NCERT Solutions Class 9th Science Chapter – 11 Sound Notes

NCERT Solutions Class 9th Science Chapter – 11 Sound

NCERT Solutions Class 9th Science Chapter – 11 Sound

Chapter – 11

Sound

Notes

Sound 

Characteristics

Reflection of sound

Range of Hearing

Application

Sound

Production of Sound

Sound is produced when object vibrates or sound is produced by vibrating objects. The energy required to make an object vibrate and produce sound is provided by some outside source (like our hand, wind etc.). Example- Sound of our voice is produced by vibration of two vocal cords in our throat [Fig. (a)]. Sound of a drum or tabla is produced by vibration of its membrane when struck [Fig. (b)]. In laboratory experiments, sound is produced by vibrating tuning fork. The vibrations of tuning fork can be shown by touching a small suspended pith ball (cork ball) with a prong of the sounding tuning fork. The pith ball is pushed away with a great force.

Sound can be produced by following methods

Propogation of Sound

• The substance through which sound travels is called a medium. • The medium may be solid, liquid or gas. • When an object vibrates, then the air particles around it also start vibrating in exactly the same way and displaced from their stable position. • These vibrating air particles exert a force on nearby air particles so they are also displaced from their rest position and start to vibrate. • This process is continued in the medium till sound reaches our ears. • The disturbance produced by sound travels through the medium (not the particles of the medium). • Wave is a disturbance which travels through a medium and carries energy. So sound travels in wave form known as mechanical waves.

Sound Waves are Longitudinal Waves

When a body vibrates then it compresses the air surrounding it and forms an area of high density called compression (C). Compression is the part of wave in which particles of the medium are closer to one another forming high pressure. This compression moves away from the vibrating body. When vibrating body vibrates back an area of low pressure is formed called rarefaction (R). Rarefaction is the area of wave in which particles of the medium are further apart from one another forming a low pressure or low density area. When body vibrates back and forth, a series of compression and rarefaction is formed in air resulting in sound wave. Propogation of sound wave is propogation of density change.

Sound needs Medium for Propogation

Sound waves are mechanical waves. It needs material medium for propogation like air, water, steel etc. It cannot travel in vaccum. An electric bell is suspended in airtight bell jar connected with vacuum pump. When bell jar is full of air, we hear the sound but when air is pumped out from the bell jar by vacuum pump and we ring the bell, no sound is heard. So medium is necessary for propagation of sound. So, sound can not be heard directly on the surface of moon/in outer space as there is no air/atmosphere at moon or in outer space. The astronauts who land on moon (or walk in outer space) talk to one another through wireless sets using radio waves.   Experiment to show that sound cannot travel through vacuum

Sound Waves are Longitudinal Waves

(i) A wave in which the particles of the medium vibrate back and forth in the same direction in which the wave is moving, is called a longitudinal wave. When we push and pull the slinky compression (number of turns are more or closer) and rarefaction (number of turns are less or farther) are formed. When a wave travels along with slinky, its each turn moves back and forth by only a small distance in the direction of wave. So the wave is longitudinal. The direction of vibrations of the particles is parallel to the direction of wave.   (ii) When one end of a slinky is moved up and down rapidly whose other end is fixed, it produces transverse wave. This wave moves along the slinky in horizontal direction, while turns of slinky (particles) vibrate up and down at right angle to the direction of wave. Thus in transverse wave particles of the medium vibrate up and down at right angles to the direction of wave. Light waves are transverse waves but they don’t need a material medium for propagation.

Characteristics of Sound Wave

The characteristics of sound waves are: wavelength, frequency, amplitude, time period and velocity. When a wave travel in air the density and pressure of air changes from their mean position. Compression is shown by crest while rarefaction is shown by trough. Compression is the region of maximum density or pressure. Rarefaction is the region of minimum density or pressure.

(i) Wavelength

(a) In sound waves the combined length of a compression and an adjacent rarefaction is called its wavelength. (b) The distance between the centres of two consecutive compressions or two consecutive rarefactions is also called its wavelength. (c) So, we can say that the minimum distance in which a sound wave repeats itself is called its wave lenght. (d) It is denoted by the Greek letter lamda 2. Its SI unit is metre.

(ii) Frequency

(iii) Time Period

(iv) Amplitude

Pitch

The pitch of sound depends on the frequency of sound (vibration). It is directly proportional to its frequency. Greater the frequency, higher is the pitch and lesser the frequency, lower is the pitch. A woman’s voice is shrill having a high pitch while a man’s voice is flat having low pitch. This is because women have shorter vocal cords, which vibrate more quickly and produce a higher pitch. On the other hand, men have longer vocal cords that vibrate with low frequencies giving then a flat deep voice. High pitch sound has large number of compressions and rarefactions passing a fixed point per unit time.

Loudness

The loudness depends on the amplitude of the sound wave. Loudness is the measure of the sound energy reaching the ear per sec. Greater the amplitude of sound wave, greater is the energy, louder the sound; short is the amplitude, less is the energy, soft is the sound. Loudness is measured in decibel ‘dB’. Mathematically, Loudness of sound is proportional to the square of the amplitude of the vibration producing the sound. Loudness & Amplitude eg – if amplitude becomes twice, the loudness increases by a factor of 4 Anything above 80dB, the sound is considered as ‘noise’.

Quality or Timbre

(v) Velocity

Example

Speed of Sound in Various Mediums

Sonic Boom

Reflection of Sound

Like light, sound also bounces back when it falls on a hard surface. It is called reflection of sound. The laws of reflection of light are obeyed during reflection of sound. (i) The incident sound wave, the reflected sound wave and normal at the point of incidence lie in the same plane. (ii) Angle of reflection of sound is always equal to the angle of incidence of sound.

Reflection of Sound

Reverberation

Methods to reduce reverberation in big halls or auditoriums

Difference between Echo and Reverberation

Applications of Reflection of Sound

(i) Megaphone, loudspeakers, bulb horns and trumpets, shehnai etc. are designed to send sound in a particular direction without spreading all around. All these instruments have funnel tube which reflects sound waves repeatedly towards audience. In this amplitude of sound waves adds up to increase loudness of sound. (ii) Stethoscope– It is a medical instrument used for listening the sounds produced in human body mainly in heart and lungs. The sound of the heartbeats reaches the doctor’s ears by the multiple reflection of the sound waves in the rubber tube of stethoscope. (iii) Sound Board– In big halls or auditoriums sound is absorbed by walls, ceiling, seats etc. So a curved board (sound board) is placed behind the speakers so that his speech can be heard easily by audiences. The soundboard works on the multiple reflection of sound. (iv) The ceiling of concert halls are made curved, so that sound after reflection from ceiling reaches all the parts of the hall.

Range of Hearing

Hearing Aid

Applications of Ultrasound

(i) It is used to detect cracks in metal blocks in industries without damaging them. (ii) It is used in industries to clean ‘hard to reach’ parts of objects such as spiral tubes, odd shaped machines etc. (iii) It is used to investigate the internal organs of human body such as liver, gall bladder, kidneys, uterus and heart. (iv) Echocardiography– These waves are used to reflect the action of heart and its images are formed. This technique is called echocardiography. (v) Ultrasonography– The technique of obtaining pictures of internal organs of the body by using echoes of ultrasound waves is called ultrasonography. (vi) Ultrasound is used to split tiny stones in kidneys into fine grains. Which then get flushed out with urine.

Sonar

‘SONAR’ (Sound Navigation And Ranging). SONAR is a device which is used to find distance, direction and speed of underwater objects. SONAR consists of a transmitter and a receptor or detector that is installed at the bottom of a ship. The transmitter produces and transmits ultrasonic waves. These waves travel through water and after striking the objects on the bottom of sea, are reflected back and received by detector.   These reflected waves are converted into electric signals by detector. The sonar device measures the time taken by ultrasound waves to travel from ship to bottom of sea and back to ship. Half of this time gives the time taken by the ultrasound waves from ship to bottom. Let the time interval between transmission and reception of ultrasound signal is t. Speed of sound through sea water is v, total distance travelled by waves = 2d. Then, 2d = v xt. This is called echo ranging. i.e, using echo to range (find out) what is around you. The sonar is used to find the depth of sea, to locate underwater hills, valleys, submarines, icebergs and sunken ships etc. Bats fly in the dark night by emitting high pitched ultrasound waves which are reflected from the obstacle or prey and returned to bats ear. The nature of reflection tells the bat where the obstacle or prey is and what it is like. This is also known as BIO SONAR. Whales and Dolphins also use this in sea.

Structure of Human Ear

The ear consists of three parts: outer ear, middle ear and inner ear. The ears are the sense organs which help us in hearing sound. The outer ear is called pinna. It collects the sound from surroundings. This sound passes through the auditory canal. At the end of auditory canal, is a thin elastic membrane called ear drum or tympanic membrane. The middle ear contains of three bones : hammer, anvil and stirrup linked with one another. Free end of hammer touches ear drum and that of stirrup linked with membrane of oval window of inner ear. The lower part of middle ear has a narrow ‘Eustachian tube’. The inner ear has a coiled tube called cochlea, which is connected with oval window. Cochlea is filled with a liquid containing nerve cells. Other side of cochlea is connected to auditory nerve which goes to brain.

Working

Working of Human ear

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