Sound - Chapter 11 PDF

C hapter 11 SOUND Everyday we hear sounds from various sources like humans, birds, bells, machines, vehicles, televisions, radios etc. Sound is a form of energy which produces a sensation of hearing in our ears...

C hapter 11 SOUND Everyday we hear sounds from various sources like humans, birds, bells, machines, vehicles, televisions, radios etc. 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, etc. 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 Fig. 11.1: Vibrating tuning fork just touching the chapter we are going to learn how sound is suspended table tennis ball. produced and how it is transmitted through a medium and received by our ears. Activity _____________ 11.2 11.1 Production of Sound 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 Activity _____________ 11.1 tuning fork, as shown in Fig. 11.2. Take a tuning fork and set it vibrating Next dip the prongs of the vibrating by striking its prong on a rubber pad. tuning fork in water, as shown in Fig. Bring it near your ear. 11.3. Do you hear any sound? Observe what happens in both the Touch one of the prongs of the vibrating cases. tuning fork with your finger and share Discuss with your friends why this your experience with your friends. happens. Now, suspend a table tennis ball or a small plastic ball by a thread from a support [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 pr ong of a vibrating tuning fork (Fig. 11.1). Observe what happens and discuss with your friends. Fig. 11.2: One of the prongs of the vibrating tuning fork touching the water surface. 2024-25  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 Fig. 11.3: Both the prongs of the vibrating tuning the medium travels through the medium and fork dipped in water not the particles of the medium. A wave is a disturbance that moves From the above activities what do you through a medium when the particles of the conclude? Can you produce sound without medium set neighbouring particles into a vibrating object? motion. They in turn produce similar motion In the above activities we have produced in others. The particles of the medium do not sound by striking the tuning fork. We can move forward themselves, but the also produce sound by plucking, scratching, disturbance is carried forward. This is what rubbing, blowing or shaking different objects. happens during propagation of sound in a As per the above activities what do we do to medium, hence sound can be visualised as a the objects? We set the objects vibrating and wave. Sound waves are characterised by the produce sound. Vibration means a kind of motion of particles in the medium and are rapid to and fro motion of an object. The called mechanical waves. sound of the human voice is produced due Air is the most common medium through to vibrations in the vocal cords. When a bird which sound travels. When a vibrating object flaps its wings, do you hear any sound? Think moves forward, it pushes and compresses the how the buzzing sound accompanying a bee air in front of it creating a region of high is produced. A stretched rubber band when pressure. This region is called a compression plucked vibrates and produces sound. If you (C), as shown in Fig. 11.4. This compression have never done this, then do it and observe starts to move away from the vibrating object. When the vibrating object moves backwards, the vibration of the stretched rubber band. it creates a region of low pressure called Activity _____________ 11.3 rarefaction (R), as shown in Fig. 11.4. As the object moves back and forth rapidly, a series Make a list of different types of of compressions and rarefactions is created in musical instruments and discuss the air. These make the sound wave that with your friends which part of the instrument vibrates to produce sound. 11.2 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 Fig. 11.4: A vibrating object creating a series of particles of the medium around it vibrating. compressions (C) and rarefactions (R) in The particles do not travel all the way from the medium. 128 SCIENCE 2024-25 propagates through the medium. The regions where the coils become closer Compression is the region of high pressure are called compressions (C) and the regions and rarefaction is the region of low pressure. where the coils are further apart are called Pressure is related to the number of particles rarefactions (R). As we already know, sound of a medium in a given volume. More density propagates in the medium as a series of of the particles in the medium gives more compressions and rarefactions. Now, we can pressure and vice versa. Thus, propagation compare the propagation of disturbance in a of sound can be visualised as propagation of slinky with the sound propagation in the density variations or pressure variations in medium. These waves are called longitudinal the medium. waves. In these waves the individual particles of the medium move in a direction parallel to Q uestion the direction of propagation of the disturbance. 1. How does the sound produced by The particles do not move from one place to a vibrating object in a medium another but they simply oscillate back and reach your ear? forth about their position of rest. This is 2. Explain how sound is produced exactly how a sound wave propagates, hence by your school bell. sound waves are longitudinal waves. 3. Why are sound waves called There is also another type of wave, called mechanical waves? a transverse wave. In a transverse wave 4. Suppose you and your friend are particles do not oscillate along the direction on the moon. Will you be able to of wave propagation but oscillate up and down hear any sound produced by about their mean position as the wave travels. your friend? Thus, a transverse wave is the one in which the individual particles of the medium move 11.2.1 SOUND WAVES ARE LONGITUDINAL about their mean positions in a direction perpendicular to the direction of wave WAVES propagation. When we drop a pebble in a pond, the waves you see on the water surface Activity _____________ 11.4 is an example of transverse wave. Light is a Take a slinky. Ask your friend to hold transverse wave but for light, the oscillations one end. You hold the other end. are not of the medium particles or their Now stretch the slinky as shown in pressure or density— it is not a mechanical Fig. 11.5(a). Then give it a sharp push towards your friend. wave. You will come to know more about What do you notice? If you move your transverse waves in higher classes. hand pushing and pulling the slinky alternatively, what will you observe? 11.2.2 CHARACTERISTICS OF A SOUND If you mark a dot on the slinky, you WAVE will observe that the dot on the slinky will move back and forth parallel to We can describe a sound wave by its the direction of the propagation of the frequency disturbance. amplitude and speed. (a) A sound wave in graphic form is shown in Fig. 11.6(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 (b) with distance, above and below the average Fig. 11.5: Longitudinal wave in a slinky. value of density and pressure. Fig. 11.6(a) and SOUND 129 2024-25 Fig. 11.6(b) represent the density and Heinrich Rudolph Hertz pressure variations, respectively, as a sound was born on 22 February wave propagates in the medium. 1857 in Hamburg, Compressions are the regions where Germany and educated at particles are crowded together and the University of Berlin. He represented by the upper portion of the curve confirmed J.C. Maxwell’s in Fig. 11.6(c). The peak represents the region electromagnetic theory by of maximum compression. Thus, his experiments. He laid the compressions are regions where density as H. R. Hertz foundation for future well as pressure is high. Rarefactions are the development of radio, telephone, telegraph regions of low pressure where particles are and even television. He also discovered the spread apart and are represented by the photoelectric effect which was later valley, that is, the lower portion of the curve explained by Albert Einstein. The SI unit of in Fig. 11.6(c). A peak is called the crest and a frequency was named as hertz in his honour. valley is called the trough of a wave. The distance between two consecutive Frequency tells us how frequently an compressions (C) or two consecutive event occurs. Suppose you are beating a rarefactions (R) is called the wavelength, as drum. How many times you are beating the shown in Fig. 11.6(c), The wavelength is drum in unit time is called the frequency of usually represented by λ (Greek letter your beating the drum. We know that when lambda). Its SI unit is metre (m). sound is propagated through a medium, the Fig. 11.6: Sound propagates as density or pressure variations as shown in (a) and (b), (c) represents graphically the density and pressure variations. 130 SCIENCE 2024-25 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 ν (Greek letter, nu). Its SI unit is hertz (symbol, Hz). 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 Fig. 11.7: Low pitch sound has low frequency and high pitch of sound has high frequency. the symbol T. Its SI unit is second (s). Frequency and time period are related as as shown in Fig. 11.6(c). For sound its unit follows: will be that of density or pressure. 1 The loudness or softness of a sound is v= T determined basically by its amplitude. The amplitude of the sound wave depends upon A violin and a flute may both be played at the force with which an object is made to the same time in an orchestra. Both sounds vibrate. If we strike a table lightly, we hear a travel through the same medium, that is, air soft sound because we produce a sound wave 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 Fig. 11.7. 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 Fig. 11.8: Soft sound has small amplitude and wave. It is usually represented by the letter A, louder sound has large amplitude. SOUND 131 2024-25 of less energy (amplitude). If we hit the table Example 11.1 A sound wave has a hard we hear a louder sound. Can you tell frequency of 2 kHz and wave length why? A sound wave spreads out from its 35 cm. How long will it take to travel source. As it moves away from the source its 1.5 km? amplitude as well as its loudness decreases. Louder sound can travel a larger distance as Solution: it is associated with higher energy. Fig. 11.8 Given, shows the wave shapes of a loud and a soft Frequency, ν = 2 kHz = 2000 Hz sound of the same frequency. Wavelength, λ = 35 cm = 0.35 m The quality or timber of sound is that We know that speed, v of the wave characteristic which enables us to distinguish = wavelength × frequency one sound from another having the same pitch v =λν and loudness. The sound which is more = 0.35 m 2000 Hz = 700 m/s pleasant is said to be of a rich quality. A sound The time taken by the wave to travel a of single frequency is called a tone. The sound distance, d of 1.5 km is 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. Thus sound will take 2.1 s to travel a distance of 1.5 km. Q uestions Q 1. Which wave property determines uestions (a) loudness, (b) pitch? 1. What are wavelength, frequency, 2. Guess which sound has a higher time period and amplitude of a pitch: guitar or car horn? sound wave? 2. How are the wavelength and frequency of a sound wave The speed of sound is defined as the related to its speed? distance which a point on a wave, such as a 3. Calculate the wavelength of a compression or a rarefaction, travels per unit sound wave whose frequency is time. 220 Hz and speed is 440 m/s in We know, a given medium. speed, v = distance / time 4. A person is listening to a tone of λ 500 Hz sitting at a distance of = T 450 m from the source of the sound. What is the time interval Here λ is the wavelength of the sound wave. It between successive compressions is the distance travelled by the sound wave in from the source? one time period (T) of the wave. Thus, The amount of sound energy passing each second through unit area is called the intensity v=λν of sound. We sometimes use the terms “loudness” and “intensity” interchangeably, or v =λν but they are not the same. Loudness is a That is, speed = wavelength × frequency. measure of the response of the ear to the sound. The speed of sound remains almost the Even when two sounds are of equal intensity, same for all frequencies in a given medium we may hear one as louder than the other under the same physical conditions. simply because our ear detects it better. 132 SCIENCE 2024-25  Q Q uestion uestion 1. Distinguish between loudness 1. In which of the three media, air, and intensity of sound. water or iron, does sound travel the fastest at a particular 11.2.3 S PEED OF SOUND IN DIFFERENT temperature? MEDIA Sound propagates through a medium at a finite 11.3 Reflection of Sound speed. The sound of a thunder is heard a little later than the flash of light is seen. So, we can Sound bounces off a solid or a liquid like a make out that sound travels with a speed rubber ball bounces off a wall. Like light, sound which is much less than the speed of light. The gets reflected at the surface of a solid or liquid speed of sound depends on the properties of and follows the same laws of reflection as you the medium through which it travels. You will have studied in earlier classes. The directions learn about this dependence in higher classes. in which the sound is incident and is reflected The speed of sound in a medium depends on make equal angles with the normal to the reflecting surface at the point of incidence, and temperature of the medium. The speed of the three are in the same plane. An obstacle of sound decreases when we go from solid to large size which may be polished or rough is gaseous state. In any medium as we increase needed for the reflection of sound waves. the temperature, the speed of sound increases. For example, the speed of sound in air is 331 Activity _____________ 11.5 m s–1 at 0 ºC and 344 m s–1 at 22 ºC. The speeds of sound at a particular temperature in various Take two identical pipes, as shown in media are listed in Table 11.1. You need not Fig. 11.9. You can make the pipes memorise the values. using chart paper. The length of the pipes should be sufficiently long as shown. Table 11.1: Speed of sound in Arrange them on a table near a wall. different media at 25 ºC Keep a clock near the open end of one of the pipes and try to hear the sound State Substance Speed in m/s of the clock through the other pipe. Adjust the position of the pipes so Solids Aluminium 6420 that you can best hear the sound of Nickel 6040 the clock. Steel 5960 Now, measure the angles of incidence and reflection and see the Iron 5950 relationship between the angles. Brass 4700 Lift the pipe on the right vertically to a small height and observe Glass (Flint) 3980 what happens. Liquids Water (Sea) 1531 (In place of a clock, a mobile phone on vibrating mode may also be used.) Water (distilled) 1498 Ethanol 1207 Methanol 1103 Gases Hydrogen 1284 Helium 965 Air 346 Oxygen 316 Sulphur dioxide 213 Fig. 11.9: Reflection of sound SOUND 133 2024-25 11.3.1 ECHO What is the distance of the cliff from the person if the speed of the sound, v is If we shout or clap near a suitable reflecting taken as 346 m s–1? object such as a tall building or a mountain,we will hear the same sound Solution: again a little later. This sound which we hear is called an echo. The sensation of Given, sound persists in our brain for about 0.1 Speed of sound, v = 346 m s–1 s. To hear a distinct echo the time interval Time taken for hearing the echo, between the original sound and the t=2s reflected one must be at least 0.1s. If we Distance travelled by the sound take the speed of sound to be 344 m/s at a = v × t = 346 m s–1 × 2 s = 692 m given temperature, say at 22 ºC in air, the In 2 s sound has to travel twice the sound must go to the obstacle and reach distance between the cliff and the back the ear of the listener on reflection after person. Hence, the distance between the 0.1s. Hence, the total distance covered by cliff and the person the sound from the point of generation to the reflecting surface and back should be = 692 m/2 = 346 m. at least (344 m/s) × 0.1 s = 34.4 m. Thus, uestion Q for hearing distinct echoes, the minimum distance of the obstacle from the source of sound must be half of this distance, that 1. An echo is heard in 3 s. What is is, 17.2 m. This distance will change with the distance of the reflecting the temperature of air. Echoes may be heard surface from the source, given that more than once due to successive or the speed of sound is 342 m s–1? multiple reflections. The rolling of thunder is due to the successive reflections of the 11.3.3 USES OF MULTIPLE REFLECTION sound from a number of reflecting surfaces, OF SOUND such as the clouds and the land. 1. Megaphones or loudhailers, horns, 11.3.2 REVERBERATION musical instruments such as trumpets A sound created in a big hall will persist and shehanais, are all designed to send by repeated reflection from the walls until sound in a particular direction without it is reduced to a value where it is no longer spreading it in all directions, as shown audible. The repeated reflection that in Fig 11.10. 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, Megaphone rough plaster or draperies. The seat materials are also selected on the basis of their sound absorbing properties. Horn Example 11.2 A person clapped his hands near a cliff and heard the echo after 2 s. Fig 11.10: A megaphone and a horn. 134 SCIENCE 2024-25  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. 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, as shown in Fig.11.11. Fig. 11.13: Sound board used in a big hall. Q uestion 1. Why are the ceilings of concert halls curved? Fig.11.11: Stethoscope 11.4 Range of Hearing 3. Generally the ceilings of concert halls, The audible range of sound for human beings conference halls and cinema halls are extends from about 20 Hz to 20000 Hz (one curved so that sound after reflection Hz = one cycle/s). Children under the age of reaches all corners of the hall, as five and some animals, such as dogs can hear shown in Fig 11.12. Sometimes a up to 25 kHz (1 kHz = 1000 Hz). As people curved soundboard may be placed grow older their ears become less sensitive to behind the stage so that the sound, higher frequencies. Sounds of frequencies after reflecting from the sound board, below 20 Hz are called infrasonic sound or spreads evenly across the width of the infrasound. If we could hear infrasound we hall (Fig 11.13). 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 5 Hz. 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. Frequencies higher than 20 kHz 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. Fig. 11.12: Curved ceiling of a conference hall. These moths can hear the high frequency SOUND 135 2024-25 squeaks of the bat and know when a bat in construction of big structures like is flying nearby, and are able to escape buildings, bridges, machines and also capture. Rats also play games by scientific equipment. The cracks or producing ultrasound. holes inside the metal blocks, which are invisible from outside reduces the strength of the structure. Ultrasonic Hearing Aid: People with hearing loss may waves are allowed to pass through the need a hearing aid. A hearing aid is an metal block and detectors are used to electronic, battery operated device. The detect the transmitted waves. If there hearing aid receives sound through a is even a small defect, the ultrasound microphone. The microphone converts the gets reflected back indicating the sound waves to electrical signals. These presence of the flaw or defect, as shown electrical signals are amplified by an in Fig. 11.14. 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. Q uestions 1. What is the audible range of the average human ear? 2. What is the range of frequencies Fig 11.14: Ultrasound is reflected back from the associated with defective locations inside a metal block. (a) Infrasound? (b) Ultrasound? Ordinary sound of longer wavelengths cannot be used for such purpose as it will 11.5 Applications of Ultrasound bend around the corners of the defective location and enter the detector. Ultrasounds are high frequency waves. Ultrasonic waves are made to reflect Ultrasounds are able to travel along well- from various parts of the heart and defined paths even in the presence of form the image of the heart. This tech- obstacles. Ultrasounds are used extensively nique is called ‘echocardiography’. in industries and for medical purposes. Ultrasound scanner is an instrument Ultrasound is generally used to clean which uses ultrasonic waves for parts located in hard-to-reach places, getting images of internal organs of the for example, spiral tube, odd shaped human body. A doctor may image the parts, electronic components, etc. patient’s organs, such as the liver, gall Objects to be cleaned are placed in a bladder, uterus, kidney, etc. It helps cleaning solution and ultrasonic waves are sent into the solution. Due to the doctor to detect abnormalities, the high frequency, the particles of such as stones in the gall bladder and dust, grease and dirt get detached and kidney or tumours in different organs. drop out. The objects thus get In this technique the ultrasonic waves thoroughly cleaned. travel through the tissues of the body Ultrasounds can be used to detect and get reflected from a region where cracks and flaws in metal blocks. there is a change of tissue density. 136 SCIENCE Metallic components are generally used 2024-25 squeaks of the bat and know when a bat in construction of big structures like is flying nearby, and are able to escape buildings, bridges, machines and also capture. Rats also play games by scientific equipment. The cracks or producing ultrasound. holes inside the metal blocks, which are invisible from outside reduces the strength of the structure. Ultrasonic Hearing Aid: People with hearing loss may waves are allowed to pass through the need a hearing aid. A hearing aid is an metal block and detectors are used to electronic, battery operated device. The detect the transmitted waves. If there hearing aid receives sound through a is even a small defect, the ultrasound microphone. The microphone converts the gets reflected back indicating the sound waves to electrical signals. These presence of the flaw or defect, as shown electrical signals are amplified by an in Fig. 11.14. 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. Q uestions 1. What is the audible range of the average human ear? 2. What is the range of frequencies Fig 11.14: Ultrasound is reflected back from the associated with defective locations inside a metal block. (a) Infrasound? (b) Ultrasound? Ordinary sound of longer wavelengths cannot be used for such purpose as it will 11.5 Applications of Ultrasound bend around the corners of the defective location and enter the detector. Ultrasounds are high frequency waves. Ultrasonic waves are made to reflect Ultrasounds are able to travel along well- from various parts of the heart and defined paths even in the presence of form the image of the heart. This tech- obstacles. Ultrasounds are used extensively nique is called ‘echocardiography’. in industries and for medical purposes. Ultrasound scanner is an instrument Ultrasound is generally used to clean which uses ultrasonic waves for parts located in hard-to-reach places, getting images of internal organs of the for example, spiral tube, odd shaped human body. A doctor may image the parts, electronic components, etc. patient’s organs, such as the liver, gall Objects to be cleaned are placed in a bladder, uterus, kidney, etc. It helps cleaning solution and ultrasonic waves are sent into the solution. Due to the doctor to detect abnormalities, the high frequency, the particles of such as stones in the gall bladder and dust, grease and dirt get detached and kidney or tumours in different organs. drop out. The objects thus get In this technique the ultrasonic waves thoroughly cleaned. travel through the tissues of the body Ultrasounds can be used to detect and get reflected from a region where cracks and flaws in metal blocks. there is a change of tissue density. 136 SCIENCE Metallic components are generally used 2024-25  These waves are then converted into examination of the foetus during electrical signals that are used to pregnancy to detect congenial defects generate images of the organ. These and growth abnormalities. images are then displayed on a monitor Ultrasound may be employed to break or printed on a film. This technique small ‘stones’ formed in the kidneys is called ‘ultrasonography’. into fine grains. These grains later get Ultrasonography is also used for flushed out with urine. 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, λ. 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 1 the frequency (ν), v =. T The speed v, frequency ν, and wavelength λ, of sound are related by the equation, v = λν. 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 0.1 s. The persistence of sound in an auditorium is the result of repeated reflections of sound and is called reverberation. SOUND 137 2024-25  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 20 Hz – 20 kHz. 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. Exercises 1. What is sound and how is it produced? 2. Describe with the help of a diagram, how compressions and rarefactions are produced in air near a source of sound. 3. Why is sound wave called a longitudinal wave? 4. Which characteristic of the sound helps you to identify your friend by his voice while sitting with others in a dark room? 5. Flash and thunder are produced simultaneously. But thunder is heard a few seconds after the flash is seen, why? 6. A person has a hearing range from 20 Hz to 20 kHz. What are the typical wavelengths of sound waves in air corresponding to these two frequencies? Take the speed of sound in air as 344 m s–1. 7. 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. 8. The frequency of a source of sound is 100 Hz. How many times does it vibrate in a minute? 9. Does sound follow the same laws of reflection as light does? Explain. 10. 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? 11. Give two practical applications of reflection of sound waves. 12. A stone is dropped from the top of a tower 500 m high into a pond of water at the base of the tower. When is the splash heard at the top? Given, g = 10 m s–2 and speed of sound = 340 m s–1. 138 SCIENCE 2024-25  13. A sound wave travels at a speed of 339 m s–1. If its wavelength is 1.5 cm, what is the frequency of the wave? Will it be audible? 14. What is reverberation? How can it be reduced? 15. What is loudness of sound? What factors does it depend on? 16. How is ultrasound used for cleaning? 17. Explain how defects in a metal block can be detected using ultrasound. SOUND 139 2024-25