Sound Class 9 Notes Science Chapter 11

Chapter Notes - Sound           

Introduction

Throughout the day, we listen the various types of sounds like our father's voice, our mother's voice, our teacher's voice, chirping of birds, ringing of a school bell, a telephone ringing, a guitar being played, a siren, a jet engine roaring in the sky, buzzing of a mosquito, a gun shot etc. These sounds stimulate the auditory nerve in the human ear and the brain interprets the sound. Now let us define sound.

Sound is a form of energy which produces the sensation of hearing in our ears.

PRODUCTION OF SOUND

Perform the following activities to produce sound.

Activity

1. Take a plastic scale or ruler from your geometry box. Hold it flat on your desk or table with about half its length protruding (stick out from the surface) over the edge. Now bend it down and release it. It will move up and down rapidly (i.e. it will vibrate) and produce the sound at the same time. The sound will last as long as the vibration (i.e. rapid up and down motion) of the scale continues.

2. Take a tuning fork. Hold it from its stem and strike it with a rubber pad or hammer. You will observe that the prongs of the tuning fork vibrate and at the same time sound is produced (Figure).

3. Place your finger lightly on your throat near the vocal cords as shown in figure. Now say "Ah" for few seconds. You will feel the vibration in your finger as long as you say "Ah".

4. Tie a thin metallic string rigidly at the two ends of a table as shown in figure. Now, pluck the string from the middle and release it. The string begins to vibrate up and down and at the same time, sound is heard.

Conclusion: From these activities, we come to the conclusion that the sound is produced by the vibrating objects or bodies.

Production of Sound in Musical Instruments

When a drum is beaten, then the skin of drum vibrates and sound is produced.

When the strings of a guitar are plucked and released, they vibrate and produce sound. When air is blown into the flute, pipe, clarinet, sexophone etc., it vibrates in the tube of the instrument and hence sound is produced. Sound is also produced when the birds flap their wings during the flight.

What is a Wave?

The movement of the disturbance through a medium due to the repeated periodic motion of the particles of the medium about their mean positions is known as a wave.

MECHANICAL WAVE

A mechanical wave is a periodic disturbance which requires material medium (i.e. solid, liquid or gas) for its propagation. In other words, waves that are characterised by the motion of particles of a medium are called mechanical waves

Examples of mechanical waves

(i) Sound waves in air

(ii) Water waves

(iii) Waves produced due to the earthquake (known as seismic waves)

(iv) Waves produced by supersonic jet planes (known as shock waves)

(v) Waves produced in a stretched string.

(vi) Waves produced in a slinky or long spring.

Types of waves

Waves are of two types : (i) Transverse Wave, (ii) Longitudinal Wave

TRANSVERSE WAVE

If the particles of a medium vibrate or oscillate about their mean positions at right angles to the direction of propagation of the disturbance then the wave is called transverse wave.

Examples : Movement of string of a sitar or violin, membrance of a tabla or dholak, movement of a kink on a rope.

Activity

Describe an activity to show the formation of a transverse wave.

Fix one end of a thin rope and give up and down jerk to the free end of the rope.

The rope oscillates or vibrates up and down as shown in figure. The disturbance travels from the free end to the fixed end but the rope vibrates up and down. This wave is known as transverse wave.

A transverse wave travelling on the surface of water is shown in figure.

When transverse wave travels through the medium, the shape of the medium changes. At some positions, the particles of the medium rise (or elevate) above their mean positions and at some positions, the particles of the medium go down (or depressed) below their mean positions.

The point on the elevation of the medium whose distance from the mean position is maximum is known as crest (C). On the other hand, the point on the depression of the medium whose distance from the mean position is maximum is known as trough(T). Thus, crests and troughs are formed when a transverse wave travels through a medium (Figure).

WAVELENGTH (OR LENGTH OF A WAVE)

The distance between two successive crests or between two successive troughs is known as thewavelength of a transverse wave.

OR

The distance between two successive particles of the medium which are in phase is called wavelength of the wave. It is denoted by l (lambda).

LONGITUDINAL WAVE

If the particles, of a medium vibrate or oscillate to and fro about their mean positions along the direction of propagation of the disturbance then the wave is called longitudinal wave.

Examples :- Sound wave, Organ pipes, Vibration on resonance appratous

Activity

Describe an activity to show the formation of longitudinal wave.

Take a slinky or a long spring which can be easily compressed and extended as shown in figure (a). Fix one end of the slinky with a rigid support. Now push the free end of the slinky in the downward direction and release it. It is observed that the slinky begins to move up and down (i.e. "to and fro") as shown in figure (b). The disturbance travels from the free end to the fixed end and the parts of the slinky vibrate along the direction of the propagation the disturbance. This wave is known as longitudinal wave.

When a longitudinal wave passes through a medium, the medium is divided into the regions ofcompressions (C) and rarefactions (R) as shown in figure (b).

Compression

The part or region of a medium, where the density of the medium is maximum or where the particles of the medium are very close to each other is known as compression. lt is denoted by C.

Rarefaction

The part or region of a medium, where the density of the medium is minimum or where the particles of the medium are far apart from each other is known as rarefaction. It is denoted by R.

PROPAGATION OF SOUND

A vibrating body produces sound. Now we shall study, how the sound travels from one place to another place.

When a body vibrates, then the particles of the medium (say air) around the vibrating body are set into vibrations. The particles of the medium which are very close to the vibrating body are pushed away from the body. These particles of the medium strike against the neighbouring particles. Hence the number of particles of the medium in the region where the displaced particles strike against the neighbouring particles is large. This region is known as compression (C). Since pressure is directly proportional to the number of particles, so the compression is a region of high pressure or high density. When the vibrating body moves backward, a region of emptiness known as rarefaction (R) or a region of low pressure or Low density is created. The displaced particles of the medium rebound into the region of low pressure or rarefaction. At the same time, compression is followed outwards. Therefore, when a body vibrates to produce sound, compressions and rarefactions follow one another as the sound waves travel through the' medium away from the vibrating body. When a sound wave travels through a medium, the particles of the medium simply vibrate about their rest positions and they do not move from one place to another place in the medium.

Figure represents the regions of compressions (or high pressures) and o rarefactions (or low pressures) as the sound propagates in the medium.

Sound needs a medium to travel

We have learnt that sound travels from one place to another place when the energy is transferred from one particle to another particle of a medium like air or gas, liquid, solid etc. It means, sound needs a material medium for its propagation. In other words, sound cannot travel through vacuum.

Demonstration to show that sound waves cannot travel through vacuum.

Put an electric bell inside a closed Bell jar connected with a vacuum pump. Initially, air from the jar is not taken out. Connect the electric bell with a battery (Figute). It rings and the sound produced is heard by us.

Now start evacuating the air from a Bell jar using a vacuum pump, we will hear less and less sound. i.e. the loudness of the sound decreases. When there is no air in the Bell jar, we do not hear sound. This activity demonstrates that sound waves require material medium (in this case air) for its propagation.

Sound waves are longitudinal waves

When a sound wave travels through the material medium, then compressions and rarefactions follow one another. The particles of the medium through which a sound wave travels vibrate to and fro about their mean positions parallel to the direction of propagation of the sound wave. Since the wave is known as longitudinal wave, if the particles of the medium vibrate to and fro about their mean positions parallel to the direction of propagation of the wave, therefore, the sound waves are longitudinal waves.

CHARACTERISTICS OF A 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 figure.

Now, we shall discuss the characteristics or quantities to describe a sound wave.

(i) Amplitude : The maximum displacement of a vibrating body or particle from its rest position (i.e. mean position) is called amplitude. In S.I., unit of amplitude is metre (m).

(ii) Wavelength (or length of a wave) : The distance between two successive 1 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. It is denoted by l (read as lambda).

In S.I., unit of wavelength is metre (m).

(iii) 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 u (read as Neu). In S.I., unit of frequency is hertz (Hz).

1 hertz = one oscillation completed by a vibrating body or a vibrating particle in one second.

(iv) Time period : Time taken by a vibrating particle or a body to complete one vibration or oscillation is known as time period. It is denoted by T.

In S.I., unit of time period is second(s).

Relation between Frequency and time period Let T = time period of a vibrating body.

Then number of oscillations completed in T second = 1

Number of oscillations completed in 1 second = 

But number of oscillations completed in 1 second = frequency (v)

(f) n = , frequency = 

(iv) Pitch or Shrillness : Pitch is the characteristic (i.e., typical feature) of a sound that depends on the frequency received by a human ear.

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 (a) and a sound wave of high pitch (i.e. high frequency) is represented by figure (b)

(v) Loudness : Loudness of a sound depends on the amplitude of the vibrating body producing the sound.

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. Loud sound and soft or feeble sound are represented as shown in Figure (a) and (b) respectively.

Loudness is a subjective quantity : It depends on the sensitivity or the response of our ears. A loud sound to a person may be a feeble sound for another person who is hard of hearing.

(vi) Timbre or quality : Quality or timbre is a characteristic (i.e., a typical feature) of a sound which enables us to distinguish between the sounds of same loudness and pitch.

This characteristic of sound helps us to recognise 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 waveforms produced by them. The waveforms produced by a vibrating tuning fork, violin and flute (Bansuri) are shown in figure.

(vii) Intensity : Intensity of a sound is defined as the sound energy transferred per unit time through a unit area placed perpendicular to the direction of the propagation of sound.

That is, intensity of sound = 

Intensity of a sound is an objective physical quantity. It does not depend on the response of our ears.

In S.I., unit of intensity of sound is joule s^_1 m^_2 or watt m^_2 ( 1Js^_1 = 1W)

Relationship between wave velocity, frequency, and wavelength for a periodic wave.

What is the relationship between wave velocity, frequency and wavelength

From the definition,

Velocity = 

So, for a wave, Wave velocity = 

A wave takes time equal to its time period (T) to travel a distance equal to its wavelength (l). So,

Wave velocity =  ...........(1)

or  .............(2)

As per definition, Frequency of the wave, 

So, Eq. (2) can be written as,

Wave velocity = Frequency of the wave × Wavelength of the wave

or 

Speed of Sound in different Media

We have seen above that sound can travel through solids, liquids and gases. The question which comes to mind is how fast does sound travel? Sound travels at different speeds in different media.

The speed of sound depends on the following factors :

The properties (or nature) of the medium. The order of the speed of sound is

Solids > Liquids > Gases

Temperature

Pressure

In any medium, the speed of sound is increases with a rise in temperature.

As per definition,

Speed of sound = 

The speed of light in the air (or more correctly in vacuum) is 3 × 10⁸ m/ s, (3lakh kilometre per second).

Conclusion

Speed of sound in solids is greater than the speed of sound in liquids and the speed of sound in liquids is greater than the speed of sound in gases.

Speed of sound in various media

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​REFLECTION OF SOUND

When a sound wave travelling in a medium bounces back to the same medium after striking the second medium, reflection of sound wave is said to take place. The reflection of sound wave is similar to the bouncing back of a rubber ball after striking a wall or the surface of a floor.

Just like light, sound is reflected by the solid and liquid surfaces. The reflection of sound obeys the laws of reflection.

The laws of reflection of sound are as follows :

(i) Incident angle = Reflected angle and (ii), The incident direction of sound, reflected direction of sound and the normal to the point of incidence all lie in the same plane.

Echo

If we clap our hands while standing at some distance from a high and huge wall or a hill, we hear the clapping of our hands again after some short interval of time. The sound of clap heard by us is known as echo. Echo is produced due to the reflection of sound.

Thus, echo is a repetition of sound due to the reflection of original sound by a large and hard obstacle.

Conditions for the production of an echo

1. Time gap between the original sound and the reflected sound

We can hear the two sounds separately if the time gap between these two sounds is more than 1/10 s or 0·1 s. The time interval equal to 0·1 s is known as persistance of hearing. This means, the impression of any sound heard by us remains for 0·1 s in our brain. If any other sound enters our ears before 0·1 s, then the second sound will not be heard by us. Thus, the echo will be heard if the original sound reflected by an obstacle reaches our ears after 0·1 s.

2. Distance between the source of sound and obstacle

Minimum distance between the observer and the obstacle for echo to be heard

Let

Distance between the observer and the obstacle = d

Speed of sound (in the medium) = v

Time after which echo is heard = t

Then, t =  or d = 

We know

Speed of soune; in air at 25°C = 343 m s^-1

For an echo to be heard distinctly,

t  0.1 s

Then d   or d 17.2 m

Thus, the minimum distance (in air at 25°C) between the observer and the obstacle for the echo to be heard clearly should be 17.2 m.

The speed of sound increases with a rise in temperature. Therefore, the minimum distance in air between the observer and the obstacle for an echo to be heard clearly at temperatures higher than 25°C is more than 17.2m. In rooms having walls less than 17.2 m away from each other, no echo can be heard.

3. Nature of the obstacle : For the formation of an echo, the reflecting surface or the obstacle .must be rigid such as a building, hill or a cliff.

4. Size of the obstacle : Echoes can be produced if the size of the obstacle reflecting the sound is quite large.

Reverberation

The repeated reflection that results in the persistence of sound in a large hall is called reverberation.

Excessive reverberation in any auditorium/hall is not desirable because the sound becomes blurred and distorted. The reverberation can be minimised/reduced by covering the ceiling and walls with sound absorbing materials such as, fiber-board, rough plaster, draperies, perforated carboard sheets etc.

Uses of multiple Reflection of sound

1. Megaphone : Megaphone is a device used to address public meetings. It is a orn-shaped. When we speak through megaphone, sound waves are reflected by the megaphone. These reflected sound waves are directed towards the people (or audience) without much spreading.

2. Hearing Aid : Hearing aid is used by a person who is hard of hearing. The sound waves falling on hearing aid are concentrated into a narrow beam of sound waves by reflection. This narrow beam of sound waves is made to fall on the diaphragm of the ear. Thus, diaphragm of the ear vibrates with large amplitude. Hence, the hearing power of the person is improved.

3. Sound boards : Sound boards are curved surfaces (concave) which are used in a big hall to direct the sound waves towards the people sitting in a hall. The speaker is (i.e. source of sound) placed at the focus of the sound board as shown in figure.

Sound waves from the speaker are reflected by die sound board and these reflected waves are directed towards the people (or audience).

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4. Stethoscope : Stethoscope is a device used by doctors to listen the sound produced by heart and lungs. The sound produced by heart beat and lungs of a patient reaches the ears of a doctor due to multiple reflection of sound.

5. Ceilings of concert halls are curved : The ceilings of concert halls and auditoriums are made curved. This is done so that the sound reaches all the parts of the hall after reflecting from the ceiling as shown in figure. Moreover, these ceilings are made up of sound absorbing materials to reduce the reverberation.

RANGE OF HEARING (AUDIBLE RANGE)

All vibrating bodies produce waves. Each wave has its own frequency. The frequency of a wave is equal to the frequency of the vibrating body producing sound. When a woman speaks, the waves produced by the vocal cords in her throat have different frequency than the frequency of the waves produced by the vocal cords of a man. Can human ears hear all the frequencies produced by the vibrating bodies ? The answer is No. In fact, normal human ears can hear only those waves whose frequency lies between 20 Hz and 20,000 Hz. The waves having frequency between 20 Hz and 20,000 Hz are known as sound waves. Thus, the audible range of frequency is 20 Hz to 20,000 Hz.

The waves having frequency less than 20 Hz and greater than 20,000 Hz cannot be heard by human ear.

Infrasonics or InFrasound

The waves of frequency less than 20 Hz are known as infrasonic waves.

The infrasonic waves are produced by large vibrating bodies.

For example, infrasonic waves are produced by the vibration of the earth's surface during the earthquake. Some animals like elephants, rhinoseroses and whales etc. also produce infrasonic waves. These waves are not audible to a human ear.

It has been observed that animals behaviour becomes unusual just before the tremor is felt. This is because the animals has the ability to detect infrasonic waves produced at the time of tremor.

Ultrasonics or Ultrasound

The waves of frequency greater than 20,000 Hz are known as ultrosonic waves or ultrasound. These waves are not audible to a human ear but they can be heard by animals and birds.

Bats can produce ultrasonic waves by flapping their wings. They can also detect these waves. The ultrasonic waves produced by the bats after reflection from the obstacles like buildings guide them to remain away from the obstacles during their flights. Hence, they can fly during night without hitting the obstacles. Bats also catch their prey during night with the help of ultrasonic waves. The ultrasonic waves produced by a bat spread out. These waves after reflecting from a prey sayan insect reach the bat. Hence, the bat can easily locate its prey.

Dolphins also produce ultrasonic waves. They can also detect the ultrasonic waves. They catch their prey like a fish due to their ability to detect the ultrasonic waves reaching them after reflecting from a fish. [CBSE 2010] ​

APPLICATIONS OF ULTRASOUND (ULTRASONIC WAVES)

Ultrasonic waves have number of uses :

1. Ultrasonic vibrations are used for homogenising milk, i.e., the milk is agitated with ultrasonic vibrators. These vibrations break down the larger particles of the fat present in milk to smaller particles.

2. Ultrasonic vibrations are used in dish washing machines. In such machines, water and detergent are vibrated with ultrasonic vibrators. The vibrating detergent particles rub against the dirty utensils and thus clean them.

3. Ultrasonic vibrations produce a sort of depression in rats and cockroaches.Ultrasonic vibrators are used to drive rats and cockroaches from godowns.

4. Ultrasonic vibrations are used for imaging internal organs of human body. In fact they are even used to study the growth of foetus in mother's womb.

5. Ultrasonic vibrations are used in relieving pain in joints and muscles.

6. Ultrasonic vibrations are used in detecting flaws in articles made from metals. They are also used in finding the thickness of various parts of a metallic component.

Sonar

SONAR stands for Sound Navigation and Ranging.

It is a device which is used in the ships to locate rocks, icebergs, submarines, old ships sank in sea ete. It is also used to measure the depth of a sea.

Principle : It is based on the principle of the reflection of sound wave (i.e. echo).

Determination of the Depth of a Sea using Sonar

A beam of ultrasonic waves from the transmitter of a SONAR fitted on the ship is sent towards the bottom of the sea. This beam is reflected back from the bottom of the sea and is received by the receiver of the SONAR 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 noted. Let it be 't' seconds. Therefore, the time taken by the ultrasonic waves to go from the ship to the bottom of the sea is t/2 seconds.

Using the following formula  .we can find the depth of the sea.

Here, u = speed of ultrasonic wave in water.

S = depth of the sea

THE HUMAN EAR

In this article we will learn about the accoustics of hearing. We will see how a human ear converts sound energy into mechanical energy and then to a nerve impulse wh!ch is transmitted to the brain.

The human ear consists of (a) the outer ear (pinna), (b) the middle ear, (c) the inner ear. Each part has a specific task to perform. The outer ear, collects the sound and guides it to the middle ear. In the middle ear sound energy is converted into mechanical energy in the form of internal vibrations of the bdne structure. These vibrations are then transferred into the inner ear which converts the vibrations into nerve impulses.

The outer ear has an approximately 2 cm tong ear canal. Here the sound is collected and amplified. It is in the form of pressure waves with alternate high pressure and low pressure regions.

The middle ear consists of eardrum (tympanic membrane) three tiny inter connected bones-the hammer (mallens), anvil (incus) and stirrup (stapes). The eardrum is a tightly stretched membrane. As the incoming pressure wave from the outer ear strikes, the ear drum starts to vibrate. A compression forces the eardrum inwards whereas a rarefaction forces the eardrum outwards. This means that the eardrum vibrates at the same frequency as that of the sound wave. The eardrum is connected to hammer which in turn is connected to anvil and stirrup. The motion of eardrum will set the hammer, anvil and stirrup into motion at the same frequency as that of eardrum. The three-bone system amplifies the sound further.

The stirrup is connected to the inner ear which consists of cochlea, semi circular canals and the auditory nerve. The vibrations are turned into electrical signals in inner ear which are sent to the brain via the auditory nerve. The brain interprets the sound by the electrical impulses it receives.

Some suggestions to keeps the ears healthy are given below:

Never insert any pointed object into the ear. It can damage the eardrum and make a person deaf.

Never shout loudly or produce a loud sound into someone's ear.

Never hit anyone hard on his/her ear.