sound class 9

 NOTES PREPARED BY

ASHAQ HUSSAIN BHAT 

GL TEACHER SCHOOL EDUCATION 

DEPARTMENT JAMMU AND KASHMIR

CHAPTER

Sound

Introduction

In daily life we hear sound everywhere, from various sources like vehicles, humans, televisions, machines and so on. Sound is a form of energy which produces a sensation of hearing in our ears. In this blog we will study how sound is produced, and how is it propagated and how is it reflected etc.

Production of Sound

Sound is produced when an object vibrates. Sound is produced when we speak, when our fingers strike the membrane of a tabla, when signals are fed to the speakers of a radio, television or music system, when brakes are applied suddenly to stop a vehicle, when utensil bang against each other, and so on. In each case the source of sound vibrates.

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Sound needs a Medium to Travel

The substance or matter through which sound is propagated is called a medium. The medium can be a solid, a liquid or a gas. When a school bell rings, the solid bell vibrates and produces sound (point of generation). The sound travels through air and finally reaches our ears (listener). So, sound moves through a medium from the point of generation to the listener. 

Let us perform an experiment to prove that sound requires a medium for its propagation. Take an electric bell, a big glass jar and vacuum pump. Place an electric bell inside a jar connected to a vacuum pump. Switch on the bell and switch on the vacuum pump to take the air molecules out of the jar. The sound becomes feeble and finally you may not hear the sound.

When there was air in the jar sound travelled through it to the wall of the jar. This caused the wall to vibrate and send the sound to you and everyone around it. But when air was removed, sound from the bell could not travel to the wall of the jar. This shows that sound needs a medium to travel. The speed of sound depends on the medium through which it travels. It is fastest in solids and slowest in air.

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Propagation of Sound

Sound is produced by vibrating objects. The object that vibrates and produces sound is called point of generation. Let us see how the sound travels from one place to another. 

When a body vibrates, 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 side ways. 

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 pressures as the sound propagates in the medium.

Following figures represent the region of compressions (or high pressures) and rarefactions (or low

High Pressure as the sound propagates in the medium

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Longitudinal Waves

A wave in which the particles of the medium oscillate (vibrate) to and fro about their mean position in the direction of propagation of the wave is called a longitudinal wave.

Longitudinal waves can be produced in any medium, viz., in solids, liquids and gases.

Examples: 

(i) Sound waves are longitudinal waves.

(ii) The waves produced in a spring (slinky) by compressing a small portion of it and releasing are longitudinal waves.

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Characteristics of a Sound Wave

Wavelength: Distance between two consecutive peaks or trough is called wavelength.

is represented by λ (lambda) and its unit is metre.

Frequency: Number of oscillations of sound wave per unit time is called its frequency. Numb

of peak and troughs being generated per unit time will give frequency. It is represented by(λ)

and its unit is hertz (Hz).

Time period: Time taken by a vibrating particle or a body to complete one vibration or oscillation is known as time period. Its unit is second and is represented by T.

Relation between frequency and time period: The number of waves produced per second is called its frequency.

v=1/t

Amplitude: The magnitude of the maximum displacement in the medium, particle on either side of the mean position is called the amplitude of the wave. It is usually represented by the letter A

Softness or Loudness of sound: Loudness of a sound depends on the amplitude of vibration particles producing that sound. If the amplitude is smaller then the sound will be softer and if it's larger then sound will be louder. Loudness of sound is measured in decibel (dB) unit.

Pitch of sound: The characteristic of sound by which a shrill note can be distinguished from a flat note is called pitch. The pitch of a sound depends upon its frequency. Higher the frequency of sound, higher is its pitch. The pitch of sound of a female is higher than that of a male.

Speed of sound : It is the distance which a compression or a rarefaction travels per unit of time.

Speed of sound = v= λ/T = Wavelength / Time Period

or v = λv ( because 1/t = v)

So, Speed = Wavelength x Frequency

The speed of sound remains almost the same for all frequencies in a given medium under the same physical conditions

Quality of sound: The quality of sound is characteristic of sound which enable us to distinguish between pattern of wave-form made by the sound produced by different sources. A pleasant sound is said to be rich in quality and makes uniform pattern of wave form. A sound of single frequency (called pure sound) is called a tone and makes uniform pattern of wave-form.

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Speed of Sound in Different Medium

The speed of sound in different media is different. This is because the molecules are packed closer in solids and liquids than in air (or gas). Since molecules undergo vibrations, they do so more efficiently ,when they are close together,

• The speed of sound depends upon :

(i) the properties (elasticity and density) of the medium through which it propagates and

(ii) temperature of the medium, 

Sound (in general) has greatest speed in solids and least in gases. For liquids, the speed of sound is intermediate between these two extremes.

In general, speed of sound in solids ＞ speed of sound in liquid  > speed of sound in gases.
However, in contain solids, the speed v) of  the sound much less than that even in gases as v for the vulcanised rubber  = 54 ms-1  and  v (for hydrogen)-1284 m s-1. The speed of sound in lead (a solid) is 1332 ms-1 and in sea water (a liquid) 1531 ms-1 The speed of sound in methyl alcohol (a liquid )-1103 ms-1
the speed of sound increases with increase in temperature of the medium in air, it increases roughly by 0.61 ms-1 with rise in 1°C in temperature. The speed of sound in air at  0 F°C is 331ms-1 and 22°C it is 334 ms-1.

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Reflection of Sound

Like light waves sound waves also get reflected when they fall on the surface of an obstacle. But unlike light waves, sound waves do not necessarily respire a polished surface for reflection, Le, for reflection of sound waves the surface may be polished or rough. The following simple experiment establishes that reflection of sound follows the same laws as those for reflection of light.

(i) Place a large plane board, AB (of a metal, cardboard or wood) in the vertical position (ie. perpendicular to the plane of the paper).

(ii) Take two hollow metallic tubes P and Q (each about 1 m long and about 8 to 10 cm in diameter) and place them in the plane of the paper and in positions inclined to the board as shown in figure. 

(iii) Hold a watch W at the free end of the tube P and try to hear the ticking sound of the watch by positioning the ear at E.

(iv) Pat a carboard screen S in between the two tubes so that sound produced by the watch does not reach the ear directly.

(v) Turn the tube Q by some angle till the ticking sound of the watch is the loudest. In this position, it is found that the tubes are inclined to S at the same angle, i.e., i (angle of incidence of sound wave) r= (angle of reflection of the sound wave).

(vi) If the tube Q is lifted slightly upwards to make it out vertical plane, no sound is heard. This implies that the reflected sound wave (OE) lies in the same plane (i.e., the plane of the paper) as the incident sound wave.

The normal OS to the surface lies in the same plane as that in which the incident and reflected

sound waves lie.

From this experiment, we obtain the following two laws for the reflection of sound waves. These 

laws are as follows.

• First law. The angle of reflection (r) is always equal to the angle of incidence (i), i.e., ∠r = ∠ i.
• Second law. The incident wave, the reflected wave and the normal (at the point of incidence),all lie in the same plane.

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Devices based on principles of reflection of sound :

(i) Speaking tube or Megaphone: You must have seen in fairs or tourist spots, people using megaphones addressing a group of people. Megaphone is simply a horn-shaped tube. The sound waves are prevented from spreading out by successive reflections by the walls of megaphone and are confined to the air in the tube. For the same reason, loud speakers also have horn-shaped openings.

(ii) Ear Trumpet or Hearing Aid : It is a device which is used by the persons who are hard of hearing. The sound waves received by the wide end of the trumpet are reflected into a much narrower area, leading it to interior of the ear. This enhances the amplitude of vibrating layer of air inside the ear and helps in improving hearing.

(iii) Stethoscope: It is an instrument used by doctors for listening sound 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 to the doctors ears by multiple reflection.

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Echo

The sound returning back towards the source after suffering reflection from a distant obstacle (a wall, a hill, a row of building etc.) is called an echo. When the sound is reflected repeatedly from a number of obstacles, more than one echo, called multiple echoes are heard. Multiple echoes may be heard one after the other when sound gets repeatedly reflected from distant high rise buildings or hills. The rolling of thunder is an example of multiple eco formation. Echo is a very familiar example of reflection of sound waves. The time gap between the two sounds - one direct and the other echo, depends upon the medium through which the sound wave travels. The two sounds - one direct and the other echo, can be heard distinctly provided the distance between second after it is produced, the observer and the reflecting surface is large enough to allow the reflected sound to reach him without interfering with the direct sound. Since the sensation of sound persists for1/10 second after it is produced  the echo can be heard distinctly only if it reaches at least  1/10   seconds after the direct sound is heard.                                         .

Reverberation

Persistence of hearing of sound after its production is stopped, is called reverberation.

When a sound is produced in a big hall, its waves reflect from the walls and travel back and forth repeated due to reflections. Due to this, energy does not reduce and the sound does not die out immediately.

A short reverberation is desirable in a concert hall (where music is being played) because it gives life to the sound. Too much reverberation confuses the programmers and must be reduced. The material having sound-absorbing properties are used for making the seats in a big hall or auditorium to reduce reverberations. Panels made of sound-absorbing materials (like compressed fibre board or felt) are put on   the walls and ceiling of big halls and auditoriums to reduce  reverberations.

Infrasonic waves or Infrasound

The waves of frequency less than 20 Hz are known as infrasonic waves. by the vibration of the earth's surface during the earthquake. Some animals like elephants, rhinoceroses The infrasonic waves are produced by large vibrating bodies. For example, infrasonic waves are produced and whales etc. also produce and detect infrasonic waves. These waves are not audible to a human ear. It has been observed that some animals behaviour becomes unusual just before the tremor is felt. This is because some animals have the ability to detect infrasonic waves produced at the time of tremor.

Audible Range

The average frequency range over which the human ear is sensitive is called audible range or

range of hearing in humans. The human ear can hear sounds having frequencies ranging between 20 Hz - 20000 Hz, these waves are called audible waves. This range of audibility gets narrower as we grow old. Children below 5 years of age can hear up to 25000 Hz.

Ultrasonic waves or Ultrasound

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

Bats can produce ultrasonic waves by flapping their wings. They can also detect these waves. The ultrasonic waves produced by bats after reflection from the obstacles like buildings guide them to remain away from impending obstacles during their flights.

Properties of Ultrasound

(i) Ultrasonic waves can travel easily in solid and liquid but not in gases. This is because the intensity of ultrasonic waves decreases fast with propagation in gases.

(ii) Good directionality: Due to high frequency of ultrasound, its wavelength is small. Therefore, these that these waves are used for scanning objects and making their image. The imaging phenomenon waves do not bend easily and therefore move along a straight line. It is because of this property also, depends on the fact that these waves can travel through opaque objects.

(iii) High power: Due to high frequency, the energy possessed by these waves is high. Therefore, if these waves are focussed at a point, a high power is generated at that point which is used in breaking and cutting objects.

Applications of Ultrasound (Ultrasonic Waves)

Ultrasonic waves have number of uses:

(1) Ultrasonic waves are used to establish ship-to-ship communication.

(ii) Ultrasonic waves are used to determine the depth of a sea. It is done with the help of a SONAR

(iii) Ultrasonic waves are used for cleaning the hidden parts of an instrument. The instrument of device whose hidden parts are to be cleaned is dipped in a liquid. The ultrasonic waves are passed through this liquid. These waves force the dirt or any other impurity out from the parts of the instrument which can not be approached directly.

(iv) Ultrasonic waves are used for welding plastic. Two plastic surfaces are pressed against each other Then ultrasonic waves are allowed to fall at a point where plastic surfaces are in contact. These waves produce heat energy. This heat energy binds the two plastic surfaces together.

(v) Ultrasonic waves are used for diagnosing the diseases in human body. Different parts of the body like bone, fat muscles and liquid have different reflective properties. Ultrasonic waves are allowed to fall on the portion of the body of a patient to be diagnosed. These waves are reflected back by different parts (like bones, tissues, liquids and muscles etc.) of that portion of the body in different manners. The varying echoes are recorded for analysing that part of the body. These echoes are converted into visuals on TV screen. The method used for diagnosing different parts of human body with the help of ultrasonic waves is known as ultrasonography. Ultrasonic waves are also used to analyse and monitor the development 

the picture taken by ultrasound. of an unborn child. Any abnormality in the growth of an unborn child can be detected by observing for a longer period of time.

(vi) Ultrasonic waves are used to kill bacteria in liquids. Thus, the liquids like milk can be preserved for a longer period of time

(vii) Ultrasonic waves are used to find faults and cracks in metals. Ultrasonic waves are made to fall on metal under investigation. The beam of ultrasonic waves reflected by the metal is investigated. The intensity of the ultrasonic waves reflected from the fault or a crack is different from the intensity of  the waves reflected from other part of metal. Thus, the position and extent of the fault or a crack in the metal can be easily detected.

It is a device which is used in ships to locate rocks, icebergs, submarines, old ships sank in sea etc.

Ultrasonic waves of high frequency are sent from a ship on the surface. These waves travel in a straight line till they hit a body or submarine from where these waves are reflected back as shown in the figure. The transmitter sending the waves records the time interval t between sending the signal and receiving it back. If d is the distance of the submarine from the ship, then the total distance travelled by the wave in time interval t is 2d,

Speed=Distance/Time   =  v = 2d/t 

: d=-1/2(v x t)

The velocity v of the ultrasonic wave in water is same as that of audible range sound in water-

The Human Ear

The human ear is a very sensitive organ. The faintest sound detected by it has a pressure variation of about 2 x  10 -5 N/m² (corresponding to an amplitude of about 10-¹ m).

The ear has the following parts

Outer ear: The part seen from outside is known as the pinna. This part collects the sound and sends it to the auditory canal. A tightly stretched membrane called eardrum separates the outer ear from the middle ear..

Middle ear: It consists of three small bones, the hammer, the anvil and the stirrup. The  amplified vibrations of sound are transmitted to the cochlea. The vibrations in the liquid in  the cochlea affect thousands of auditory nerves which send messages to the brain.

Inner ear: The inner ear contains the organs of hearing which perceive sound and send a message to the brain via the auditory nerve. Our ears are very delicate and sensitive organs. Proper care must be taken to keep them in healthy state. Some suggestions to keep them healthy are given below:

• Never insert any pointed objects into the ear. If can damage the eardrum and can make person deaf.

• Never shout loudly in to some ones ear

• never hit any thing hard on the ear