CBSE NOTES CLASS 9 SCIENCE CHAPTER 12

SOUND

Sound

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

Source of sound

Sound is produced by vibrating motion of an object. Examples, vibrating tuning fork, oscillating pendulum, a taught string, vibrating air column etc.

• Describe an activity for producing sound.

(a) Take a tuning fork and set it vibrating by striking its prong on a rubber pad.

(b) On bringing it near our ear, we hear a sound.

(c) If we touch one of the prongs of the vibrating tuning fork with our finger, we feel the tuning fork vibrating and the energy being passed to the hand.

(d) If we touch a suspended plastic ball gently with the prong of a vibrating tuning fork, we observe that the ball also start vibrating. That is the energy is being passed from the tuning fork to the ball.

Propagation of Sound

Sound needs medium to travel.

The matter or substance through which sound is transmitted is called a medium. It can be solid, liquid or gas.

• Describe an activity to show that sound cannot travel in vaccum.

Take an electric bell and an airtight glass bell jar. The electric bell is suspended inside the airtight bell jar. The bell jar is connected to a vacuum pump. If we press the switch we will be able to hear the bell. Now start the vacuum pump. When the air in the jar is pumped out gradually, the sound becomes fainter, although the same current is passing through the bell.

After some time when very little air is left inside the bell jar we hear a very feeble sound.

If the air is removed completely, we do not hear hear the sound of the bell?

Hence we can conclude that sound cannot travel through vaccum.

Mechanism of Propagation of Sound

Sound moves through a medium from the point of generation to the listener through alternate compressions and rarefactions in the medium.

Compression: When a vibrating object moves forward, it pushes and compresses the air in front of it creating a region of high pressure. This region is called a compression (C).

This compression starts to move away from the vibrating object.

Rarefaction: When the vibrating object moves backwards, it creates a region of low pressure called rarefaction (R).

As the object moves back and forth rapidly, a series of compressions and rarefactions is created.

This process continues in the medium till the sound reaches the ear.

• The particles do not travel but simply vibrate and exert a force on the adjacent particle.

Q: Why does sound travel faster in solids compared to liquids or gases?

The closer the molecules are to each other and the tighter their bonds, the less time it takes for them to pass the sound to each other and the faster sound can travel. It is easier for sound waves to go through solids than through liquids because the molecules are closer together and more tightly bonded in solids.

Wave

A wave is a periodic disturbance that moves and transfers energy from one place to another. All waves do not need medium to travel.

Types of waves

(A) Mechanical waves

A mechanical wave is a periodic disturbance which requires a material medium for its propagation. On the basis of motion of particles the mechanical waves are classified into two parts.

(a) Transverse wave

When the particles of the medium vibrate in a direction perpendicular to the direction of propagation of the wave, the wave is known as the transverse wave. For example, waves produced in a stretched string.

(b) Longitudinal wave

When the particles of the medium vibrate along the direction of propagation of the wave then the wave is known as the longitudinal wave. For example sound wave in air, or wave produced in a slinky.

(B) Electromagnetic waves

The waves which do not require medium for propagation are called electromagnetic waves these waves can travel through vacuum also. For example, light waves, X-rays.

Characteristics of a sound wave

(i) Frequency

(ii) Amplitude

(iii) Speed

(i) Frequency

The number of vibrations or oscillations per second is called frequency of the sound wave.

The SI unit of frequency is hertz or Hz.

The symbol is f or ν

Pitch is the interpretation of the frequency of an emitted sound by the brain and is the characteristic which distinguishes a shrill (or sharp) sound from a deep (or flat) sound. The pitch of a sound is determined by the rate of vibration, or frequency, of the sound wave.

• Higher the frequency, higher is the pitch.

• Tight strings produce higher pitch than loose ones

• Thicker objects (strings) produce lower pitch than thin ones.

• Smaller strings produce higher pitch

Time Period

The time taken by two consecutive compressions or rarefactions to cross a fixed point is called the time period of the wave. In other words, the time taken for one complete oscillation in the density of the medium is called the time period of the sound wave. Measured in seconds and symbol is T.

or

Wavelength (λ)

Wavelength is the distance between identical points in the adjacent cycles of a waveform, e.g., the distance between two adjacent crests or two adjacent troughs.

(ii) Amplitude

The magnitude of the maximum disturbance in the medium on either side of the mean value is called the amplitude of the wave. Symbol is A.

The loudness or softness of a sound is determined by its amplitude.

Loudness is a measure of the sound energy reaching the ear per second

Larger the amplitude louder is the sound.

The amplitude of the sound wave depends upon the force with which an object is made to vibrate.

Loud sound can travel a larger distance as it is associated with higher energy.

Amplitude and loudness decrease with the distance from the source.

Intensity

The amount of sound energy passing each second through unit area is called the intensity of sound.

Difference between loudness and intensity

The terms “loudness” and “intensity” are not the same.

Loudness is a measure of the response of the ear to the sound, whereas intensity is the amount of sound energy passing each second through unit area. Even when two sounds are of equal intensity, we may hear one as louder than the other simply because our ear detects it better.

(iii) Velocity of wave (v)

The distance travelled by a wave in one second is called velocity of the wave (or speed of the wave). The S.I. unit for the velocity of a wave is metres per second (m/s or ms-1).

Velocity v = ν λ

and

• The speed (or velocity) of sound depends on the properties of the medium through which it travels. The speed of sound decreases when we go from solid to gaseous state.

• The speed of sound in a medium also depends on temperature and pressure of the medium. In any medium as we increase the temperature the speed of sound increases.

• The speed of sound in air is 331 m s–1 at 0 ºC and 344 m s–1 at 22 ºC.

Sonic Boom

When the speed of any object exceeds the speed of sound it is said to be travelling at supersonic speed. Bullets, jet aircrafts etc. often travel at supersonic speeds.

When a sound, producing source moves with a speed higher than that of sound, it produces shock waves in air. These shock waves carry a large amount of energy. The air pressure variation associated with this type of shock waves produces a very sharp and loud sound called the “sonic boom”.

The shock waves produced by a supersonic aircraft have enough energy to shatter glass and even damage buildings.

Timber

The quality or timber of sound is that characteristic which enables us to distinguish one sound from another having the same pitch and loudness. The sound which is more pleasant is said to be of a rich quality.

A sound of single frequency is called a tone. The sound which is produced due to a mixture of several frequencies is called a note and is pleasant to listen to.

Noise is unpleasant to the ear! Music is pleasant to hear and is of rich quality.

Reflection of Sound

Bouncing back of waves like sound from a surface is called reflection.

Reflection of sound follows the laws of reflection,

1. The incident wave, the reflected wave, and the normal to the surface all lie in the same plane.

2. The angle of reflection (r) is equal to the angle of incidence (i).

Echo

Echo is a reflection of sound from tall buldings or mountains that arrives at the listener with a delay after the direct sound.

If t is the time at which an echo is heard, d is the distance between the source of sound and the reflecting body and v is the speed of sound, the total distance travelled by the sound is 2d.

Then v = $\frac{2\mathrm{d}}{\mathrm{t}}$ or d = $\frac{\mathrm{v}\mathrm{t}}{2}$

Persistence: The sensation of sound persists in our brain for about 0.1s. The phenomenon is called persistence.

For hearing distinct echoes,

• The reflected sound (echo) should arrive after minimum of 0.1 second.

• The minimum distance of the obstacle from the source of sound must be 17.2 m (calculate).

• The wavelength of sound should be less than the height of the reflecting body.

• The intensity of sound should be sufficient so that it can be heard after reflection.

Echoes may be heard more than once due to successive or multiple reflections.

The rolling of thunder is due to the successive reflections of the sound from a number of reflecting surfaces, such as the clouds and earth.

Reverberation

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

In an auditorium or big halls reverberation is undesirable, because it causes disturbance in hearing.

To reduce reverberations, the roof and walls of the auditorium are generally covered with sound-absorbent materials like compressed fibre board, rough plaster or draperies. The seat materials are also selected on the basis of their sound absorbing properties.

Uses of Multiple Reflection of Sound

1. Megaphones or loudhailers, horns, musical instruments such as trumpets and shehanais, are all designed to send sound in a particular direction without spreading it in all directions. In these instruments, a tube followed by a conical opening reflects the sound successively to guide most of the sound waves from the source in the forward direction towards the audience.

2. Stethoscope is a medical instrument used for listening to sounds produced within the body, mainly in the heart or lungs. In stethoscopes the sound of the patient’s heartbeat reaches the doctor’s ears by multiple reflection of sound.

3. Generally the ceilings of concert halls, conference halls and cinema halls are curved so that sound after reflection reaches all corners of the hall.

Sometimes a curved soundboard may be placed behind the stage so that the sound, after reflecting from the sound board, spreads evenly across the width of the hall

Range of Hearing

The audible range of sound for human beings extends from about 20 Hz to 20000 Hz.

Children under the age of five can hear up to 25 kHz (1 kHz = 1000 Hz).

Infrasonic Waves

Sounds of frequencies below 20 Hz are called infrasonic sound or infrasound. Example - waves due to oscillating pendulum.

Rhinoceroses, whales and elephants produce and hear sound in the infrasound range.

Some animals get disturbed before earthquakes. Earthquakes produce low-frequency infrasound before the main shock waves begin which possibly alert the animals.

Ultrasonic waves or ultrasounds

Waves with frequencies higher than 20 kHz are called ultrasonic waves or ultrasounds.

Ultrasound is produced and heard by dolphins, dogs, bats, porpoises, rats, moths etc.

How bats navigate and catch their prey?

Bats search out prey and fly in dark night by emitting and detecting reflections of ultrasonic waves. The high-pitched ultrasonic squeaks of the bat are reflected from the obstacles or prey and returned to bat’s ear. The nature of reflections tells the bat where the obstacle or prey is and what it is like.

Porpoises also use ultrasound for navigation and location of food in the dark.

Moths of certain families have very sensitive hearing equipment. They can hear the high frequency squeaks of the bat and know when a bat is flying nearby, and are able to escape capture.

Applications of ultrasound

• The ultrasound is commonly used for medical diagnosis and therapy, and also as a surgical tool.

• Ultrasounds can be used to detect cracks and flaws in metal blocks.

Technique

Ultrasonic waves are allowed to pass through the metal block and detectors are used to detect the transmitted waves. If there is even a small defect, the ultrasound gets reflected back indicating the presence of the flaw or defect.

• Ultrasound is used to clean parts located in hard-to-reach places, for example, spiral tube, odd shaped parts, electronic components etc

• Ultrasonic waves are made to reflect from various parts of the heart and form the image of the heart. This technique is called ‘echocardiography’.

• Ultrasound scanner is an instrument which uses ultrasonic waves for getting images of internal organs of the human body. A doctor may image the patient’s organs such as the liver, gall bladder, uterus, kidney, etc. It helps the doctor to detect abnormalities, such as stones in the gall bladder and kidney or tumours in different organs.

Technique

In this technique the ultrasonic waves travel through the tissues of the body and get reflected from a region where there is a change of tissue density. These waves are then converted into electrical signals that are used to generate images of the organ. These images are then displayed on a monitor or printed on a film. This technique is called ‘ultra sonography’.

• Ultra sonography is also used for examination of the foetus during pregnancy to detect congenial defects and growth abnormalities.

• Ultrasound may be employed to break small ‘stones’ formed in the kidneys into fine grains. These grains later get flushed out with urine.

SONAR

SONAR stands for SOund Navigation And Ranging.

In this device, sound waves (ultrasonic) are used [since microwaves are absorbed by water]. Ultrasound waves are emitted by a source. These waves travel in water with velocity v. The waves reflected by targets (like submarine bottom sea) are detected.

The distance (d) of the under-water object is calculated from the time (t) taken by the echo to return with speed (v) is given by 2d = v × t. This method of measuring distance is also known as ‘echo-ranging’.

The SONAR is used for detecting the presence of unseen underwater objects, such as a submerged submarine, a sunken ship, sea rock or a hidden iceberg, and locating them accurately.

Structure and Woking of Human Ear

It is a highly sensitive part of the human body which enables us to hear a sound. It converts the pressure variations in air with audiable frequencies into electric signals which travel to the brain via the auditory nerve and the brain interprets the sound.

The human ear has three main parts,

(i) Outer ear

The outer ear is called ‘pinna’. It collects the sound from the surroundings. The shape is such that it enables maximum collection of sound. 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 compression of the medium produced due to vibration of the object 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. In this way the ear drum vibrates.

(ii) Middle ear

The vibrations are amplified several times by three bones (the hammer, anvil and stirrup) in the middle ear which act as levers. The middle ear transmits the amplified pressure variations received from the sound wave to the inner ear. A narrow passage leading from the pharynx to the cavity of the middle ear, permitting the equalization of pressure on each side of the eardrum, is called eustachian tube.

(iii) 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.