Solved examples for class 8 science : Chapter 13

    Sound

    I. Multiple Choice Questions (MCQs).

    Q.1. The maximum displacement of a vibrating body from its mean position is called its
    a) amplitude
    b) frequency
    c) time period
    d) none of these
    Answer. (a) amplitude

    Q.2. SI unit of frequency is
    a) m
    b) Hz
    c) s
    d) $s ^{-1}$
    Answer. (b) Hz

    Q.3. Musical instruments can be played at different pitches by changing the _______ of the wires.
    a) Length
    b) thickness
    c) tightness
    d) all of these

    Answer. d) all of these

    Q.4. Sound cannot propagate through
    a) iron
    b) water
    c) vacuum
    d) rubber
    Answer. (c) vacuum

    Q.5. The correct statement among the following is/ are:
    a) Dogs can hear ultrasonic sound waves.
    b) Sound with frequency less than 20 Hz are known as infrasonic.
    c) In SONAR, ultrasonic waves are used.
    d) All of these
    Answer. d) All of these

    II. Fill in the blanks by selecting appropriate words from the given list:
    List : longitudinal, material, larynx, water, percussion, air

    Q.1. Sound waves in air travel as…………………………. waves.
    Q.2. In humans, the sound is produced by the………………………….
    Q.3. Sound requires a………………………… medium for propagation.
    Q.4. …………………………………… instruments have a stretched skin whose vibrations produce sound.
    Q.5. Sound travel about 5 times faster in ……………………….. than in………………………

    Answer.
    1- longitudinal,
    2- larynx,
    3- material,
    4- percussion,
    5- water, air

    III. Match the items of Column A with those of Column B.


    Answer. 1-e, 2-a, 3-d, 4-b, 5- c

    IV. Write ‘T’ for the true and ‘F’ for the false statements.
    Q.1. Sound helps us to communicate with one another.
    Q.2. In all cases vibrations of objects are easily visible to us.
    Q.3. Men, women and children have different vocal cord sizes.
    Q.4. The loudness of the sound does not depend on the quantity of air that is made to vibrate.
    Q.5. A high-pitched sound appears shrill, and a low-pitched sound appears bass to our ears.

    Answer. 1-T, 2-F, 3-T, 4-F, 5-T

    V. Very Short Answer Questions.

    Q.1. Name an instrument which is used to produce sound for experiments in a laboratory.
    Answer. Tuning fork

    Q.2. Give an example of transverse wave.
    Answer. Waves in water are the example of transverse waves.
    Q.3. Which instrument is used to produce a certain fixed frequency sound? Answer. Tuning fork is used to produce a certain fixed frequency sound.

    Q.4. Name two examples of stringed instrument.
    Answer. Guitar and violin
    Q.5. How the loudness of sound is related to the amplitude of the vibrations producing the sound?

    Answer. Loudness of sound is proportional to the square of the amplitude of the vibrations producing the sound.

    VI. Short Answer Questions.
    Q.1. How is sound produced?
    Answer. Vibrating objects make sound. Every type of sound is produced by something in a state of vibration. In some cases, the vibrations are easily visible to us but in some cases they can only be felt and not seen. However, we can feel them.

    Q.2. Why does one sound differ from the other?
    Answer: One sound differs from the other different sound has different pitch. The shrillness or pitch of the sound produced is determined by the frequency of the vibrating body. Higher the frequency of a sound, higher is its pitch. Lower the frequency of a sound, Lower is its pitch. A high-pitched sound appears shrill, and a low-pitched sound appears deep to our ears.

    Q.3. Write differences between music and noise.
    Answer:


    Q.4. List two uses of ultrasonic waves.
    Answer. two uses of ultrasonic waves are:
    • Ultrasonic wave is 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;

    Q.5. Discuss musical instruments and their kinds with proper examples.
    Answer: According to sound of the vibrations they are classified into three types :

    i. 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.

    ii. Stringed Instrument: The instrument where vibrations in wires produces sound.
    The frequency of vibration is changed by changing the length, thickness and tightness of the wires. For example, guitar, violin, sitar, veena, etc.

    iii. Percussion Instruments: They have a stretched skin whose vibrations to produce sound. Drum, tabla, mridangam, etc., are percussion instruments.

    Q.6. How can you prove that sound needs a material medium for propagation? Discuss with an activity.

    Answer: To show that sound requires a material medium for propagation. We need a glass jar, an electric bell, stopper, vacuum pump.

    Procedure : Take a glass jar and put or hang an electric bell inside it through stopper. Connect vacuum pump to the other end of the glass jar. Connect the wires of the bell and vacuum pump to switch.
    Now switch on the bell. You hear sound of ringing bell. Now switch on the vacuum pump to take out the molecules of air from jar. 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 vacuum.


    VII. Long Answer Questions.

    Q.1. (a) Describe characteristics of sound. How can you differentiate sounds on the basis of their amplitude and frequency?

    (b) A cracker is burst at a distance of 660 m from a person. After how much time will he hear the sound? (Speed of sound in air = 330 m/s)

    Answer. a) Following are the characteristics of sounds

    Loudness: Loudness of sound depends on its amplitude, i.e., larger the amplitude, more is the loudness. Loudness of sound is proportional to the square of the amplitude of the vibrations producing the sound. For example, if the amplitude of the vibration becomes thrice, the Loudness increases nine times. Similarly, if the amplitude becomes four times, the loudness increases sixteen times. The Loudness of the sound also depends on the quantity of air that is made to vibrate. Loudness of sound is measured in decibel. Decibel is represented as dB.

    Pitch: The shrillness or pitch of the sound produced is determined by the frequency of the vibrating body. Higher the frequency of a sound, higher is its pitch. Lower the frequency of a sound, Lower is its pitch. A high-pitched sound appears shrill, and a low-pitched sound appears deep to our ears. For example, the voice of a child or a woman has higher frequency. Therefore, the voice of a child or a woman is more shrill. On the other hand, the voice of a man has low frequency. Therefore, the voice of man is deep.

    Quality or Timbre: The quality of sound is also known as the timbre depends on the source of sound. When two people sing in a chorus having the same pitch and loudness, we can still able to distinguish their voices. This is due to the different quality or timbre of the sound produced.

    b) Distance of cracker bursting from the person = 660 m Speed of sound = 330 m/s
    Time = Distance/Speed of Sound in Air = 660 m/ 330 $ms^{-1}$ = 2 s

    Thus, the person will hear the sound after 2 s.

    Q.2. Discuss the structure of human ear with well labelled diagram. Also explain function of each part.
    Answer. Ear has three parts – outer ear, middle ear and inner ear.


    • 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 reach the ear 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.

    • 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 semi-circular canals, and the auditory nerve. The eardrum sends vibrations to the auditory nerve in inner ear. The auditory nerve takes the signal to the brain and we can then hear the sound.

    Q.3. (a) What do you mean by audible and inaudible sounds? Discuss noise pollution and its harmful effects.
    (b)Describe measures to reduce noise pollution.

    Answer. 20 Hz to 20,000 Hz, range is called the audible range and the sound is called audible sound or sonic sound. Sound of frequencies less than or higher than this range cannot be detected by humans. Such sounds are called 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.

    Noise pollution: 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.

    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 high noise level can cause permanent deafness by rupturing the ear drum.
    • High noise can cause heart attack and death.

    b) To reduce noise pollution, you must control the sources of noise. This can be done by the following activities:
    • 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 craft engines, 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 tree along the roadside helps to reduce noise pollution.
    • T.V. and music systems should not be played loudly.

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