Class-8, Science, Chapter - 13 Sound Notes, Ncert Solutions & Frequently Asked Questions
Science
Class - 8, Chapter - 13
Sound
Notes, NCERT Solutions & Frequently Asked Questions
-- Notes --
What is sound?
A sound is a form of energy produced when air molecules vibrate in a particular pattern called waves. Hence, the sound is a wave.
Vibration can be described as a back and forth motion of an object.
Sound cannot be produced without any vibration.
Figure 1 Production of Sound through Vibrations
Figure 2 Different instruments produce different sounds
How do humans produce sound?
Human beings have a voice box or larynx which is present in their throat on the upper side of the windpipe.
The larynx has two vocal cords which have a narrow slit between them so that air can pass through it.
As the lungs throw the air out of the windpipe, it passes through the slit and hence allows the production of sound as the vocal cords start vibrating.
The vocal cord muscles also play a role in the production of sound.
Their thickness and tightness describe the quality or type of voice a person has.
The vocal cords in males are of length 20 mm and females have 15mm long vocal cords. Children have very short length vocal cords. Hence, the voices, their quality and their type are always different in women, men and children.
Figure 3 Larynx or Voice box
Propagation of sound through a medium
Sound always requires a medium to travel from the source of its production to the receiver end. Different mediums of propagation of sound are air, liquid and solids.
Sound cannot travel through a vacuum as it lacks any medium of transmission.
you can not talk on moon because there is no atmosphere.
Figure 4 Sound Waves
How sound travels in a medium?
Sound needs some vibration of molecules to travel.
Solids, liquids and gases all have molecules present in them which allow the propagation of sound.
Solids have tightly packed particles in them and hence they allow fast propagation of sound through them as the vibrations can be carried easily from one particle to another.
Liquids have slightly loosely packed particles and hence it takes a little time for sound to travel in water or through a liquid.
Gases have completely loosely packed particles and hence sound takes the most time in travelling through the air.
| Sound through water travels 4 times faster than through the air. Sound through solids travels 13 times faster than through the air. |
How do we hear?
Sounds are produced as waves in the air or any other medium.
As these sound waves travel to our ears they convert them into electrical signals or messages that our brain can understand.
There are three major parts of the human ear:
Figure 5 The Human Ear
The outer ear (Pinna): It catches the sound waves and forwards them to middle ear.
The middle ear: It converts the sound waves into vibrations that then travel to the inner ear. It can do this with the help of the eardrum.
The eardrum is a thin rubber-like sheet present in the Middle ear. As the sound waves reach the eardrum, it vibrates and these vibrations propagate to the inner ear.
The inner ear (cochlea): It receives the vibrations sent by the eardrum. It contains a liquid substance and vibrations that enter the inner ear moves through this liquid. There are tiny hairs present inside the inner ear that turn these vibrations into signals for the brain and pass them to the brain through the hearing nerve. As the brain receives the signal it interprets the sound. his whole process is so quick that we cannot notice it.
Frequency, Time Period and Amplitude of a Sound
Figure 6 Displacement of Particles by Production of Sound and Representation of A Sound Wave
| Oscillatory motion | When an object travels in a ‘to and fro’ motion, that is, when an object vibrates it is said to have an oscillatory motion. |
| Oscillation | One oscillation is said to be the movement between the two endpoints or extreme points of the motion of the object. |
| Frequency | The number of oscillations an object takes per second is called its frequency. The SI unit of frequency is Hertz (Hz). 1 Hz = 1 oscillation per second 20 Hz = 20 oscillations per second |
| Time Period | The time taken for one complete oscillation of a sound wave is called the time period of the sound wave. |
| Amplitude | The amplitude of a sound wave is defined as the maximum displacement of the particles from their mean position due to the vibrations. |
The loudness of a sound
The loudness of a sound depends upon the amplitude of the sound.
The higher the amplitude, the higher is the displacement of the particles and the higher is the loudness of the sound.
SI unit for measuring the loudness of a sound is decibels (dB).
Pitch of the sound
Every person has a different sound quality.
The sensation of a frequency is known as the pitch of a sound. A high pitch sound corresponds to a high frequency sound wave and a low pitch sound corresponds to a low frequency sound wave.
The pitch of a sound depends upon the frequency of the sound wave.
The pitch would be higher if the frequency of the sound is high.
Figure 12 Loudness and Pitch of a Sound
How Loudness and Pitch are different from each other?
| Pitch | Loudness |
| It depends upon the frequency of the sound. | The loudness of a sound depends upon the amplitude of the sound wave. |
| Pitch of a sound is how our ears react to the frequency of a sound. | The loudness of sound also depends upon the energy of the sound wave. |
| Pitch of the sound helps in recognizing whether the sound is shriller or flatter. | The loudness of a sound helps in recognizing whether a sound is loud or feeble. |
Audible and Ianudible sounds:-
Sounds that can be heard by human ears are called audible sounds. The range of audible sounds is from 20 Hz to 20 KHz.
Sounds that cannot be heard by human ears are called inaudible sound. Any sounds having a frequency less than 20 Hz or greater than 20 KHz are categorized as inaudible sounds.
The inaudible sounds can be divided into two categories as infrasound and ultrasound.
| Infrasound | Ultrasound |
| The sounds having a frequency range below 20 hertz are called infrasound. | The sound waves having a frequency range above 20 kilohertz are called ultrasounds. |
| Infrasound cannot be heard by humans. | Having too high frequency, they cannot be heard by human beings as well. |
| Animals like giraffe, rhinoceros and whales use infrasound to communicate. | Bats, dogs and cats use ultrasounds to communicate. |
| Infrasound waves are used to treat myopia in kids | Ultrasounds can be used for various purposes such as identifying abnormalities in the human body or calculating distances under the water through SONAR. |
Figure 16 Infrasound and Ultrasound
Noise and Music
Noise can be regarded as an unpleasant sound. Noise consists of sound waves of varied frequencies which have no particular periodic pattern of repetition. Noise is regarded as a mixture of sound waves with irregular frequencies.
Music is a pleasant sound that has a clear pitch. Musical sound can be created by arranging and combining different sounds in a particular order. The frequencies of a musical sound are harmonious in nature.
Figure 17 Noise And Music
Noise pollution
Noise pollution is the presence of undesirable and unpleasant sounds in the earth's environment.
Human beings can bear sounds ranging up to 85 decibels only. Above that, any noise can damage our hearing power.
Generally, any sound that has a frequency of more than 30 dB is considered noise.
Causes of noise pollution
Transport noise: The sound of the traffic on roads, railways and aircraft leads to noise pollution.
Industrial noise: Industries, factories and other commercial businesses cause high-intensity sounds that pollute the environment.
Neighbour-hood noise: The noise from the radios, televisions, air conditioners, coolers, kitchen applications and other electrical appliances used in houses lead to noise.
Construction noise: Construction of houses, industries and various architectures also lead to sound pollution.
Political activities: Noise pollution is also created due to the rallies and demonstrations conducted in cities and rural areas.
Bursting of crackers and fireworks: People burst crackers that lead to sound pollution in the neighbourhoods.
Natural sounds: The environment of the earth also sometimes leads to unpleasant sounds due to lightning, thunderstorms, earthquakes, volcanic eruptions, sounds of various animals etc.
Effects of Noise Pollution
Excessive noise in the surroundings can lead to serious health problems such as hypertension, lack of sleep or insomnia, anxiety, lack of memory, stress, irritation and even nervous breakdown.
It can lead to temporary or permanent hearing loss in human beings as well as animals.
Excessive noise leads to increased blood pressure.
It increases cholesterol levels in the blood and therefore increases the chances of cardiovascular diseases in a person.
If sound intensity is more than 180 dB it can lead to the death of a person.
Noise pollution also leads to a decrease in the production of digestive juices by our body.
It can affect animals as well and can lead to their death and loss of habitats.
Prevention of Noise Pollution
Factories and other industries should be set up at a distance from residential areas.
Silencing devices should be incorporated into heavy vehicles such as aircraft, industrial equipment, machinery and other home appliances.
We should always play television, radio and other music systems at a low sound so that it does not harm the neighbourhood.
Use of horns should be minimized especially near public places such as hospitals, religious places and schools.
Sound proofing systems should be installed in industries, party halls and other buildings that produce a high amount of unwanted sounds.
Trees should be planted in huge numbers as they can absorb unwanted noise from the environment.
--NCERT Solutions --
Choose the correct answer.
Question 1.- Sound can travel through
a) gases only b) solids only
c) liquids only d) solids, liquids, and gases
Answer:- d) solids, liquids, and gases.
Question 2.- Voice of which of the following is likely to have a minimum frequency?
a) Baby girl b) Baby boy
c) A man d) A woman
Answer:- c) A man
Question 3.- In the following statements, tick ‘T’ against those which are true and ‘F’ against those which are false.
a) Sound cannot travel in a vacuum. (T/F)
b) The number of oscillations per second of a vibrating object is called its time period. (T/F)
c) If the amplitude of vibration is large, the sound is feeble. (T/F)
d) For human ears, the audible range is 20 Hz to 20,000 Hz. (T/F)
e) The lower the frequency of vibration, the higher is the pitch. (T/F)
f) Unwanted or unpleasant sound is termed as music. (T/F)
g) Noise pollution may cause partial hearing impairment. (T/F)
Answer:-
a) True
b) False
c) False
d) True
e) False
f) False
g) True
Question 4.- Fill in the blanks with suitable words.
a) Time taken by an object to complete one oscillation is called _______
b) Loudness is determined by the ________ of vibration.
c) The unit of frequency is ________
d) Unwanted sound is called _______
e) The shrillness of a sound is determined by the ______ of vibration.
Answer:-
a) Time period
b) Amplitude
c) Hertz (Hz)
d) Noise
e) Frequency
Question 5.- A pendulum oscillates 40 times in 4 seconds. Find its time period and frequency.
Answer:- Time period is defined as the time required to complete one oscillation.
Frequency is defined as the number of oscillations per unit time.
Here, Time,t = 4s
Number of oscillations,n = 40
Frequency,f = n/t
f = 40/4
f = 10 Hz
Time period is the inverse of frequency of an object.
Therefore,
T = 1/f
T = 1/10
T = 0.1s
Question 6.- The sound from a mosquito is produced when it vibrates its wings at an average rate of 500 vibrations per second. What is the time period of the vibration?
Answer:-
Time period is defined as the time taken to complete one oscillation. It is also the inverse of frequency.
Time period = 1
Oscillation frequency = 500 Hz
Time period = 1/500 = 0.002 s
Question 7.- Identify the part which vibrates to produce sound in the following instruments.
a) Dholak
b) Sitar
c) Flute
Answer:-
a) Stretched membrane
b) String of sitar
c) Air column
Question 8.- What is the difference between noise and music? Can music become noise sometimes?
Answer:- The type of sound which are unpleasant to listen is known as noise whereas music is a pleasant sound, which produces a sensation.
Yes, music can become noise when it’s too loud.
Question 9.- List the sources of noise pollution in your surroundings.
Answer:- Following are the major sources of noise pollution:
• Sound of vehicles
• Sound of kitchen appliances
• Sound of bursting crackers
• Sound of loudspeakers, TV, transistors
Question 10.- Explain in what way noise pollution is harmful to humans.
Answer:- Noise pollution causes:
a) Lack of sleep
b) Anxiety
c) Hypertension
and these are harmful to health.
Question 11.- Your parents are going to buy a house. They have been offered one on the roadside and another three lanes away from the roadside. Which house would you suggest your parents should buy? Explain your answer.
Answer:- I would suggest my parents buy a house three lanes away from the roadside because house on the roadside would be much noisy in both days and night due to running vehicles. Whereas, a house three lanes away would be comparatively quieter as the intensity of noise decreases with the distance between the source and the listener.
Question 12.- Sketch larynx and explain its function in your own words.
Answer:- Larynx is also known as voice box. It is at the upper end of the windpipe. Two vocal cords are stretched across the voice box or larynx in such a way that it leaves a narrow slit between them for passage of air (Fig. 13.12). When lung force air through the slit, the vocal cords vibrate, producing sound. Muscles attached to the vocal cords can make the cords tight or loose.
When the vocal cords are tight and thin, the type or quality of voice is different from that when they are loose and thick.
Question 13.- 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 the speed of sound. Thus, due to more speed of light it reaches us before sound. So, lightning is seen earlier and thunder is heard later
-- Frequently Asked Questions --
Question 1.- What are ‘Transverse waves’?
Answer:- Transverse wave is where the motion in all points on a wave oscillate along paths at right angles to the direction of the wave’s advance.
Question 2.- What is the definition of ‘Amplitude’?
Answer:- The definition of amplitude refers to the length and width of waves, such as sound waves, as they move or vibrate.
Question 3.- What are some ways to control ‘Noise pollution’?
Answer:-
1. Turn off unnecessary appliances
2. Use earplugs
3. Follow limits of noise level
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