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Sound Class 8 Science Chapter 13 as per NCERT Book used in CBSE and other Schools. The lesson covers the complete explanation of class 8 Chapter 13 Sound. Topics covered are sound, types of sound, sound as wave and wave types. Also, topics like human ear, Sound produced by human and animals, Sound wave propagation through states of matter, audible and inaudible sound, noise, music, instruments and Noise Pollution have been discussed. The lesson covers all important questions and NCERT solutions to book questions have also been provided for convenience of the students.
Sound is a form of energy that produces sensation of hearing. We can also say that it is a form of energy and it comes from vibrations. These vibrations create sound waves which move through medium such as air and water before reaching our ears. All sounds are a combination of pure frequencies. A stretched rubber band when plucked, vibrates and produces sound. We have also heard a variety of sounds from sitar, violin, veena, ringing of temple bells, chirping of birds, vehicles honking in traffic, dogs barking, pressure cooker whistling, crackers bursting, rattling and breaking of objects. Also, we hear our favourite song playing in the iPod. While some of these sounds may be pleasant and soothing, some could be annoying. But all these are known as sounds.
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There are different types of sounds which we can hear, written as follows:
Sound is a kind of wave
Wave is defined as a periodic disturbance in a medium (air, water, and gas) that transfers energy from one particle to another.
When we throw a stone in any water body, like pond or lake, we see ripples and this stone hinders the particles of water due to which disturbances occur in a wide range at a regular interval of time.
It is that wave which needs a medium to travel. This wave does not pass through vacuum.
It is the wave which can travel through a medium as well as through vacuum.
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It is that wave in which particles vibrate to and fro. We can also say that in this case, particles of the medium oscillate along the direction of propagation of the wave.
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Longitudinal waves further consist of two regions:
It is a region where particles are closer to each other. To examplify, when a vibrating object moves forward, it pushes and compresses the air in front of it, forming a region of high pressure called compression (C).
Compressions are the regions of high pressure and density where the particles are crowded and are represented by the upper portion of the curve called crest.
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It is a region where particle are far from each other.
When the vibrating object moves backward, it forms a region of low pressure called rarefaction (R). A vibrating object producing a series of compressions (C) and rarefaction (R). In these waves the particles move back and forth parallel to the direction of propagation of the disturbance. Such waves are called longitudinal waves.
Rarefactions are the regions of low pressure and density where the particles are spread out and are represented by the lower portion of the curve called trough.
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It is the wave in which particles vibrate up and down. We can also say that it is the wave in which the particles of the medium oscillate in a direction perpendicular to the direction of propagation of the wave. Light waves are transversal waves. They do not require medium for their propagation. Light can travel through vaccum. Light is an electromagnetic and transverse wave.
The upward movement is known as crest or an elevation is called as crest. Alternatively, a downward movement in wave is called as trough or a depression in wave is said to be trough.
Sound is a kind of longitudinal wave and mechanical wave.
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Oscillation
It is defined as the complete movement of particle about its mean position.
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Amplitude is the maximum displacement of a particle from its mean position in either direction. It is a measure of the intensity of the sound. Amplitude determines the loudness of sound. Loudness is proportional to the square of the amplitude.
We can also say that the amplitude of a sound wave is the height of the crest or tough. It is represented by the letter ‘A’.
The SI unit is the same as that of density or pressure.
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It is defined as the number of sound waves produced per second. Frequency is expressed in hertz. Its symbol is Hz. Frequency determines the pitch of the sound. Higher the frequency, higher the pitch or it is said that the sound is shrilling. Lower the frequency, lower the shrill or pitch.
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It is the time taken to complete one oscillation. Its unit is second.
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It is the distance between two consecutive crests or troughs. Its unit is centimetre (cm). It is denoted by lamda.
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It is the speed with which sound travels. Its unit is m/sec. It depends upon the nature of the medium through which it passes.
Conditions necessary to produce sound:
A vibrating body
Sound is produced only when the source of sound vibrates (Oscillates) at its mean position in to and fro direction. Sometimes, vibrations can be seen and sometimes they are invisible.
Examples of a vibrating body
To show that sound needs a medium to travel, let us take an example-
There are different characteristics of sound which are as follows:
Loudness
It is a characteristic of sound, the degree of sensation of sound. It depends upon amplitude. Amplitude determines the loudness of sound. Loudness is proportional to the square of the amplitude of the vibrating body which produces sound. More is the amplitude, louder is the sound. It is measured in unit ‘decibel’.
The more the amplitude of vibration, louder the sound. The lesser the amplitude of vibration, the sound gets feeble.
Pitch
It is the property that determines the shrillness (sharpness) or depth of sound. It depends upon the frequency. More the frequency, more the voice is shrilled. Less the frequency, deep is the sound. For example: a baby’s voice has more frequency and thereby, it is shriller than an adult’s voice.
Quality
It is the property by which one can distinguish between two different sounds of same pitch and loudness. Like in a dark room where there is no light and 100 people are present, we can easily recognize the sound of our friend among all the people due to the particular quality of its sound.
Intensity
It is defined as the number of sound waves passing 1 meter square area in one second. It depends upon the square of amplitude.
Sound can travel through all 3 states of matter that is solids, liquids and gases.
Speed of sound depends upon the density (density is defined as mass per unit volume) of medium (it increases with increase in density of mediums).
Speed of sound is maximum in solids and least in gases as particles are closer in solids. So, transfer of sound energy occurs faster. Speed of sound in air is 340 metre per second.
Question: Why speed of sound is more on a humid day as compared to a dry day?
Answer: The speed of sound is more on a humid day as compared to a dry day because on a humid day, particles are comparatively closer, so, the transfer of energy occurs faster.
Question: Let us compare speed of sound and speed of light in air?
Answer: Light travels faster than sound. The speed of light is 3 x 10⁸ m/sec.
Speed of sound is 340 m/sec.
Question: On a stormy day, why lightning is seen first and thunder is heard later?
Answer: On a stormy day, we usually see the lightning first and thunder is heard later because in air, the speed of light is faster as compared to the speed of sound.
Question: “Sound travels through a medium, it cannot travel in a vacuum”. Justify the statement.
Answer: Sound propagates by causing the particles in the medium to vibrate. A vibrating source transfers the vibrations to the surrounding medium, which can be solid, liquid or gaseous. The resulting vibrations create varying pressure regions by compressing and decompressing the particles in the medium, and through this process of compression and decompression, sound waves propagate through the medium. During the propagation of sound, the particles of the medium receive these vibrations and transfer them to the surrounding particles, allowing sound to travel.
In a vacuum, there are no particles that can transfer and carry vibrations, so sound cannot travel.
Question: A pendulum oscillates 50 times in 6 seconds. Find its time period and frequency?
A pendulum makes 27 oscillations in 3 seconds. what is the frequency of the pendulum?
Answer: As we know that the number of oscillations made by a pendulum in 1 second is called its frequency.
Now,
In 6 seconds, pendulum makes – 50 oscillations
In 1 second, pendulum makes – 50/6 = 25/3 oscillations
Thus, the frequency of this pendulum is 25/3
Time period T = 1/F = 3/25
Question: How is a sound produced?
Answer: Sound is produced when something vibrates. The vibrating body causes the medium (water, air, etc.) around it to vibrate. Vibrations in the air are called travelling longitudinal waves, which we can hear. Sound waves consist of areas of high and low pressure called compressions and rarefactions, respectively.
Question: What are the three characteristics of sound?
Answer: Following are some of the characteristics of sound:
Sound produced by man is through a voice box called larynx, it consists of two flap – like ligaments called vocal cords with a narrow slit spacing them.
When we speak, air is forced out (exhaled). This exhaled air forces the vocal chords to vibrate due to which sound is produced.
Length of vocal cord in males is 20mm and females is 15mm. That is the reason that females and males have different sounds.
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Sound produced by animals
Animals produce a wide range of sounds. Like sparrow, lion, elephant all of them have different sounds. Many animals use lungs to blow air that make their vocal cords vibrate and sound is produced.
Lower organisms use their wings to produce buzzing sound.
Fish use airbladder to produce sound.
Let us conduct an activity to show that sound can travel through liquids
Question: Why astronomers fail to hear the sound of each other on the surface of the moon?
Answer: Astronomers fail to hear the sound of each other while they are on the surface of the moon because there is no air present on the moon and we know that sound cannot travel through vacuum. It needs medium to travel.
Question: How whales and dolphins are able to communicate?
Answer: Whales and dolphins are able to communicate in liquids as sound can travel through liquids.
Question: State the functions of the human ear.
Answer: While hearing is the obvious function of the ear, it is the primary organ for our sense of balance as well. Physically, the ear consists of three parts: the outer ear, the middle ear, and the inner ear. The outer and middle ears mostly collect and transmit sound.
Question: Why are the voices of men, women and children different?
Answer: The vocal cords in men are about 20 mm long. In women, these are about 15 mm shorter. Children have very short vocal cords. This is the reason why the voices of men, women and children are different.
Question: Who has a high pitched sound, a man or a woman? What is the reason for the difference between the two?
Answer: One of the first and most obvious biological differences between men and women’s speech is the pitch. This is partly because men are normally larger than women (sexual dimorphism), which means their vocal folds are larger. Thus, men have lower pitched voice.
Question: Is there any animal that can hear the sounds of frequencies higher than 20,000Hz? For what purpose is it used?
Answer: A bat can hear the sounds of frequencies higher than 20,000Hz. Bats produce ultrasonic sound during screaming. These ultrasonic waves help the bat to locate its way and its prey.
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Audible sound: It is the range of the frequency of sound that can be heard by humans. The human ear can detect sounds in the range of 20Hz-20KHz.
Inaudible sound: It is the range of the frequency of sound that cannot be heard by humans. The range below 20Hz and greater than 20KHz cannot be heard by the human ear.
Question: What is Galton whistle?
Answer: These are the special whistles that produce sound of frequency higher than 20,000 Hertz. Dogs have the ability to hear it but humans cannot. Ultrasound scans produce sounds in this range. It is used for investigation purpose.
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Human earIt is an organ that helps in hearing.
It is divided into 3 parts:
Outer part: It consist of ear pinna, auditory canal, and ear drum. The outer ear is the most external portion of the ear called the pinna or auricle, the ear canal.
Middle part: The middle ear is an air-filled cavity. It includes three interlocked bones: the malleus (or hammer), incus (or anvil), and stapes (or stirrup).
Inner part: This contains the sensory organs for balance and motion, with the help of semicircular canals. This also contains cochlea, the sensory organ for hearing. Inner ear consists of eustachian tube.
A coiled organ filled with fluid (Cochlea). The sound produced by a vibrating body travels in the form of vibrations. It is collected by ear pinna and it directs into ear canal where it travels and hits the ear drum.
On receiving vibrations, the ear drum starts vibrating. Once the ear drum vibrates, it passes vibrations to 3 interlocked bones of middle ear which further increases the amplitude of vibrations.
Inner ear vibrations are received by a coiled organ. On receiving the vibrations its fluid also starts vibrating and sensory cells present in them transform the vibrations into electrical signals which are carried by auditory nerve to brain for interpretation and understanding.
Time to check your knowledge by answering the following questions-
Fill in the blanks with suitable words –
Answers:
Mark the following statements as true or false-
There are two types of sound which are as follows:
Music
It is the sound that gives pleasant effect or soothing effect. It is used for relaxation and for enjoyment.
Musical Instrument: An instrument that produce a musical sound. Like guitar, drums, flute.
A musical instrument uses the following things to produce a pleasant effect:
There are 3 main types of instruments:
Stringed instruments
These are the instruments which use strings to produce a pleasant sound. These can be plucked, bowed or struck. The instruments with plucked strings have varied lengths. The examples of these include sitar, violin, guitar, etc. These all instruments are bowed to produce sound of different pitches.
Here are few other examples:
Violin: Violin produces the sound of highest pitch.
Double bass: Double bass produces the sound of lowest pitch.
Wind instruments
These instruments are mainly made of brass and wood but sometimes they can be made of silver. They all differ due to the varied length of air column. For example: Flute, Shehnai, Been.
Percussion Instruments
These instruments struck themselves as they include vibrating membranes as used in drums of various kinds. For example: Tabla and dholak.
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Noise
It is defined as the sound that is unpleasant and has an irritating effect. For example: Sound of a horn, sound near the site of construction work, sound of an aeroplane, etc.
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It is defined as the contamination of air with unpleasant and irritable effect. Loud and excessive sound is unbearable for our ears and is called noise. The unwanted and excessive sound in our environment creates noise pollution. Sounds of crackers, factories, vehicles, desert coolers, air conditioners, aeroplane, transistors or television with high volume, loudspeakers, etc. create sound pollution.
Major causes of noise pollution are as follows:
Harms of Noise pollution are as follows:
Measures to limit Noise pollution are as follows:
Multiple choice questions
1. Eardrum is a part of which of the following-
a. Sound producing organ
b. Skeletal system
c. Hearing organ
d. Reproductive organ
2. What is the hearing range of human ear?
a. 20 Hz to 20,000 Hz
b. Less than20 Hz
c. More than 20,000 Hz
d. 20 Hz to 25,000 Hz
3. What is the other name of voice box ?
a. Stomach
b. Heart
c. Larynx
d. Mouth
4. Large amplitude of sound vibrations will produce:
a. Loud sound
b. Meak sound
c. Slow sound
d. Shreak
5. The pitch of sound depends on which of the following?
a. frequency
b. amplitude
c. both of these
d. none of these
6. Sound is kind of
a. Work
b. Energy
c. Force
d. None
7. To and fro motion of an object is called as?
a. Waves
b. Amplitude
c. Vibration
d. All of the above
8. Voice box or larynx of human process
a. Sound
b. Wind
c. Loudness
d. None
9. In which of the following sound propagates maximum ?
a. Gas
b. Liquid
c. Solid
d. All
10. Noise pollution is harmful for
a. Human
b. Cat
c. Bird
d. All
ANSWERS
Question: Explain harmful effects of noise pollution?
Answer: The harmful effects of noise pollution are as follows:
Question: What is the difference between noise and music? Can music become noise sometimes?
Answer: The sound which is unpleasant for our ears is called noise while music is the sound which is pleasant for our ears. Music becomes noise sometimes, when it crosses the bearable range of sound for our ears.
Question: Lightning and thunder take place in the sky at the same time and at the same distance from us. Lightning is seen earlier and thunder is heard later. Can you explain why?
Answer: The speed of light is more than that of sound. Due to more speed the light reaches us before the sound does. So Lightning is seen earlier and thunder is heard later.
Question: List sources of noise pollution in your surroundings.
Answer: There are different sources of noise pollution which includes honking of horns, loudspeakers, loud sound of machines in factories, loud sound of TV, radio, domestic etc.
Question: Why should we never put a sharp, pointed or hard, thing into our ear?
Answer: We should not put anything like pin, pencil or pen etc. inside our ears because they can hurt the eardrum and can make a person completely deaf.
Question: Why are we not able to hear the screams of a bat?
Answer: Bat produces ultrasonic sound during screaming and that is the frequency of a sound is more than 20,000Hz, it cannot be heard by us. The sound of these frequencies is inaudible. So we are not able to hear the screams of a bat.
Question: Write the full form of dB ?
Answer: Decibel is the full form of dB.
Question: What is the name the substance which vibrates in a flute to produce sound?
Answer: Flute uses air as the primary vibrating medium for the production of sound.
Multiple Choice Questions
1. The speed of sound in solids is the speed of sound in liquids.
(a) same as
(b) less than
(c) greater than
(d) sometimes greater and sometimes smaller
Answer: (c) greater than
2. Bats detect the obstacles in their path by receiving the reflected?
(a) infrasonic waves
(b) electromagnetic waves
(c) radio waves
(d) ultrasonic waves
Answer: (d) ultrasonic waves
3. Which of the following is not a characteristic of a musical sound?
(a) Pitch
(b) Quality
(c) Wavelength
(d) Loudness
Answer: (a) Pitch
4. Sound waves do not travel through which of the following?
(a) solids
(b) gases
(c) liquids
(d) vacuum
Answer: (d) vacuum
5. Which one of the following can hear an infrasonic sound?
(a) Dog
(b) Rhinoceros
(c) Bat
(d) Humans
Answer: (c) Bat
6. What kind of musical instrument is a sitar?
(a) String
(b) Membrane
(c) Wind
(d) None
Answer: (a) String
7. In a ‘dholak’ sound is produced due to?
(a) stretched membrane
(b) air column
(c) stretched strings
(d) the vibration of air
Answer: (a) stretched membrane
8. The pitch of sound depends on the which of the following of the vibrating body?
(a) amplitude
(b) frequency
(c) noise
(d) the medium of propagation
Answer: (b) frequency
9. The human ear can hear sounds having frequency range:
(a) between 200-20,000Hz
(b) between 200-2000Hz
(c) between 20-2000Hz
(d) between 20-20,000Hz
Answer: (d) between 20-20,000Hz
10. What is the SI unit of time period is
(a) second
(b) minute
(c) hour
(d) nanosecond
Answer: (a) second
11. Vibrations inside the ear are amplified by the three bones namely the in the middle ear.
(a) Hammer, anvil and stirrup
(b) Hammer, cochlea and stirrup
(c) Hammer, anvil and pinna
(d) Auditory bone, anvil and stirrup
Answer: (a) Hammer, anvil and stirrup
12.What is the SI unit of pitch ?
(a) Hertz
(c) metre
(c) m/sec
(d) second
Answer: (a) Hertz
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