Class 9 Physics Sound Notes for CBSE
Wave motion
Introduction
When a pebble is thrown in a pond of still water, circular ripples called waves or pulses move outward on the surface of water as shown in the figure. These waves are in the form of disturbance that travels outward, and no portion of the medium (water in this case) is transported from one part to another part of the medium. The particles of the medium simply vibrate about their mean positions.
Figure
Thus, wave is a form of disturbance which travels through a material medium due to the repeated periodic motion of the particles of the medium about their mean positions. The disturbance is handed over from one particle to another particle of the medium without the actual movement of the particles of the medium.The velocity associated with the particle of the medium is called particle velocity and the velocity associated with wave propagation is called wave velocity.
Definition
A wave motion is a means of transferring energy from one point to another without any actual transportation of matter between these points.
In a wave motion, disturbance travels through some medium, but the medium does not travel along with the disturbance.
Classification of waves
(A) Depending on medium requirement, waves can be classified as :
(i) Mechanical waves
(ii) Non Mechanical or Electromagnetic waves
(B) Depending upon the direction of vibration of medium, particles waves are classified as :
(i) Transverse waves
(ii) Longitudinal waves
Mechanical wave
Those waves which need a material medium (like solid, liquid or gas) for their propagation, are called mechanical waves or elastic waves.
A mechanical wave cannot travel through vacuum.
A mechanical wave involves the vibrations of the particles of the medium about their mean position. The vibrating particles possess energy and it is this energy which moves forward in the form of a mechanical wave.
Examples of mechanical waves :
(i) Sound waves in air.
(ii) Water waves.
(iii) Waves produced in a stretched string.
(iv) Waves produced in spring.
Requirement for propagation of a mechanical wave
(i) Source which produces disturbance.
(ii) An elastic medium (inertia medium). The disturbance from one point to another is transferred due to the elasticity of the medium.
Electromagnetic waves
Those waves which do not need a material medium for their propagation and can travel even through a vacuum, are called electromagnetic waves because they do not require a material medium (like solid, liquid or gas) for their propagation, they can travel even through vacuum. An electromagnetic wave involves the electric and magnetic fields of the empty space (or vacuum). Thus, the main difference between mechanical waves (or elastic waves) and electromagnetic waves is that the mechanical waves (or elastic waves) require a material medium (like solids, liquids or gases) for their propagation whereas electromagnetic waves can propagate even through vacuum. Examples of electromagnetic waves are (i) Radio waves (ii) Infra-red waves (iii) Visible (light) waves.
Difference between mechanical or elastic waves and electromagnetic waves
The main points of difference between elastic waves and electromagnetic waves are given below. Please note that elastic waves include sound waves and water waves whereas electromagnetic waves include light waves and radio waves.
(i) Elastic waves (or mechanical waves) are due to the vibrations of the particles of the medium through which they pass whereas electromagnetic waves are due to the varying electric and magnetic fields in space.
(ii) Elastic waves (or mechanical waves) have a low speed. On the other hand, all the electromagnetic waves have a high speed of 3 × 108 m/s in vacuum.
(iii) Elastic waves (or mechanical waves) have usually low frequency and large wavelength. On the other hand, electromagnetic waves have very high frequency and extremely short wavelength.
(iv) Elastic waves (or mechanical waves) can be transverse waves or longitudinal waves but electromagnetic waves are only transverse waves.
Transverse wave
A wave motion in which an individual particle of the medium vibrates in a direction at right angles to the direction of propagation of wave is called transverse wave motion.
For transverse wave motion to be set up, a medium must posses
(i) elasticity i.e. a tendency to gain back their normal position when disturbed.
(ii) inertia so that the particle overshoots the mean position.
(iii) force of cohesion so that the motion is gradually handed over from one particle to the next.
(a) Water waves show crests and troughs
(b) Transverse waves on a rope or stretched string
In the case of waves formed over the surface of water, the individual particles of water oscillate in a direction at right anglesto the direction of propagation of wave (figure (a)). Similarly, if a heavy rope with one of its ends tied to a hook H in the wall is stretched along the length of the room and is given an upward and downward jerk at the free end A, a wave is seen to travel along the length of the room as shown in figure (b). Every part of rope vibrates up and down while wave train travels along the rope.
Longitudinal wave
A wave motion in which the particles of the medium vibrate about their mean position along the direction of propagation of the wave is called longitudinal wave motion. For example, sound wave in air (340 m/s).
When a longitudinal wave travels in a medium then the particles of the medium vibrate back and forth in the same direction in which the wave travels.
At any instant there are points in space where pressure or density is maximum, called as compression and there are points where pressure or density is minimum called as rarefaction. These compressions and rarefactions occur one after the other. From a compression to a rarefaction the pressure or density continually varies from a maximum to a minimum. Figure below shows the propagation of a longitudinal wave in, say, air.
A longitudinal wave
Differences between transverse and Longitudinal wave
| S. No. | Transverse waves | Longitudinal waves |
| 1 | Particles of medium vibrate perpendicular to direction of wave propagation | Particles of medium vibrate in the direction of wave propagation |
| 2 | Transverse mechanical waves can propagate through solids and over liquid surfaces | They can propagate through solids; liquids and gases |
| 3 | Examples are electromagnetic waves, water, waves, etc. | Examples are sound waves, waves on slinky etc. |
Wave Terminology
(1) Amplitude (A) : It is the maximum displacement of the particle of a medium from its equilibrium state, during the propagation of a wave. In S.I., unit of amplitude is meter (m).
(2) Wavelength (\lambda ) : The distance between two nearest (adjacent) crests or two nearest (adjacent) troughs of a wave is called its wavelength or the distance between any two nearest particles of the medium vibrating in the same manner is called wavelength.
In S.I., unit of wavelength is meter (m).
Smaller units of wavelength are cm., nanometer (nm), angstrom (
).
1 nanometer (nm) = 10–9 m.
1 angstrom () = 10–10 m.
(3) Time-period (T) : Time taken by a vibrating particle or a body to complete one vibration or oscillation is known as time period.
(4) Frequency (f) : Number of complete wavecycles that pass a given point in one second is called its frequency.
f = \frac{1}{T} 1 Hertz = \frac{1}{{{\rm{Second}}}} = 1 cycle per second (cps)
In SI, unit of frequency is hertz (Hz).
Relation between time period and Frequency
We know that time-period is equal to the “time required to produce one wave” and frequency is equal to the “number of waves produced in one second”. This means that “time-period” is equal to the “reciprocal of frequency”. i.e.
{\rm{Time}}\,{\rm{Period = }}\frac{{\rm{I}}}{{{\rm{Frequency}}}}or T = \frac{I}{f}
where T = Time-period of the wave
and f = frequency of the wave
Velocity of Wave
The distance travelled by a wave in one second is called velocity of the wave (or speed of the wave). The velocity of a wave is represented by the letter v. The S.I. unit for measuring the velocity of a wave is metres per second (m/s or ms-1).
Relationship between Velocity, Frequency and Wavelength of a wave
We know that, {\rm{Velocity = }}\frac{{\,{\rm{Distance}}\,\,{\rm{travelled}}}}{{{\rm{Time}}\,\,{\rm{Taken}}}}
Suppose a wave travels a distance lambda, \lambda (which is its wavelength) in time T, then :
v = \frac{\lambda }{T}Here T is the time taken by one wave. We know that \frac{1}{T} becomes the number of waves per second and this is known as frequency (f) of the wave. So, we can write f in place of \frac{1}{T} in the above relation.
Thus, v = f \times \lambda
where v = velocity of the wave
f = frequency
and \lambda = wavelength
In other words, Velocity of a wave = Frequency × Wavelength
Thus, the velocity (or speed) of a wave in a medium is equal to the product of its frequency and wavelength. The formula v = f \times \lambda is called wave equation. It applies to all types of waves, transverse wave (like water waves), longitudinal waves (like sound waves) and electromagnetic waves (like light waves and radio waves). The wave equation has three quantities in it, so if we know the values of any two quantities then the value of third quantity can be calculated. We will use this formula to solve numerical problems.
Characteristics of Wave Motion
(1) Wave is a disturbance travelling through the medium.
(2) Only energy (and no particle) is transferred in a wave motion.
(3) Energy transfer takes place with a constant speed, if medium properties are homogenous.
(4) Wave velocity (v = f\lambda ) is constant throughout the medium while the velocity of particle is different at different positions (maximum at mean, zero at extreme position).
(5) There is a continuous phase difference amongst the successive particles of the medium.
(6) Wave motion is possible in a medium which possesses the property of elasticity and inertia.
Sound Waves
Definition
Sound is a form of energy which produces the sensation of hearing. This sensation is produced by longitudinal waves in an elastic medium.
Production of sound waves
In laboratory, sound is produced by a tuning fork when hit on a rubber pad, plucking a stretched string of sitar, by hitting table, by flute etc.
A vibrating body produces sound. When a vibrating body moves forward then the particles of the medium (say air) are set into vibration. The particles of the medium which are very close to the vibrating body are pushed away from the body. These particles of the medium bump against the neighbouring particles. Hence the number of particles of the medium in the region where the displaced particles bump against the neighbouring particles is large. Points in the region where the density is maximum are called as compression (C). Since pressure is directly proportional to number of particles, so the compression is a point where pressure is also maximum.
When the vibrating body moves backward, a region of emptiness is created. The points where the density or pressure is minimum are called as rarefaction (R).
When a body vibrates to produce sound, compressions and rarefactions follow one another as the sound travels through the medium away from the vibrating body. Pressure or density from a compression to a rarefaction continually varies.
Let us brief this phenomenon by a tuning fork.
We have drawn three pictures of a tuning fork to help you visualize how air molecules might look around a tuning fork. Look at tuning fork #1 (figure). When the tuning fork is at rest, the fork is surrounded by molecules of the air.
Figure : Tuning fork
As the tuning fork’s prongs move apart because of a vibration, the molecules ahead of it are crowded together. (See tuning fork #2) (figure). They look as if they are being pushed together. They bump each other.
Figure : Tuning fork
As the tuning fork’s prongs come back together, (see tuning fork #3) (figure), they leave a region that has fewer molecules than usual.
As a tuning fork vibrates, it causes molecules in the air to move. The molecules bump into other molecules nearby, causing them to move. This process continues from molecule to molecule. The result is a series of compressions and rarefactions that make up sound waves. As these regions of varying pressure fall on an ear, they cause ear drum to vibrate and produce the sensation of hearing.
Figure : Tuning fork
Note : Unlike light waves, sound waves do not travel through a vacuum. They need matter to travel. That is why sound can travel through a wall.
Sound Needs medium to Travel
(1) Put an electric bell inside a closed jar having air. Connect the bell with battery, it rings.
(2) Now start evacuating air from the jar using a vacuum pump. Less and less sound is heard. When there is no air, no sound is heard. Sound needs air/medium to travel.
Figure
Speed of Sound in different media
Sound travels with different speed in different media like solid, liquid and gas. This is because, sound travels in a medium due to the transfer of energy from one particle to another particle of the medium.
Solid : Since the particles of solid are close to each other, transfer of energy from one particle to another takes place in less time (i.e. faster). Hence speed of sound in solids is large.
Liquid : Speed of sound in liquids is less than in solids since the particles are away from each other as compared to solids.
Gas : Speed of sound in gases is less than the speed of sound in liquids and solids as the particles are far away as compared to solids and liquids.
Figure
Effect of Temperature on The Speed of Sound
Sound travels faster as the temperature of the medium increases and vice-versa. This happens because as temperature increases, the particles of the medium collide more frequently and hence the disturbance spreads faster.
Speed of sound in air increases by 0.61 m/s with every 1°C increase in temperature. For e.g. if speed of sound in air at 0°C is 330 m/s then its speed at 25°C will be 345 m/s.
Characteristics of Sound Wave
When a sound wave travels through a material medium then the density or pressure of the medium changes continuously from maximum value to minimum value and vice-versa. Thus, the sound wave propagating in a medium can be represented as shown in the figure.
Figure
Now, we shall discuss the characteristics or quantities to describe a sound wave.
Amplitude
The maximum displacement of a vibrating body or particle from its mean position is called amplitude. S.I.unit of amplitude is metre (m). In figure “A” represents the amplitude of sound wave.
Wavelength
The distance between two successive regions of high pressure or high density (or compressions) or the distance between two successive regions of low pressure or low density (or rarefactions) is known as wavelength of a sound wave or distance between any two particles vibrating in same phase is called wavelength of a sound wave. It is denoted by ‘’(read as lambda). (see figure)
S.I. unit of wavelength is metre (m).
Frequency
The number of oscillations or vibrations made by a vibrating body or particles of a medium in one second is known as the frequency of a wave. It is denoted by ‘f’ SI, unit of frequency is hertz (Hz). 1 hertz means that one oscillation is completed by a vibrating body or a vibrating particle in one second.
Figure : Sound wave having frequency 1 Hz.
Time-period
Time taken by a vibrating particle or a body to complete one vibration or oscillation is known as its time period. It is denoted by T.
Let T = time period of a vibrating body
Then number of oscillation completed in T second = 1
number of oscillations completed in 1 second = \frac{1}{T}
But number of oscillation completed in 1 second = frequency (f)
and, f = \frac{1}{T}
Figure : A sound wave having ‘T’ time period
Pitch
Pitch is the characteristic of a sound that depends on the frequency received by a human ear.
Thus a sound wave of high frequency has high pitch and a sound wave of low frequency has a low pitch. You must have noticed that the voice of a woman has higher pitch than the voice of a man. Thus, the frequency of woman’s voice is higher than the frequency of man’s voice.
A sound wave of low pitch (i.e., low frequency) is represented by figure and a sound wave of high pitch (i.e. high frequency) is represented in figure.
Figure
Loudness
Loudness of a sound depends on the amplitude of the vibrating body producing the sound. Thus, a sound produced by a body vibrating with large amplitude is a loud sound. On the other hand, a sound produced by a body vibrating with small amplitude is a feeble or soft sound. Soft or feeble sound and loud sound are represented as shown in the figure respectively.
Figure
Activity : Strike the school bell harder with a hammer. You will hear the loud sound. This is because the bell vibrates with a large amplitude when struck harder. Now, you strike the school bell gently. You will hear a feeble or soft sound. This is because the bell vibrates with a small amplitude when struck gently.
Timbre or quality
Quality or timbre is a characteristic of a sound which enables us to distinguish between the sounds of same loudness and pitch. This characteristic of sound helps us to recognize our friend from his voice without seeing him. The quality of two sounds of same loudness and pitch produced by two different sources are distinguishable because of different wave forms produced by them. For example, the violin and flute (Bansuri).
Figure
Range of hearing
Audible range
The human ear and brain are sensitive to sound waves confined to the frequency range from about 20 Hz to 20 KHz. This range is called audible range.
Audible waves may be initiated by vibrating string (sitar, guitar, etc), vibrating air columns (organ pipes, flutes, Shehnai etc.) and vibrating rods (harmonium etc.).
Ultrasonic waves
A longitudinal wave whose frequency is above the upper limit of audible range i.e, 20 KHz is called ultrasonic wave.
This may be generated by very small sources. For example, a quartz crystal.
Infrasonic waves
The waves of frequency less than 20 Hz are known as infrasonic waves. The infrasonic waves are produced by large vibrating bodies. These waves are not audible to a human ear. For example, infrasonic waves are produced by the vibration of the earth’s surface during the earthquake. Some animals like elephants, rhinocerous, whales etc. also produce infrasonic waves.
It has been observed that animals’ behaviour becomes unusual just before the tremor is felt. This is because the animals have the ability to detect infrasonic waves produced at the time of tremor.
Ultrasound
Definition
Sound of frequency above maximum audible range (ultrasonic range) is called ultrasound.
Production
These are produced by electronic oscillator (a very useful electronic device) using high frequency vibrations of quartz crystal.
Properties
Sound waves of all frequencies carry energy with them. With increase in frequency, vibrations become faster and energy content also increases. When ultrasound travels in a solid, liquid or gas (longitudinal waves can travel through all of them), it subjects the particles of matter to face large force and energy. Subsequently the behaviour of the particles and hence of the matter changes. Because of the smallness of waves, large force and high energy contents, ultrasound has wide range of applications.
Applications
(i) Investigation of structure of matter
Sending ultrasound through bulk of matter and studying the variation in their velocity inside it, valuable information regarding the constitution of complex molecules can be obtained.
(ii) Cleaning
Dirty odd shaped parts, spiral tubes, electronic components are placed in a cleaning solution and ultrasonic waves are passed through it. Due to high frequency vibrations, particles of dirt get detached and drop out. The objects are thoroughly cleaned.
(iii) Detection of flaws (cracks) in metals
Ultrasound is sent through the metallic structure. They pass through unobstructed if the structure is homogeneous. In case of a crack inside the structure, the ultrasound will be reflected and received by the observer. Existence of reflection, confirms the existence of a crack. Due to high frequency and short wavelength, even fine cracks can be detected.
(iv) SONAR (Depth of Seas and Oceans)
Ultrasound oscillator producing waves of frequency 40 kHz is provided inside ships. An ultrasound receiver is also provided inside the ship.
The transmitter oscillator transmits the waves towards bottom and reflected waves are received by the receiver. A recorder finds the time interval (t). Knowing velocity (v) of ultrasound through sea water, the depth (d) can be calculated d = \frac{{Vt}}{2} as . This phenomenon is termed as sound navigation and ranging.
Figure
(v) Emulsions of immiscible liquids
When a strong beam of ultrasound is passed through a liquid, it is heated to a very high temperature. This fact is utilized in preparing homogeneous stable emulsion of immiscible liquids. Ultrasound treated honey does not crystallise.
(vi) Medical and biological effects
(a) When ultrasound is passed through a body part having pain or rigid joints, their high frequency vibrations produce soothing effect and relieve the pain.
(b) Ultrasound sent through brain cures a mental patient.
(c) A newly developed technique of three dimensional photographs with the help of ultrasound (ultrasonography) is being used by the physicians to locate the exact position of an eye tumour and its removal giving normal vision to the patient.
(d) Harmful insects are killed by exposing them to ultrasound.
(vii) Ultrasound in nature
Bats are provided with natural ultrasound equipment. During flight, bat produces ultrasound in a series of pulses. It also receives the sound reflected (echo) from the objects (obstacles or insects) in the neighbourhood. From the time interval, it interprets the distance between itself and the obstacle or insect (reflector producing the echo). By judging the phase difference of the echoes at its two ears, it is able to estimate the postion of the obstacle or insect. Thus, it avoids collision with the obstacle or catches the insect (prey) even in darkness.
Figure : Ultrasound is emitted by a bat and it is reflected back by the prey or an obstacle.
Laws of Reflection for sound
(i) Angle of incidence is equal to the angle of reflection.
(ii) The incident wave, the reflected wave and the normal, all lie in same plane.
Verification of laws of Reflection : Take a smooth polished large wooden board and mount it vertically on the table. At right angle to the board, fix a wooden screen. On each side of the screen, place a long, narrow tube that is highly polished from inside. Place a clock at the end of the tube A. Move the tube B slightly left to right, till a distinct tick of clock is heard. Measure the ∠PCN and ∠RCN between tubes and wooden screen. It is found ∠PCN = ∠RCN. (see figure)This experiment shows that sound waves follow the laws of reflection.
Figure
Megaphone
When we have to call someone at a far off distance (say 100m), we cup our hands around our mouth and call the person with maximum sound we can produce. The hands prevent the sound energy from spreading in all directions. In the same way, the people use horn shaped metal tubes, commonly called megaphones. The loud speakers have horn shaped openings. In all these devices, the sound energy is prevented from spreading out by successive reflections from the horn shaped tubes.(see figure).
Figure
Sound board
The sound waves obey the laws of reflection on the plane as well as curved reflecting surfaces. In order to spread sound evenly in big halls or auditoriums, the speaker (S) is fixed at the principal focus of the concave reflector. This concave reflector is commonly called sounding board . The sound waves striking the sounding board get reflected parallel to the principal axis.(see figure).
Figure
Stethoscope
It is an instrument used by the doctors for listening sounds produced within the body, especially in the heart and lungs. In the stethoscope, the sound produced within the body of a patient is picked up by a sensitive diaphragm and then reaches the doctors ears by mutiple reflection.
Figure : Tube of stethoscope
Echo
Echo is based on the reflection of sound. An echo is defined as repetetion of sound due to reflection. There are a number of tourist places where echo points are marked. If you speak something from there loudly you will hear back your sound after sometime. This is called an echo. At some places, you might listen a number of echos one after the other. This is called as multiple echo. It is not that you will hear an echo at any place. There are certain conditions required for an echo to be heard. Before discussing these conditions we will firstly talk about the term persistence of sound.The impact of any sound heard by us does not vanish immediately. It is due to this that a person can’t hear two sounds if the time delay between them is less than the minimum required. It is found by scientists that if the time delay between the sounds is less than 1/10 sec, they are heard as single sound. Thus to hear two sounds as different sounds the time delay must be at least 1/10 sec. This forms the basis of an important condition needed to hear an echo.
Conditions for formation of an echo
(a) The minimum distance between the source of sound and the reflecting body should be 11 metres.
(b) The wavelength of the sound should be less than the height of the reflecting body.
(c) The intensity of sound should be sufficient so that it can be heard after reflection.
Reverberation
When a sound is produced in a big hall, its waves reflects from the walls and travel back and forth. Due to this the sound does not vanish at once but it fades away gradually, that is the sound persists even after its production has been stopped. A small amount of reverberation is desirable in large halls or cinemas as it makes the sound pleasant and more effective. However too much reverberation is undesirable as it makes the sound confusing. To reduce reverberation the roof and walls of the hall are covered with sound absorbing materials like rough plaster and thick curtains. One may define reverberation as the persistence of sound due to repeated reflection and its gradual fading away.
Figure:
Ear
(1) It is an important organ that percepts sound.
(2) It is the organ that allows us to convert pressure variations in air with audible frequency into electric signals that travel to the brain via the Auditory Nerve.
Structure of Human Ear
Figure
The outer ear is called pinna. It collects the sound from the surroundings. The collected sound passes through the auditory canal. At the end of the auditory canal, there is a thin membrane called the ear drum or tympanic membrane. When a compression of the medium reaches the ear drum, the pressure on the outside of the membrane increases and forces the eardrum inward. Similarly, the eardrum moves outward when a rarefaction reaches it. In this way the eardrum vibrates. The vibrations are amplified several times by three bones (the hammer, anvil and stirrup) in the middle ear. The middle ear transmits the amplified pressure variations received from the sound wave to the inner ear. In the inner ear, the pressure variations are turned into electrical signals by the cochlea. These electrical signals are sent to the brain via the auditory nerve, and the brain interprets them as sound.
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