Welcome dear students! Today we are going to learn about Sound from Class 9 Science.
Everyday we hear sounds from various sources like humans, birds, bells, machines, vehicles, televisions, radios and others. Sound is a form of energy which produces a sensation of hearing in our ears. There are also other forms of energy like mechanical energy, light energy, and others. We have talked about mechanical energy in the previous chapters. You have been taught about conservation of energy, which states that we can neither create nor destroy energy. We can just change it from one form to another. When you clap, a sound is produced. Can you produce sound without utilising your energy? Which form of energy did you use to produce sound? In this chapter we are going to learn how sound is produced and how it is transmitted through a medium and received by our ears.
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Let us begin with section eleven point one, Production of Sound. Activity eleven point one. Take a tuning fork and set it vibrating by striking its prong on a rubber pad. Bring it near your ear. Do you hear any sound? Touch one of the prongs of the vibrating tuning fork with your finger and share your experience with your friends. Now, suspend a table tennis ball or a small plastic ball by a thread from a support. To do this, take a big needle and a thread, put a knot at one end of the thread, and then with the help of the needle pass the thread through the ball. Touch the ball gently with the prong of a vibrating tuning fork. In Figure eleven point one, we see a vibrating tuning fork just touching a suspended table tennis ball. Observe what happens and discuss with your friends.
Activity eleven point two. Fill water in a beaker or a glass up to the brim. Gently touch the water surface with one of the prongs of the vibrating tuning fork, as shown in Figure eleven point two. Next dip the prongs of the vibrating tuning fork in water, as shown in Figure eleven point three. Observe what happens in both the cases. Discuss with your friends why this happens. From the above activities what do you conclude? Can you produce sound without a vibrating object? In the above activities we have produced sound by striking the tuning fork. We can also produce sound by plucking, scratching, rubbing, blowing or shaking different objects. As per the above activities what do we do to the objects? We set the objects vibrating and produce sound. Vibration means a kind of rapid to and fro motion of an object. The sound of the human voice is produced due to vibrations in the vocal cords. When a bird flaps its wings, do you hear any sound? Think how the buzzing sound accompanying a bee is produced. A stretched rubber band when plucked vibrates and produces sound. If you have never done this, then do it and observe the vibration of the stretched rubber band.
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Activity eleven point three. Make a list of different types of musical instruments and discuss with your friends which part of the instrument vibrates to produce sound.
Now we move to section eleven point two, Propagation of Sound. Sound is produced by vibrating objects. The matter or substance through which sound is transmitted is called a medium. It can be solid, liquid or gas. Sound moves through a medium from the point of generation to the listener. When an object vibrates, it sets the particles of the medium around it vibrating. The particles do not travel all the way from the vibrating object to the ear. A particle of the medium in contact with the vibrating object is first displaced from its equilibrium position. It then exerts a force on the adjacent particle. As a result of which the adjacent particle gets displaced from its position of rest. After displacing the adjacent particle the first particle comes back to its original position. This process continues in the medium till the sound reaches your ear. The disturbance created by a source of sound in the medium travels through the medium and not the particles of the medium. A wave is a disturbance that moves through a medium when the particles of the medium set neighbouring particles into motion. They in turn produce similar motion in others. The particles of the medium do not move forward themselves, but the disturbance is carried forward. This is what happens during propagation of sound in a medium, hence sound can be visualised as a wave. Sound waves are characterised by the motion of particles in the medium and are called mechanical waves.
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Air is the most common medium through which sound travels. 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, denoted as C, as shown in Figure eleven point four. This compression starts to move away from the vibrating object. When the vibrating object moves backwards, it creates a region of low pressure called rarefaction, denoted as R, as shown in Figure eleven point four. As the object moves back and forth rapidly, a series of compressions and rarefactions is created in the air. These make the sound wave that propagates through the medium. Compression is the region of high pressure and rarefaction is the region of low pressure. Pressure is related to the number of particles of a medium in a given volume. More density of the particles in the medium gives more pressure and vice versa. Thus, propagation of sound can be visualised as propagation of density variations or pressure variations in the medium.
Let us pause to answer the questions from this section. Question one. How does the sound produced by a vibrating object in a medium reach your ear? Answer. Sound is produced by vibrating objects. When an object vibrates, it sets the particles of the surrounding medium vibrating. A particle in contact with the vibrating object is displaced from its equilibrium position and exerts a force on the adjacent particle, displacing it. After displacing the adjacent particle, the first particle returns to its original position. This process continues through the medium until the disturbance reaches our ears. The disturbance travels, not the particles themselves.
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Question two. Explain how sound is produced by your school bell. Answer. When the school bell is struck, it vibrates. These vibrations set the surrounding air particles into motion, creating a series of compressions and rarefactions that propagate through the air as sound waves until they reach our ears. Question three. Why are sound waves called mechanical waves? Answer. Sound waves are called mechanical waves because they require a material medium to propagate. They are characterised by the motion of particles in the medium, which transfer energy from one particle to the next through mechanical vibrations. Question four. Suppose you and your friend are on the moon. Will you be able to hear any sound produced by your friend? Answer. No, you will not be able to hear any sound. The moon has no atmosphere, meaning there is no material medium like air, water, or solid to carry the sound vibrations. Since sound requires a medium to propagate, it cannot travel through the vacuum of space.
Now, section eleven point two point one, Sound Waves Are Longitudinal Waves. Activity eleven point four. Take a slinky. Ask your friend to hold one end. You hold the other end. Now stretch the slinky as shown in Figure eleven point five part a. Then give it a sharp push towards your friend. What do you notice? If you move your hand pushing and pulling the slinky alternatively, what will you observe? If you mark a dot on the slinky, you will observe that the dot on the slinky will move back and forth parallel to the direction of the propagation of the disturbance. In Figure eleven point five, we see a longitudinal wave in a slinky. The regions where the coils become closer are called compressions and the regions where the coils are further apart are called rarefactions. As we already know, sound propagates in the medium as a series of compressions and rarefactions. Now, we can compare the propagation of disturbance in a slinky with the sound propagation in the medium.
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These waves are called longitudinal waves. In these waves the individual particles of the medium move in a direction parallel to the direction of propagation of the disturbance. The particles do not move from one place to another but they simply oscillate back and forth about their position of rest. This is exactly how a sound wave propagates, hence sound waves are longitudinal waves. There is also another type of wave, called a transverse wave. In a transverse wave particles do not oscillate along the direction of wave propagation but oscillate up and down about their mean position as the wave travels. Thus, a transverse wave is the one in which the individual particles of the medium move about their mean positions in a direction perpendicular to the direction of wave propagation. When we drop a pebble in a pond, the waves you see on the water surface is an example of transverse wave. Light is a transverse wave but for light, the oscillations are not of the medium particles or their pressure or density, it is not a mechanical wave. You will come to know more about transverse waves in higher classes.
Next, section eleven point two point two, Characteristics of a Sound Wave. We can describe a sound wave by its frequency, amplitude, and speed. A sound wave in graphic form is shown in Figure eleven point six part c, which represents how density and pressure change when the sound wave moves in the medium. The density as well as the pressure of the medium at a given time varies with distance, above and below the average value of density and pressure. Figure eleven point six part a and part b represent the density and pressure variations, respectively, as a sound wave propagates in the medium. Compressions are the regions where particles are crowded together and represented by the upper portion of the curve in Figure eleven point six part c. The peak represents the region of maximum compression. Thus, compressions are regions where density as well as pressure is high.
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Rarefactions are the regions of low pressure where particles are spread apart and are represented by the valley, that is, the lower portion of the curve in Figure eleven point six part c. A peak is called the crest and a valley is called the trough of a wave. The distance between two consecutive compressions or two consecutive rarefactions is called the wavelength, as shown in Figure eleven point six part c. The wavelength is usually represented by lambda, the Greek letter lambda. Its SI unit is metre. Let me share an interesting historical note. Heinrich Rudolph Hertz was born on twenty two February eighteen fifty seven in Hamburg, Germany and educated at the University of Berlin. He confirmed James Clerk Maxwell electromagnetic theory by his experiments. He laid the foundation for future development of radio, telephone, telegraph and even television. He also discovered the photoelectric effect which was later explained by Albert Einstein. The SI unit of frequency was named as hertz in his honour.
Frequency tells us how frequently an event occurs. Suppose you are beating a drum. How many times you are beating the drum in unit time is called the frequency of your beating the drum. We know that when sound is propagated through a medium, the density of the medium oscillates between a maximum value and a minimum value. The change in density from the maximum value to the minimum value, then again to the maximum value, makes one complete oscillation. The number of such oscillations per unit time is the frequency of the sound wave. If we can count the number of the compressions or rarefactions that cross us per unit time, we will get the frequency of the sound wave. It is usually represented by nu, the Greek letter nu. Its SI unit is hertz, symbol Hz.
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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, we can say that the time taken for one complete oscillation is called the time period of the sound wave. It is represented by the symbol T. Its SI unit is second. Frequency and time period are related as follows: nu equals one divided by T. A violin and a flute may both be played at the same time in an orchestra. Both sounds travel through the same medium, that is, air and arrive at our ear at the same time. Both sounds travel at the same speed irrespective of the source. But the sounds we receive are different. This is due to the different characteristics associated with the sound. Pitch is one of the characteristics. How the brain interprets the frequency of an emitted sound is called its pitch. The faster the vibration of the source, the higher is the frequency and the higher is the pitch, as shown in Figure eleven point seven. In Figure eleven point seven, we see two wave shapes plotted against time. The top wave has a lower frequency and is labelled low pitched sound. The bottom wave has a higher frequency and is labelled high pitched sound. Thus, a high pitch sound corresponds to more number of compressions and rarefactions passing a fixed point per unit time. Objects of different sizes and conditions vibrate at different frequencies to produce sounds of different pitch.
The magnitude of the maximum disturbance in the medium on either side of the mean value is called the amplitude of the wave. It is usually represented by the letter A, as shown in Figure eleven point six part c. For sound its unit will be that of density or pressure. The loudness or softness of a sound is determined basically by its amplitude. The amplitude of the sound wave depends upon the force with which an object is made to vibrate. If we strike a table lightly, we hear a soft sound because we produce a sound wave of less energy, meaning less amplitude. If we hit the table hard we hear a louder sound. Can you tell why? A sound wave spreads out from its source. As it moves away from the source its amplitude as well as its loudness decreases. Louder sound can travel a larger distance as it is associated with higher energy. Figure eleven point eight shows the wave shapes of a loud and a soft sound of the same frequency. The top wave has a smaller amplitude and is labelled soft sound. The bottom wave has a larger amplitude and is labelled louder sound.
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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. Let us answer the questions here. Question one. Which wave property determines loudness and pitch? Answer. Loudness is determined by the amplitude of the wave, and pitch is determined by the frequency of the wave. Question two. Guess which sound has a higher pitch: guitar or car horn? Answer. A guitar typically produces a higher pitch sound compared to a car horn, as its strings vibrate at higher frequencies.
The speed of sound is defined as the distance which a point on a wave, such as a compression or a rarefaction, travels per unit time. We know, speed v equals distance divided by time, which equals lambda divided by T. Here lambda is the wavelength of the sound wave. It is the distance travelled by the sound wave in one time period T of the wave. Thus, v equals lambda times nu, because one divided by T equals nu. Or v equals lambda times nu. That is, speed equals wavelength multiplied by frequency. The speed of sound remains almost the same for all frequencies in a given medium under the same physical conditions.
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Let us solve Example eleven point one. A sound wave has a frequency of two kilohertz and wave length thirty five centimetres. How long will it take to travel one point five kilometres? Solution. Given, Frequency, nu equals two kilohertz, which is two thousand hertz. Wavelength, lambda equals thirty five centimetres, which is zero point three five metres. We know that speed, v of the wave equals wavelength times frequency. v equals lambda times nu equals zero point three five metres times two thousand hertz equals seven hundred metres per second. The time taken by the wave to travel a distance, d of one point five kilometres is t equals d divided by v equals one point five times one thousand metres divided by seven hundred metres per second equals fifteen divided by seven seconds equals two point one seconds. Thus sound will take two point one seconds to travel a distance of one point five kilometres.
Now answer these questions. Question one. What are wavelength, frequency, time period and amplitude of a sound wave? Answer. Wavelength is the distance between two consecutive compressions or two consecutive rarefactions. Frequency is the number of complete oscillations per unit time. Time period is the time taken for one complete oscillation. Amplitude is the magnitude of the maximum disturbance in the medium on either side of the mean value. Question two. How are the wavelength and frequency of a sound wave related to its speed? Answer. The speed of sound is the product of its wavelength and frequency, given by the equation v equals lambda times nu.
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Question three. Calculate the wavelength of a sound wave whose frequency is two hundred twenty hertz and speed is four hundred forty metres per second in a given medium. Answer. Using v equals lambda times nu, we get lambda equals v divided by nu. Lambda equals four hundred forty divided by two hundred twenty equals two metres. Question four. A person is listening to a tone of five hundred hertz sitting at a distance of four hundred fifty metres from the source of the sound. What is the time interval between successive compressions from the source? Answer. The time interval between successive compressions is the time period T. Since frequency nu is five hundred hertz, T equals one divided by nu equals one divided by five hundred equals zero point zero zero two seconds.
The amount of sound energy passing each second through unit area is called the intensity of sound. We sometimes use the terms loudness and intensity interchangeably, but they are not the same. Loudness is a measure of the response of the ear to the sound. Even when two sounds are of equal intensity, we may hear one as louder than the other simply because our ear detects it better. Question. Distinguish between loudness and intensity of sound. Answer. Intensity is an objective physical quantity defined as the amount of sound energy passing each second through unit area. Loudness is a subjective physiological response of the human ear to that intensity. Two sounds of equal intensity may be perceived with different loudness depending on the ear sensitivity.
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Moving to section eleven point two point three, Speed of Sound in Different Media. Sound propagates through a medium at a finite speed. The sound of a thunder is heard a little later than the flash of light is seen. So, we can make out that sound travels with a speed which is much less than the speed of light. The speed of sound depends on the properties of the medium through which it travels. You will learn about this dependence in higher classes. The speed of sound in a medium depends on temperature of the medium. The speed of sound decreases when we go from solid to gaseous state. In any medium as we increase the temperature, the speed of sound increases. For example, the speed of sound in air is three hundred thirty one metres per second at zero degrees Celsius and three hundred forty four metres per second at twenty two degrees Celsius. The speeds of sound at a particular temperature in various media are listed in Table eleven point one at twenty five degrees Celsius. Let me read them out for you. In solids: Aluminium is six thousand four hundred twenty metres per second, Nickel is six thousand forty metres per second, Steel is five thousand nine hundred sixty metres per second, Iron is five thousand nine hundred fifty metres per second, Brass is four thousand seven hundred metres per second, and Glass Flint is three thousand nine hundred eighty metres per second. In liquids: Sea water is one thousand five hundred thirty one metres per second, distilled water is one thousand four hundred ninety eight metres per second, Ethanol is one thousand two hundred seven metres per second, and Methanol is one thousand one hundred three metres per second. In gases: Hydrogen is one thousand two hundred eighty four metres per second, Helium is nine hundred sixty five metres per second, Air is three hundred forty six metres per second, Oxygen is three hundred sixteen metres per second, and Sulphur dioxide is two hundred thirteen metres per second. You need not memorise the values. Question. In which of the three media, air, water or iron, does sound travel the fastest at a particular temperature? Answer. Sound travels fastest in iron, as it is a solid and has tightly packed particles that transmit vibrations more quickly than liquids or gases.
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Now, section eleven point three, Reflection of Sound. Sound bounces off a solid or a liquid like a rubber ball bounces off a wall. Like light, sound gets reflected at the surface of a solid or liquid and follows the same laws of reflection as you have studied in earlier classes. The directions in which the sound is incident and is reflected make equal angles with the normal to the reflecting surface at the point of incidence, and the three are in the same plane. An obstacle of large size which may be polished or rough is needed for the reflection of sound waves. Activity eleven point five. Take two identical pipes, as shown in Figure eleven point nine. You can make the pipes using chart paper. The length of the pipes should be sufficiently long as shown. Arrange them on a table near a wall. Keep a clock near the open end of one of the pipes and try to hear the sound of the clock through the other pipe. Adjust the position of the pipes so that you can best hear the sound of the clock. Now, measure the angles of incidence and reflection and see the relationship between the angles. Lift the pipe on the right vertically to a small height and observe what happens. In place of a clock, a mobile phone on vibrating mode may also be used. In Figure eleven point nine, we see a wall, a table, a clock, two pipes arranged at angles, a normal line drawn at the point of reflection, and an ear listening through the second pipe. You will observe that the angle of incidence equals the angle of reflection, and when the right pipe is lifted vertically, the sound is no longer heard because the reflected wave no longer travels along the pipe to your ear.
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Section eleven point three point one, Echo. If we shout or clap near a suitable reflecting object such as a tall building or a mountain, we will hear the same sound again a little later. This sound which we hear is called an echo. The sensation of sound persists in our brain for about zero point one seconds. To hear a distinct echo the time interval between the original sound and the reflected one must be at least zero point one seconds. If we take the speed of sound to be three hundred forty four metres per second at a given temperature, say at twenty two degrees Celsius in air, the sound must go to the obstacle and reach back the ear of the listener on reflection after zero point one seconds. Hence, the total distance covered by the sound from the point of generation to the reflecting surface and back should be at least three hundred forty four metres per second multiplied by zero point one seconds, which equals thirty four point four metres. Thus, for hearing distinct echoes, the minimum distance of the obstacle from the source of sound must be half of this distance, that is, seventeen point two metres. This distance will change with the temperature of air. 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 the land.
Section eleven point three point two, Reverberation. A sound created in a big hall will persist by repeated reflection from the walls until it is reduced to a value where it is no longer audible. The repeated reflection that results in this persistence of sound is called reverberation. In an auditorium or big hall excessive reverberation is highly undesirable. To reduce reverberation, the roof and walls of the auditorium are generally covered with sound absorbent materials like compressed fibreboard, rough plaster or draperies. The seat materials are also selected on the basis of their sound absorbing properties.
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Let us solve Example eleven point two. A person clapped his hands near a cliff and heard the echo after two seconds. What is the distance of the cliff from the person if the speed of the sound, v is taken as three hundred forty six metres per second? Solution. Given, Speed of sound, v equals three hundred forty six metres per second. Time taken for hearing the echo, t equals two seconds. Distance travelled by the sound equals v multiplied by t equals three hundred forty six metres per second multiplied by two seconds equals six hundred ninety two metres. In two seconds sound has to travel twice the distance between the cliff and the person. Hence, the distance between the cliff and the person equals six hundred ninety two metres divided by two equals three hundred forty six metres. Question. An echo is heard in three seconds. What is the distance of the reflecting surface from the source, given that the speed of sound is three hundred forty two metres per second? Answer. Given speed v equals three hundred forty two metres per second and time t equals three seconds. Total distance travelled by sound equals v times t equals three hundred forty two times three equals one thousand twenty six metres. Since this is the distance to the surface and back, the distance to the reflecting surface is half of this, which is one thousand twenty six divided by two equals five hundred thirteen metres.
Section eleven point three point three, Uses of Multiple Reflection of Sound. First, 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, as shown in Figure eleven point ten. In Figure eleven point ten, we see a megaphone and a horn, both having a tube followed by a conical opening. In these instruments, a tube followed by a conical opening reflects sound successively to guide most of the sound waves from the source in the forward direction towards the audience.
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Second, 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 heartbeat reaches the doctor ears by multiple reflection of sound, as shown in Figure eleven point eleven. Figure eleven point eleven shows a stethoscope with a chest piece, tubing, and earpieces. Third, generally the ceilings of concert halls, conference halls and cinema halls are curved so that sound after reflection reaches all corners of the hall, as shown in Figure eleven point twelve. 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, as shown in Figure eleven point thirteen. Figure eleven point twelve shows a curved ceiling in a conference hall. Figure eleven point thirteen shows a curved soundboard behind a stage with a sound source. Question. Why are the ceilings of concert halls curved? Answer. The ceilings of concert halls are curved so that sound, after reflection from the ceiling, spreads evenly and reaches all corners of the hall, ensuring uniform sound distribution for the audience.
Now, section eleven point four, Range of Hearing. The audible range of sound for human beings extends from about twenty hertz to twenty thousand hertz. Children under the age of five and some animals, such as dogs can hear up to twenty five kilohertz. As people grow older their ears become less sensitive to higher frequencies. Sounds of frequencies below twenty hertz are called infrasonic sound or infrasound. If we could hear infrasound we would hear the vibrations of a pendulum just as we hear the vibrations of the wings of a bee. Rhinoceroses communicate using infrasound of frequency as low as five hertz. Whales and elephants produce sound in the infrasound range. It is observed that some animals get disturbed before earthquakes. Earthquakes produce low frequency infrasound before the main shock waves begin which possibly alert the animals.
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Frequencies higher than twenty kilohertz are called ultrasonic sound or ultrasound. Ultrasound is produced by animals such as dolphins, bats and porpoises. Moths of certain families have very sensitive hearing equipment. These moths can hear the high frequency squeaks of the bat and know when a bat is flying nearby, and are able to escape capture. Rats also play games by producing ultrasound. Hearing Aid. People with hearing loss may need a hearing aid. A hearing aid is an electronic, battery operated device. The hearing aid receives sound through a microphone. The microphone converts the sound waves to electrical signals. These electrical signals are amplified by an amplifier. The amplified electrical signals are given to a speaker of the hearing aid. The speaker converts the amplified electrical signal to sound and sends to the ear for clear hearing. Questions. One. What is the audible range of the average human ear? Answer. The audible range of the average human ear is from twenty hertz to twenty thousand hertz. Two. What is the range of frequencies associated with infrasound and ultrasound? Answer. Infrasound refers to sound waves with frequencies below twenty hertz. Ultrasound refers to sound waves with frequencies above twenty thousand hertz.
Section eleven point five, Applications of Ultrasound. Ultrasounds are high frequency waves. Ultrasounds are able to travel along well defined paths even in the presence of obstacles. Ultrasounds are used extensively in industries and for medical purposes. First, ultrasound is generally used to clean parts located in hard to reach places, for example, spiral tube, odd shaped parts, electronic components, and others. Objects to be cleaned are placed in a cleaning solution and ultrasonic waves are sent into the solution. Due to the high frequency, the particles of dust, grease and dirt get detached and drop out. The objects thus get thoroughly cleaned.
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Second, ultrasounds can be used to detect cracks and flaws in metal blocks in construction of big structures like buildings, bridges, machines and also scientific equipment. The cracks or holes inside the metal blocks, which are invisible from outside reduces the strength of the structure. 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, as shown in Figure eleven point fourteen. In Figure eleven point fourteen, we see a metal block with a defect or flaw inside. Ultrasound enters from one side, hits the defect, and reflects back to detectors. Ordinary sound of longer wavelengths cannot be used for such purpose as it will bend around the corners of the defective location and enter the detector. Third, ultrasonic waves are made to reflect from various parts of the heart and form the image of the heart. This technique is called echocardiography. Fourth, 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 organs, such as the liver, gall bladder, uterus, kidney, and others. It helps the doctor to detect abnormalities, such as stones in the gall bladder and kidney or tumours in different organs. 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 ultrasonography. Ultrasonography is also used for examination of the foetus during pregnancy to detect congenial defects and growth abnormalities. Fifth, ultrasound may be employed to break small stones formed in the kidneys into fine grains. These grains later get flushed out with urine.
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Let us review what you have learnt. Sound is produced due to vibration of different objects. Sound travels as a longitudinal wave through a material medium. Sound travels as successive compressions and rarefactions in the medium. In sound propagation, it is the energy of the sound that travels and not the particles of the medium. The change in density from one maximum value to the minimum value and again to the maximum value makes one complete oscillation. The distance between two consecutive compressions or two consecutive rarefactions is called the wavelength, lambda. The time taken by the wave for one complete oscillation of the density or pressure of the medium is called the time period, T. The number of complete oscillations per unit time is called the frequency, nu, where nu equals one divided by T. The speed v, frequency nu, and wavelength lambda, of sound are related by the equation, v equals lambda times nu. The speed of sound depends primarily on the nature and the temperature of the transmitting medium. The law of reflection of sound states that the directions in which the sound is incident and reflected make equal angles with the normal to the reflecting surface at the point of incidence and the three lie in the same plane. For hearing a distinct sound, the time interval between the original sound and the reflected one must be at least zero point one seconds. The persistence of sound in an auditorium is the result of repeated reflections of sound and is called reverberation. Sound properties such as pitch, loudness and quality are determined by the corresponding wave properties. Loudness is a physiological response of the ear to the intensity of sound. The amount of sound energy passing each second through unit area is called the intensity of sound. The audible range of hearing for average human beings is in the frequency range of twenty hertz to twenty kilohertz. Sound waves with frequencies below the audible range are termed infrasonic and those above the audible range are termed ultrasonic. Ultrasound has many medical and industrial applications.
Now, let us carefully solve all the exercises. Exercise one. What is sound and how is it produced? Answer. Sound is a form of energy which produces a sensation of hearing in our ears. It is produced due to the vibration of different objects. When an object vibrates, it sets the surrounding medium particles into motion, creating compressions and rarefactions that propagate as sound waves.
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Exercise two. Describe with the help of a diagram, how compressions and rarefactions are produced in air near a source of sound. Answer. When a vibrating object moves forward, it pushes and compresses the air in front of it, creating a region of high pressure called a compression. When it moves backward, it creates a region of low pressure called a rarefaction. As the object vibrates rapidly, a continuous series of compressions and rarefactions is formed in the air, propagating the sound wave. Exercise three. Why is sound wave called a longitudinal wave? Answer. Sound waves are called longitudinal waves because the individual particles of the medium vibrate back and forth in a direction parallel to the direction of propagation of the disturbance, creating compressions and rarefactions. Exercise four. Which characteristic of the sound helps you to identify your friend by his voice while sitting with others in a dark room? Answer. The quality or timber of sound helps us identify a specific voice. It is determined by the mixture of frequencies and harmonics present in the sound, making each person voice unique even if pitch and loudness are similar. Exercise five. Flash and thunder are produced simultaneously. But thunder is heard a few seconds after the flash is seen, why? Answer. Light travels much faster than sound. The speed of light is approximately three times ten to the power eight metres per second, while the speed of sound in air is only about three hundred forty four metres per second. Hence, the light from the flash reaches our eyes almost instantly, while the sound of thunder takes longer to travel the same distance.
Exercise six. A person has a hearing range from twenty hertz to twenty kilohertz. What are the typical wavelengths of sound waves in air corresponding to these two frequencies? Take the speed of sound in air as three hundred forty four metres per second. Answer. Using v equals lambda times nu, lambda equals v divided by nu. For twenty hertz, lambda equals three hundred forty four divided by twenty equals seventeen point two metres. For twenty thousand hertz, lambda equals three hundred forty four divided by twenty thousand equals zero point zero one seven two metres.
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Exercise seven. Two children are at opposite ends of an aluminium rod. One strikes the end of the rod with a stone. Find the ratio of times taken by the sound wave in air and in aluminium to reach the second child. Answer. Speed of sound in aluminium is six thousand four hundred twenty metres per second. Speed of sound in air is three hundred forty six metres per second. Time equals distance divided by speed. Since distance is the same, ratio of times t air divided by t aluminium equals v aluminium divided by v air equals six thousand four hundred twenty divided by three hundred forty six, which is approximately eighteen point five five to one. Exercise eight. The frequency of a source of sound is one hundred hertz. How many times does it vibrate in a minute? Answer. Frequency is one hundred oscillations per second. In one minute, which is sixty seconds, the number of vibrations equals one hundred multiplied by sixty equals six thousand times. Exercise nine. Does sound follow the same laws of reflection as light does? Explain. Answer. Yes, sound follows the same laws of reflection as light. The incident sound wave, the reflected sound wave, and the normal at the point of incidence all lie in the same plane. Also, the angle of incidence equals the angle of reflection.
Exercise ten. When a sound is reflected from a distant object, an echo is produced. Let the distance between the reflecting surface and the source of sound production remains the same. Do you hear echo sound on a hotter day? Answer. On a hotter day, the speed of sound increases. Since the distance remains the same, the time taken for the sound to travel to the obstacle and back decreases. If this time falls below zero point one seconds, the echo will merge with the original sound and will not be heard distinctly. Exercise eleven. Give two practical applications of reflection of sound waves. Answer. First, megaphones and horns use multiple reflections to direct sound in a specific direction. Second, stethoscopes use multiple reflections to transmit internal body sounds to the doctor ears.
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Exercise twelve. A stone is dropped from the top of a tower five hundred metres high into a pond of water at the base of the tower. When is the splash heard at the top? Given, g equals ten metres per second squared and speed of sound equals three hundred forty metres per second. Answer. First, calculate time for stone to fall. Using s equals u t plus half g t squared, with u equals zero, five hundred equals half times ten times t squared. So t squared equals one hundred, t equals ten seconds. Time for sound to travel up equals distance divided by speed equals five hundred divided by three hundred forty equals one point four seven seconds. Total time equals ten plus one point four seven equals eleven point four seven seconds. Exercise thirteen. A sound wave travels at a speed of three hundred thirty nine metres per second. If its wavelength is one point five centimetres, what is the frequency of the wave? Will it be audible? Answer. Wavelength lambda equals one point five centimetres equals zero point zero one five metres. Frequency nu equals v divided by lambda equals three hundred thirty nine divided by zero point zero one five equals twenty two thousand six hundred hertz. Since this is greater than twenty thousand hertz, it is ultrasonic and will not be audible to humans. Exercise fourteen. What is reverberation? How can it be reduced? Answer. Reverberation is the persistence of sound in an enclosed space due to repeated reflections from walls and ceilings. It can be reduced by covering roofs and walls with sound absorbent materials like compressed fibreboard, rough plaster, or draperies, and by using sound absorbing seat materials.
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Exercise fifteen. What is loudness of sound? What factors does it depend on? Answer. Loudness is a physiological response of the ear to the intensity of sound. It depends mainly on the amplitude of the sound wave, which in turn depends on the force with which the object is made to vibrate. It also decreases with distance from the source. Exercise sixteen. How is ultrasound used for cleaning? Answer. Objects to be cleaned are placed in a cleaning solution. Ultrasonic waves are sent into the solution. The high frequency vibrations cause particles of dust, grease, and dirt to detach from the objects and drop out, thoroughly cleaning even hard to reach areas. Exercise seventeen. Explain how defects in a metal block can be detected using ultrasound. Answer. Ultrasonic waves are passed through the metal block. Detectors on the other side monitor the transmitted waves. If there is a crack or flaw, the ultrasound gets reflected back instead of passing through. The detectors pick up this reflection, indicating the presence and location of the defect.
Thank you for listening! Keep revising and practicing. Goodbye! [CHAPTER_COMPLETE]