Sound - PDF

# Sound ## Learning Objectives At the end of this chapter, students will be able to understand: - How is sound produced? - The characteristics of vibrations - Musical instruments - The concept of sound produced by animals - Sound needs a medium for propagation - The working of the human ear - The...

# Sound ## Learning Objectives At the end of this chapter, students will be able to understand: - How is sound produced? - The characteristics of vibrations - Musical instruments - The concept of sound produced by animals - Sound needs a medium for propagation - The working of the human ear - The characteristics of sound - Distinguish between audible and inaudible sound - Technological applications of ultrasonic sound - Distinguish between noise and music - The harmful effects of noise pollution ## Description Of Image An image of a book page about sound, containing descriptions and an image of various musical instruments on the right side of the page. ## Overview Of Sound Our environment is full of sounds. Some sounds are pleasant to hear while others are noisy. A car horn is noisy but the sound of music is pleasant. It is your ears that help you hear and differentiate between the two sounds. In nature, we hear different kinds of sound. The sound of an alarm clock, the chirping of birds, the whispering of the trees, the crying of infants, the barking of dogs, the humming of insects and mosquitoes, the voices of classmates and teachers, and so on. Sound helps us to communicate with one another. Can you recognize your friends by just listening to their voices? Yes, you can because each sound has a different character. ## How is sound produced? Put your finger tips in front of your throat. What do you feel? Again touch it when you speak. What do you feel now? Your finger tips feel some vibrations while you speak. So, vibrating objects make sound. Every type of sound is produced by something in a state of vibration. Vibrations are produced by to and fro motion of a body. Let's perform these activities for better understanding. ### Try and Learn-10.1 **Aim:** To demonstrate that a vibrating body produces sound. **Requirements**: A shallow pan, a stick, and rope. **Procedure:** Take a shallow pan (easily available at home). The pan should be hung with the help of rope at a convenient place in such a way that it does not touch a wall. Now strike the pan with a stick. You hear the sound of beating the shallow pan. Now touch the pan gently with your finger. What do you feel? You feel the vibrations. Again strike the pan with a stick and hold it tightly with your hands immediately after striking. What do you observe? You observe that as your hands come in contact with the pan, the vibrations produced in the pan stop and no sound is produced. Again touch the pan when it stops producing sound. What do you feel? You feel that there is no vibration at this time. ### Try and Learn-10.2 **Aim:** To prove that sound is produced by a stretched rubber band. **Requirements:** A rubber band, pencil box containing at least two pencils. **Procedure:** Take a rubber band and put it around the longer side of a pencil box. Insert two pencils at both the ends between the box and the stretched rubber. Now pluck the rubber band from the middle with the fingers of one hand and pressing the pencil box with the fingers of the other hand. What do you observe or feel? You observe that the rubber band vibrates, and you hear the sound. From the above activities, we can conclude that when a tightly stretched rubber band is plucked or a shallow pan is struck, they vibrate and produce sound. When they stop vibrating, they do not produce any sound. Thus, sound is a vibration that is capable of being heard. A vibration is a rapid to and fro or back and forth motion of an object. This type of motion is known as oscillatory motion. ## Characteristics of Vibrations (Oscillations) Amplitude, frequency, and the time period are the three important characteristics of the vibrations (or oscillations). - **Amplitude:** The maximum displacement of a vibrating body from its mean position is called its amplitude. Its Sl unit is a meter (m). In figure let PQ be a vibrating string. RS is the amplitude of vibration. - **Frequency:** The number of vibrations of the vibrating body in one second is called its frequency. Its Sl unit is Hertz (Hz) in honor of the German physicist Heinrich R. Hertz. If any vibrating body makes 60 vibrations per second, its frequency is 60 Hz. - **Time Period:** The time taken by the vibrating body to complete one vibration is called the time period of vibration. Its Sl unit is seconds (s). ## Musical Instruments Sound is produced not only by vibrations in strings but also by vibrations in stretched membranes, as well as by vibrations in air columns. Some instruments such as manjira (cymbals), the noot (mud-pots), the ghatam, and the kartal produce sound simply when beaten or struck. These instruments are commonly used in different parts of India. Some of these are very simple and can be made at home. Let’s try to make some musical instruments by performing the following activities. ### Try and Learn-10.4 **Requirements:** A hollow coconut shell or an earthen pot, strong string, split bamboo cane, small wooden brick (bridge), string, and tuning peg. **Procedure:** Take a hollow coconut shell and to it attach a split bamboo cane as a neck. Now tightly stretch string over an opening cut in the coconut shell. Fix one tuning peg at the end of the bamboo cane. Now stretch the string from the hook at the end of the coconut shell, over the bridge, to the tuning peg. Your Ektara is ready to use. The string of the Ektara is plucked with one finger, the pitch can be changed continuously by stretching the string with the help of the tuning peg. ## Musical Instruments Classifications Musical instruments are classified according to the way they produce sound, they are: 1. **Wind Instrument:** The instrument where vibrations in air columns produce sound. The frequency is changed by changing the length of the vibrating air column. For example, shehnai, flute, clarinet, etc. 2. **Stringed Instrument:** The instrument where vibrations in wires produce sound. The frequency of vibration is changed by changing the length, thickness and tightness of the wires. For example, guitar, violin, sitar, veena, etc. 3. **Percussion Instruments:** They have a stretched skin whose vibrations produce sound. The pitch or frequency of vibration can be increased by stretching the skin more. Drum, tabla, mridangam, etc. are percussion instruments. Identify the vibrating parts of the various musical instruments given in the table, and fill them in the table. ## Table 10.1 Musical instruments and their vibrating parts | S.No. | Musical Instrument | Vibrating parts which produce sound | | :---- | :------------------- | :------------------------------------ | | 1. | Flute | Air-column | | 2. | Tabla | | | 3. | Veena | | | 4. | Sitar | | | 5. | Sarod | | | 6. | Shehnai | | | 7. | Drum | | | 8. | Mridangam | | ## Let's Recall Fill in the blanks: (i) Sound is produced when a body **(vibrates / remains same)**. (ii) When we touch a source of sound, we can feel the **(air / vibration)**. (iii) A rapid back and forth movement of a body about a central position is called **(vibration / compression)**. (iv) A sitar is a **(wind / string)** instrument. ## Sound Produced By Animals You are familiar with the sound produced by many animals such as dog barks, birds, chirp etc., The hyena makes sound which resemble the laughter of a human. These animals have a voice organ and are able to communicate with others. Frogs also have vocal chords with which they make loud sounds. But some animals do not have voice organs, they make sounds with different parts of their bodies. Snakes make hissing sounds by forcing air out of their mouths. Grasshoppers produce sound by scraping the rough edges of their wings against the upper part of their legs. In humans, the sound is produced by the voice box or the larynx. A hard bump in the throat which seems to move when you swallow is known as the voice box. Two ligaments commonly known as vocal cords are stretched across the larynx in such a way that it leaves a narrow slit between them for the passage of air. When the air from the lungs is forced through the slit, it makes the vocal cords vibrate and thus sound is produced. When the vocal cords are tight and thin, the type of voice is different from that when they are loose and thick. Let us see how vocal cords work. ## Science Bits! Men, women and children have different vocal cord sizes. The male vocal cords are between 17 mm to 25 mm in length. The women vocal cords are between 12.5 mm to 17.5 mm in length. Children have very short vocal cords. The difference in vocal cord size between men, women and children means that they have differently pitched voices. ### Try and Learn-10.6 **Aim:** To show the working of vocal cords in humans. **Requirements:** Two identical rubber strips, a piece of paper with a narrow slit. **Procedure:** Take two identical rubber strips and place them one above the other. Now stretch these pieces tightly by holding them near the mouth. Now blow air through the gap between them. What happens? As the air blows through the stretched rubber strips, a sound is produced. Again take a piece of paper with a narrow slit and hold it between your fingers. Now blow air through the slit. What happens? Do you hear any sound? As the air blows through the slit, a sound is produced. Human’s vocal produces sound in a similar way. ## Sound Needs A Medium For Propagation We know that vibrating objects produce sound on vibration. Hence, the air surrounding the object gets disturbed and the air molecules in contact with the object begin to vibrate. In this process, they gain energy and in turn transfers this energy to the next air molecule in contact, causing them to vibrate. This sets up a chain reaction. Thus sound wave travels from the source to the detector through a medium such as air. Sound needs molecules to travel. Thus sound can travel through solids, liquids, and gases but not through a vacuum. Speed of the sound depends upon the medium through which it travels. The speed of sound is maximum in solids, lesser in liquids, and least in gases. ### Try and Learn-10.7 **Aim:** To show that sound requires a material medium for propagation. **Requirements:** A glass jar, an electric bell, stopper, vacuum pump. **Procedure:** Take a glass jar and put or hang an electric bell inside it through the stopper. Connect the vacuum pump to the other end of the glass jar. Connect the wires of the bell and vacuum pump to the switch. Now switch on the bell. Do you hear any sound? Yes! you hear the sound of the ringing bell. Now switch on the vacuum pump to take out the molecules of air from the jar. What happens to the sound of the bell? As more and more air is removed from the jar, the sound gets feebler and finally is heard very faintly. Now switch off the vacuum pump and allow the air to enter the jar. The sound of the bell is heard again. **Conclusion:** From this activity, you can conclude that: (i) Sound requires a material medium for propagation. (ii) Sound cannot propagate through a vacuum. We know that sound can travel through air. Does sound travel in liquids and solids? ### Try and Learn-10.8 **Aim:** To show that sound can travel in liquids. **Requirements:** A bath tub, clean water, bell. **Procedure:** Take a bath tub and fill it with clear water. Take a bell in one hand and dip this into the water. Shake the bell to produce sound by taking care that the bell does not touch any side of the bath tub. Now, place your one ear gently on the surface of water by taking care that the water should not enter your ear. What do you feel? Can you hear the sound of the bell? You feel some vibrations on the ear, and hear the sound of the bell. This shows that sound travels in liquids. ### Try and Learn-10.9 **Aim:** To show that sound can travel through solids. **Requirements:** A wooden table, meter scale. **Procedure:** (a) Take a wooden table. Place your one ear at one end of the wooden table and close the other ear with your hand. Now ask your friend to gently tap on the table at the other end of the table. Can you hear the sound of tapping? Yes! you can hear the sound of tapping. Now lift your head in such a way that your ear is not touching the table. Again ask your friend to tap on the table at the same place. Can you hear the sound of taping? Yes! you can hear the sound of taping but the sound appears less loud in this case. (b) Take a metre scale. Hold one end of the metre scale to your ear and ask your friend to gently tap on the other end of the scale. Can you hear the sound of tapping? Yes! You can hear the sound of tapping. Now keep the metre scale away from your ear and again ask your friend to tap on the metre scale at the same place. Can you hear the sound of tapping again? Yes! You can hear the sound of taping but the sound appears less loud. **Conclusion:** From this activity, you can conclude that sound can travel through solids. Can you say that sound travels through strings? Let's perform this activity. ### Try and Learn-10.10 **Aim:** Make a toy telephone. **Requirements:** Two empty coca-cola cans, nail, hammer, cutter, and a string. **Procedure:** Take two empty coca-cola cans. Remove the covers of the cans with the help of a cutter. Now punch a hole with the help of a nail and hammer in the bottom of each can. Pass the string through each of the holes and tie a knot at its end. Ask your friend to hold one of the cans and you hold the other. Ask your friend to move away from him till the string is fully stretched Put the can on your ear and ask your friend to speak in his can. Do you hear your friend? Yes! How is the sound passing from one can to the other? It is because sound travels through the string. In fact, sound can travel through any solid. ## Speed Of Sound In Different Media We have learnt that sound waves cannot travel in a vacuum. They can travel only through material mediums e.g., solids, liquids, and gases. During a thunderstorm, lightning and thunder occur at the same time but you hear the sound of thunder a little after you see the lightning flash. During the firing of explosives, a person at a distance first sees the flash of fire and then hears the sound of explosion. From the above phenomena, we conclude that the speed of light is much faster than speed of sound in air is only 330 m/s, whereas the speed of light is 3 x 108 m/s. It explains why the light is seen almost at the same instant as it is produced, but the sound is heard much after as evident in the above examples. ## Table Of Speed Of Sound | Medium | Speed in ms | | :-------------- | :---------- | | Rubber | 1600 to 1650 | | Copper | 3700 | | Air (Dry) | 330-345 | | Air (Wet) | 400 | | Wood | 4000 to 5000 | | Steel | 5000 to 7000 | | Water | 1500 | | Glass | 5000 | We know that, Speed of sound = Distance travelled by the sound / Time taken **Example 1:** A cracker is burst at a distance of 600 m from a person. After how much time will he hear the sound? (speed of sound in air = 330 m/s). **Solution:** Speed of sound in air = 330 m/s. Distance travelled = 600 m. Applying formula, Speed = Distance / Time We get, 330 m/s = 600 m / T or T = 600 m / 330 m/s = 1.8 s. **Example 2:** A gun was fired 3 km away from a person. The time taken for the sound to reach the person was 6 seconds. Find the speed of sound. **Solution:** Distance travelled = 3 km = 3000 m. Time taken = 6 s. Applying formula speed = Distance / Time taken. We get, Speed = 3000 m / 6 s = 500 m/s. ## Perception Of Sound We have learnt that a vibrating body produces sound. These vibrations propagate in all directions in a medium. The question which comes to mind is how do we hear it. Our ears are organs that detect sound. ## The Human Ear Ear has three parts: outer ear, middle ear, and inner ear. Each part of the ear serves a purpose in the task of detecting and interpreting sound. - **Outer ear:** The outer ear consists of an ear flap (pinna) and an approximately 2 cm long ear canal (ear tube). The shape of pinna is like a funnel. The outer ear also channels sound waves to reach the eardrum through the ear canal to the eardrum of the middle ear. The eardrum is a tightly stretched membrane that separates it from the middle ear. The ear drum is like a stretched balloon. Sound vibrations make the eardrum vibrate. To understand the working of an ear drum, let us perform the following activity. ### Try and Learn-10.11 **Aim:** To show the working of an ear drum. **Requirements:** A plastic or tin can, rubber balloon, cutter, wheat grains, rubber band. **Procedure:** Take a tin can or plastic can and cut its ends with the help of a cutter. Now stretch a piece of balloon across one end of the can and tie it with a rubber band. Put some grains on the stretched balloon. Now ask some one to speak loudly from the open end by holding the can vertically. **What do you observe?** You observe that grains jump up and down. This is due to vibrations produced by human voice. - **Middle ear:** The middle ear is an air-filled cavity behind the eardrum. It consists of three tiny, interconnected bones - the hammer, anvil, and stirrup. - **Inner ear:** The inner ear consists of a cochlea, the semicircular canals, and the auditory nerve. The eardrum sends vibrations to the auditory nerve in the inner ear. The auditory nerve takes the signal to the brain and then we can hear the sound. ## Science Bits! Italian physicists GIOVANI ALFONSO BORELLI and VINCENZO VIVIANI, in the 1660s, first calculated the speed of sound. They measured the time between seeing the flash of a gun and hearing its sound over a long distance on a still day. Their calculation was: 350 metres per second. Never put a sharp pointed and hard object into your ear. It can damage the ear drum which can impair hearing. ## Characteristics of Sound We have learnt about amplitude, time period, and frequency of vibrations. Amplitude and frequency determine the characteristics of sound produced. Can you differentiate sounds on the basis of their amplitude and frequency? ## Loudness Loudness depends upon the amplitude of the vibrating body. Loudness does not change with the change in frequency. Loudness of sound is determined by the amount of sound energy received by the ear in unit time. It means, more the energy received, the louder is the sound. ## Pitch The pitch depends upon the frequency of the vibrating body. The pitch changes with the change in the frequency. A lower frequency will produce bass sound, whereas a higher frequency will produce shrill sound. The pitch does not depend upon the amount of energy received by the ear. ## Audible And Inaudible Sounds Although it is true that all vibrating bodies produce sound waves, can you hear all of them? No, because our ears are sensitive to a range of frequencies 20 Hz to 20,000 Hz. This range is called the audible range and the sound is called audible sound or sonic sound. Sound frequencies less than or higher than this range cannot be detected by humans. Such sounds are inaudible sounds. The sound of frequencies less than about 20 Hz is called infrasonic or subsonic sound. The sound of frequencies higher than 20,000 Hz is called ultrasonic sound. However, certain animals such as dogs have the ability to hear very high-pitched sounds, i.e., ultrasonic sounds. Dolphins use ultrasonic sound to locate their prey. ## Technical Applications Of Ultrasonic Sound Ultrasonic waves have higher frequency than 20,000 Hz. So, we cannot hear ultrasonic waves. However, ultrasonic waves are used in various technologies. Here are some of them: - Ultrasonic waves are used to study the growth of the foetus in the uterus of a woman by means of ultrasonography. - Ultrasonic waves are used for homogenizing milk. - Ultrasonic waves are used to detect flaws in metals and structures. - Most important use of ultrasonic waves is in SONAR for detecting the presence of submarines, icebergs, sunken ships, etc., and to map the sea beds. ## Noise And Music We hear different types of sounds around us. Some of them are pleasant to the ear, whereas some are not. ## Science Bits! Bats make high-pitched sounds while flying, which bounce off objects in the form of echoes. They give the bats information about the direction and distance of the objects. Sounds which are pleasing to the ear are called music and those which are unpleasant to the ear are called noise. Musical sound is produced by regular vibrations and noise is produced by irregular vibrations. Sound produced by a harmonium is a musical sound, but sound produced by all the students together of a class is noise. If any musical sound becomes too loud, it acts as noise. ## Noise Pollution The world today is suffering from pollution such as water pollution, air pollution, noise pollution, etc. Like water pollution and air pollution, noise pollution is also very dangerous to mankind. Noise pollution is defined as the presence of excessive or unwanted sounds in the environment. Noise pollution is caused by vehicles, explosions including bursting of crackers, machines, loudspeakers, television, transistors, radio at high volumes, some kitchen appliances, desert coolers, air conditioners, etc. ## Table 10.4: Differences between music and noise | S.No. | Music | Noise | | :---- | :-------------------------------- | :---------------------------------------------------------- | | 1. | It has a pleasing effect on the ears. | It has the irritating and disturbing effect on the ears. | | 2. | It is produced by regular and periodic | It is produced by irregular vibrations in a body. | | 3. | vibrations in a material. | The amplitude of vibration and its frequency may change suddenly. | | | The amplitude of vibration and its frequency do not change suddenly. | | ## Harmful Effects Of Noise Pollution The presence of excessive noise in the surroundings can affect human health in a number of ways: - High noise pollution can cause hypertension (high blood pressure), lack of sleep, anxiety and many more health disorders. - Sudden exposure to a high noise level can cause permanent deafness by rupturing the ear drum. - High noise can cause a heart attack and even death. ## Measures to Reduce Noise Pollution To reduce noise pollution, you must control the sources of noise. This can be done by the following preventive measures: - The use of loud speakers should be restricted - Setting up of noise-producing industries away from the residential areas - Noise pollution could be reduced to a great extent by installing silencing devices in air conditioners, transport vehicles, industrial machines, and home appliances - Use of automobile horns should be minimised. - Using sound absorbing materials like cushions and curtains inside the home and planting along the roadside helps to reduce noise pollution. - T.V and music systems should not be played loudly. ## Science Bits! Increasing public awareness of noise by providing factual information about the effect of noise is a view to influencing behaviour to avoid noise generation. All these measures can help in reducing the harmful effects of noise pollution. - **Hearing Impairment** is the inability of an individual to hear sounds adequately. This may be due to improper development, damage or disease to any part of the hearing mechanism. Hearing is a prerequisite for the development of normal speech and language. A child learns to speak by hearing the speech of others in the family and surroundings. Technological devices for the hearing-impaired have made it possible for such persons to improve their quality of life. - **The Golconda Fort:** 11 km west of the city of Hyderabad. Andhra Pradesh (India), is one of the most magnificent forts in India. It was built for defense from invading Mughals from the north by early emperors. The Golconda Fort has a unique acoustical system by which a hand clap sounded at the Fort’s main gate was heard at the top of the Fort, situated on a 300 feet (91 m) high granite hill. This is one of the fascinating features of the fort. ## Let's Recall Fill in the blanks: (i) Excess noise around us causes noise **pollution**, too much of which causes **sound pollution**. (ii) The loudness of sound is measured in **decibels**. (iii) Irregular vibrations produce **noise**. (iv) Music is produced by **regular** vibrations in a body. ## Important Terms - **Amplitude:** The maximum displacement from the mean position in an oscillation. - **Audible:** The sound which can be heard. - **Eardrum:** A tightly stretched membrane that separates outer ear from middle ear. - **Hertz (Hz):** SI unit of frequency. - **Larynx:** A hard bump in the throat which seems to move when any one swallows. - **Loudness:** The property of sound that depends upon the amplitude of vibration of a vibrating body. - **Noise:** An undesirable, varying sound - **Oscillation:** Movement of a body from one extreme position to the other and back. - **Pitch:** The characteristic of sound that depends upon the frequency of a vibrating body. - **Time period:** The time taken for one oscillation. - **Vibration:** Fast to and fro movements or the oscillatory motion of a body, which may produce sound. - **Decibel:** The unit used to measure the loudness of sound.