriums to reduce reverberation, the roofs and walls are covered by
sound absorbing materials like compressed fibre boards, rough plaster or draperies.
Uses Of Multiple Reflection Of Sound
i)Megaphones, horns, musical instruments like trumpets, etc. are deigned to send sound by
multiple reflection in a particular direction without spreading in all directions.
ii) Doctors listen to sounds from the human body through a stethoscope. The sound of
heartbeat reaches the doctor’s ears by multiple reflection.
iii) Generally the ceilings of cinema halls and auditoriums are curved so that sound after
multiple reflection reaches all parts of the hall.
Sometimes a curved sound board is placed behind the stage so that sound after multiple
reflection spreads evenly across the hall.
Speed of sound in Different Medium
Sound travels with different speeds in different mediu.The speed of sound also depends u pon the temperature of the medium.Sound travels faster in solids than in air.The speed of sound in solids is much more than the speed of sound in liquids or gases.
Average speed of sound at room temperature is 332 m/s
Application of Ultrasound
They are used to establish ship to ship communication.
They are used to find the depth of a sea.
They are used to diagnose the disease in a human body
They are used to kill bacteria in liquids.
They are used for welding plastics
SONAR
SONAR stands for Sound Navigation and Ranging.It is based on the principle of the reflection of sound wave.The ultrasonic waves from the transmitter of SONAR are sent towards the bottom of the sea whose depth is to be measured.These waves are reflected back from the bottom of the seas and are received by the receiver of the SONAR the transmitter and the receiver are suitably placed on the ship.The time taken by the ultrasonic waves to go from the ship to the bottom of the sea and then back to the ship is recorded
Time taken by the waves to go from the ship to the bottom of the sea is =t/2.If V is the velocity of wave, the the required distance S is given by.
s= v(t/2)
Human Ear
The ear mainly consists of three basic parts the outer ear, the middle ear, the the inner ear.Each part of the ear serves a specific purpose in the task of the detecting and interpreting sound.The outer ear serves to collect and channel sound to the middle ear.The middle ear serves to transform the energy of a sound wave into the internal vibrations of the bone structure and ultimately transform these vibrations into a compressional waves in the inner ear.The inner ear serves to transform the energy of a compressional wave with the inner ear fluid into nerve impulses which can be transmitted to the brain.
Production of Sound
Sound is produced due to the vibration of objects. Vibration is the rapid to and fro motion of an object. Vibrating objects are the source of all sounds Irregular, chaotic vibrations produce noise Regular, controlled vibration can produce music All sound is a combination of pure frequencies A stretched rubber band when plucked vibrates and produces sound.
Propagation of Sound
When an object vibrates, the particles around the medium vibrate. The particle in contact with the
vibrating object is first displaced from its equilibrium position
The disturbance produced by the vibrating body travels through the medium but the particles do not
move forward themselves.
A wave is a disturbance which moves through a medium by the vibration of the particles of the
medium. So sound is considered as a wave.Sound waves Require medium for transmission.Sound
waves are called mechanical waves. When a vibrating object moves forward, it pushes and
compresses the air in front of it forming a region of high pressure called compression (C). When the
vibrating object moves backward, it forms a region of low pressure called rarefaction (R).
A vibrating object producing a series of compressions (C) and rarefaction (R)
In these waves the particles move back and forth parallel to the direction of propagation of the
disturbance. Such waves are called longitudinal waves.
Compressions are the regions of high pressure and density where the particles are crowded and
are represented by the upper portion of the curve called crest.
Rarefactions are the regions of low pressure and density where the particles are spread out and
are represented by the lower portion of the curve called trough
Sound is a Mechanical Wave:
Sound requiers medium for its propagation it can travel through solid, liquid and gas but not vaccum. The waves which require a material medium for their propagation are called mechanical waves.
Sound is Longitudnal Wave
The wave in which the particles of the medium oscillate along the direction of propagation of the wave is called a longitudinal wave
Transverse wave
The wave in which the particles of the medium oscillate in a direction perpendicular to the direction of propagation of the wave is called a transverse wave.Light waves are transverse waves.They do not require medium for their propagation.Light can travel through vaccum.
Pulse: A wave of short duration which is confined to a small portion of a medium at any given time is known as a pulse.
Characteristics of a sound wave
Frequency of sound waveThe number of oscillations per unit time is called the frequency of the sound wave.
It is represented by the symbol ٧ (Greek letter nu). Its SI unit is hertz (Hz)
The pitch of a sound depends upon the frequency, higher the frequecy higher will be the pitch and vice versa.
Time Period:Time taken to complete one oscillation
Time period= 1/f
Amplitude of the Sound: The maximum displacement of particles of the medium from their mean positions during the propagation of a wave is known as amplitude of the wave.The amplitude of sound wave is the height of the crest or tough.It is represented by the letter A.
The SI unit is the same as that of density or pressure.
Velocity:The distance travelled by a wave in one second is called wave velocity.It depends upon the nature of the medium through which it passes.
v=frequency X Wavelength
Audible Frequency: The human ears can hear only those waves whose freqnecy lies between 20 Hz and 20,000HZ.
Infrasonic Waves:The waves having freqnecy less than 20 Hz are infrasonic
Ultrasonic Waves:The waves having frequency more than 20.000 Hz are ultrasonic waves
Reflection of Sound
Sound gets reflected at the surface of a solid or liquid and follows the laws of reflection.
i) The angle of incidence is equal to the angle of reflection.
ii) The incident ray, the reflected ray and normal at the point of incidence all lie in the
same plane.
Echo
If we shout or clap near a reflecting surface like tall building or a mountain, we hear the
same sound again. This sound which we hear is called echo. It is caused due to the reflection
of sound.
To hear an echo clearly, the time interval between the original sound and the echo
must be at least 0.1 s.
Since the speed of sound in air is 344 m/s, the distance travelled by sound in 0.I s = 344
m/s x 0.1 s = 34.4 m
So to hear an echo clearly, the minimum distance of the reflecting surface should be
half this distance that is 17.2 m.
Reverberation
Echoes may be heard more than once due to repeated or multiple reflections of sound
from several reflecting surfaces. This causes persistence of sound called reverberation.
In big halls or auditoriums to reduce reverberation, the roofs and walls are covered by
sound absorbing materials like compressed fibre boards, rough plaster or draperies.
Uses Of Multiple Reflection Of Sound
i)Megaphones, horns, musical instruments like trumpets, etc. are deigned to send sound by
multiple reflection in a particular direction without spreading in all directions.
ii) Doctors listen to sounds from the human body through a stethoscope. The sound of
heartbeat reaches the doctor’s ears by multiple reflection.
iii) Generally the ceilings of cinema halls and auditoriums are curved so that sound after
multiple reflection reaches all parts of the hall.
Sometimes a curved sound board is placed behind the stage so that sound after multiple
reflection spreads evenly across the hall.
Speed of sound in Different Medium
Sound travels with different speeds in different mediu.The speed of sound also depends u pon the temperature of the medium.Sound travels faster in solids than in air.The speed of sound in solids is much more than the speed of sound in liquids or gases.
Average speed of sound at room temperature is 332 m/s
Application of Ultrasound
They are used to establish ship to ship communication.
They are used to find the depth of a sea.
They are used to diagnose the disease in a human body
They are used to kill bacteria in liquids.
They are used for welding plastics
SONAR
SONAR stands for Sound Navigation and Ranging.It is based on the principle of the reflection of sound wave.The ultrasonic waves from the transmitter of SONAR are sent towards the bottom of the sea whose depth is to be measured.These waves are reflected back from the bottom of the seas and are received by the receiver of the SONAR the transmitter and the receiver are suitably placed on the ship.The time taken by the ultrasonic waves to go from the ship to the bottom of the sea and then back to the ship is recorded
Time taken by the waves to go from the ship to the bottom of the sea is =t/2.If V is the velocity of wave, the the required distance S is given by.
s= v(t/2)
Human Ear
The ear mainly consists of three basic parts the outer ear, the middle ear, the the inner ear.Each part of the ear serves a specific purpose in the task of the detecting and interpreting sound.The outer ear serves to collect and channel sound to the middle ear.The middle ear serves to transform the energy of a sound wave into the internal vibrations of the bone structure and ultimately transform these vibrations into a compressional waves in the inner ear.The inner ear serves to transform the energy of a compressional wave with the inner ear fluid into nerve impulses which can be transmitted to the brain.
No comments:
Post a Comment