Sound for class 8 CBSE

Here we come with Sound for class 8 CBSE. In previous post we learn Force, Pressure and Friction. Now we learn Sound for class 8 CBSE. For that we have to clear some questions regarding Sound.

What is sound?

As per your syllabus of Sound for class 8 CBSE, In simple word we can say that sound is a form of energy which produces the sensation of hearing in our ears.

How sound is produced?

Sound for class 8 CBSE

Vibrating Bodies Produce Sound. Cut a rubber band. Hold one end of it in your mouth and the other end in your hand and stretch it. Now, pull it with the other hand and release it. What happens to the rubber band? You will notice that it moves to and fro or vibrates and it also produces sound.

How sounds produced by musical instruments?

There are basically three families of musical instruments:


Stringed instruments have taut strings mounted over specially designed wooden frames, which are partially hollow from within. When the strings are plucked or struck or played with a bow they vibrate to produce a musical sound of some particular frequency. The pitch of the sound of a musical instrument can be changed by altering its length.


Wind instruments make use of vibrating air columns. In these instruments, the air is blown in, either directly or through the reeds. Flute, Shahnai, Bag pipes, Bugles, etc., are some of the examples of wind instruments.


All percussion instruments have a taut skin over a hollow metal or wooden frame. When the skin is struck, it produces a musical sound. Dholak, tabla, mridangam and drums, etc., are some of the examples of percussion instruments. There are some other musical instruments which are exclusively used in our country, not belong to any of the above three kinds of musical instruments. They can be placed in a fourth class of musical instruments.


These instruments are simply beaten or struck in a rhythm.

Manjira (cymbals), the ghatam (mud pots), jal tarang, etc., are some of the Ghana Vadya. These instruments are commonly used in all parts of our country.

Characteristic of Vibrations?

In sound producing objects, the vibrations take place very fast…so fast, in fact, that they cannot be seen very clearly with the unaided human eye (slow-motion video is one practical solution that is routinely used in scientific and industrial applications). We can produce slower vibrations, which are also called oscillations, by an arrangement called a simple pendulum. It consists of a ball or any weight, such as a stone, wooden top, steel bolt, etc. which is here called a bob, and which hangs down vertically.

Sound for class 8 CBSE

Figures show a oscillating simple pendulum and a vibrating string depicting the mean position and amplitude. by means of a thread. It hangs vertically because of the force of gravity acting upon it. When the bob is given a small push, it performs a to-and-fro motion for some time and ultimately stops. The position where the bob stops is called the mean position (A) or the equilibrium position.


The maximum displacement of the bob from the mean position during oscillation is called amplitude of the oscillation. In the case of a simple pendulum, as show in the figure, the amplitude is AB (or AC).


The number of oscillations produced by the vibrating body in one second is called its frequency. It is denoted by the letter ‘f’. The unit of frequency in SI system is Hertz (Hz or s–1). When a vibrating body produces 10 vibrations (oscillations) in one second, its frequency is said to be 10 hertz. If the frequency of a tuning fork is written as 200 Hz, it means that it produces 200 vibrations in one second.


When a bob moves from one position and returns to the same position, such that it repeats the motion, it is said to complete on oscillation. The time taken by a vibrating body to complete one oscillation is the time period.

Different types of waves?

On the basis of the need of medium,and the syllabus of Sound for class 8 CBSE, waves are of two types :

 (a) Mechanical wave

 (b) Electromagnetic wave

 (a) Mechanical Wave : A mechanical wave is a periodic disturbance which requires a material medium for its propagation. The properties of these waves depend upon the medium so they are known as elastic waves, such as sound waves, water waves, waves in stretched string. On the basis of motion of particles the mechanical waves are classified into two parts : (i) transverse wave (ii) Longitudinal wave

 (i) Transverse wave : A wave motion in which the particles of the medium oscillate about their mean positions at right angles to the direction of propagation of the wave is called transverse wave. These waves can propagate through solids and liquids but not through gases because gases do not possess elastic properties. E.g. – Vibration in strings, ripple on water surface etc. These waves travel in the form of crests and troughs.

(ii) Longitudinal wave : A wave motion in which the particles of the medium oscillate about their mean position is the direction of propagation of wave, is called longitudinal wave. These waves can propagate through solids, liquids and gases.

E.g. – Wave produced by compressing a spring, sound waves etc. They travel in the form of compression and rarefaction.

How sound propagate?

Unlike light, sound can travel only through a material medium. Hence a material medium is necessary for the propagation of sound. Let us do some activities to prove that sound can travel through solid, liquids and gases.

Sound Travels Through SolidsSound for class 8 CBSE

Take a wooden stick and press your ear at one end of it. Ask a friend to gently knock at the other end (fig). You will be able to hear the sound very clearly. This shows that sound can travel through solids.

Sound Travels Through Liquids

Place a squeaking toy in a polythene bag and hold it in a bucket of water (Fig). Can you hear its squeak, when you squeeze it? Place your ear against the side of the bucket and squeeze the toy again. Do you hear its squeak? In which case did you hear the sound better? This activity shows that sound can travel through liquids as well.

Sound Travels Through Gases

Fill a balloon with air and press it to your ear scratch the other end of it with your fingers (Fig). You will note that the sound reaches your ear. This activity shows that sound can also travel through gases.

What are the characteristics of a sound?

a) Loudness : When the string is plucked softly, it produces a feeble sound and when it is plucked hard, it produces a louder sound. In other words, in the first case vibrations with small amplitudes are produced and in the second case vibrations with large amplitudes are produced. This means that the loudness of a sound depends on the amplitude of vibration. It measured in decibels (dB).

b) Pitch : The shrillness or flatness of a sound is known as pitch. We can distinguish between a man’s voice and woman’s voice of the same loudness without even seeing whether the speaker is a man or a woman. A man’s voice is flat and has a low pitch, Whereas a woman’s voice shrill and has a high pitch. The pitch of a sound depends on the frequency of vibration.

 The higher the frequency of a sound, the higher will be its pitch. In other words, high pitched sounds (such as the shrill whistling of a kettle or a jet engine) can created by high frequencies. This is obviously why we whistle for our dog; the high pitched sound attracts him. You can get a special ‘dog whistle’ that emits a very high pitched sound at a frequency much beyond 20,000 Hz; humans cannot hear it, but your dog will come bounding up to you when he hears the sound (inaudible to you).

c) Quality or Timber : The characteristic of sound which enables us to distinguish between two sounds of the same pitch and loudness, produced by two different sources is called its quality or timber. For example, we can recognize a person by hearing his voice, we can also distinguish the sound of a guitar from that of a sitar or harmonium.

How we hear sound through our ears?

Human Ear :

We have learnt that vibrating objects produce sound which may carried in all directions in a medium. How do we hear sound? Our ears help us to hear sound. Human ear has three important parts. Only one of its parts can be seen and felt by you, which is the outer ear. The rest of the ear remains deep inside the skull.

(i) Outer Ear : The outer ear consists of the pinna and the eartube. The shape of the outer part of the ear is like a funnel. When sound enters the ear, it travels down a canal at the end of which a thin membrane is stretched tightly. The eardrum, which performs a very important function actually a tightly stretched membrane.

(ii) Middle Ear : It has three very tiny interlocked bones. The innermost bone mostly joined to the inner ear.

(iii) Inner Ear :It has a coiled organ of hearing semicircular canals and the auditory nerve. A vibrating body causes air molecules to vibrate. These vibrations reach out ear and collected by the pinna and then funneled into the ear tube. These vibrations strike the eardrum that start vibrating with the same frequency. It then forces the interlocked bones to vibrate. The hearing organ passes the vibrations to the auditory nerve, which takes the signal to the brain. This enables us to hear the sound.

How sound produced by humans?

As per your syllabus Sound for class 8 CBSE, in a human being, sound is produced by a voice box (larynx). Two vocal cords are stretched across the voice box such that a passage for air remain between the cords. When we speak lungs force air through the passage due to which the cords start vibrating and produce sound. The muscles connected to vocal cords controlled the tightness of the cords .

Sound for class 8 CBSE

What is audible and inaudible sound?

It has been proved that the human ear is not sensitive to vibrations of all frequencies. In other words, we can only hear sounds that fall within a certain range of frequencies; any sound outside that frequency range is inaudible to our ears. We can hear sounds within frequencies ranging from 20 Hz to 20,000 Hz. This frequency range generally called Audible frequency range of sound. Such vibrations named sonic vibrations or sound vibrations.

Sound frequencies less than 20 Hz are Infrasonic sound and sound frequencies more than 20,000 Hz (20 k Hz) are Ultrasonic sound. Human beings cannot perceive either infrasonic or ultrasonic sounds. Hence, these sounds called ‘Inaudible’ sounds, i.e., sounds we cannot hear. Dogs, on the other hand, have much ‘sharper’ hearing, since they can hear sounds within the frequency range of 50 Hz to 45000 Hz ! That is why your dog starts braking a welcome long before you have reached your front door; he has heard your footfalls or recognized the distinctive sound of your car as it enters the gate. Almost all animals have a wide range of hearing. Bats can detect frequencies as high as 100,000 Hz.

What are the uses of ultrasonic vibrations?

  1. Dogs can hear up-to the frequency of 40,000 Hz as compared to humans who can hear upto 20,000 Hz. Dogs got specially training to respond to a whistle which produces ultrasonic vibrations. And it commonly called Galton whistle. The sound of this whistle cannot heard by humans.
  2. Bats produce vibrations in the frequency range of 50 Hz to 80,000 Hz. Bats have very weak eyes. When the bats fly at night, they produce ultrasonic vibrations which cannot be heard by the humans or the insects. When these vibrations strike the insects, they reflected back after reflection. On receiving the reflected sound, the bat can locate the insect and eat it.
  1. Ultrasonic vibrations can used for homogenizing milk, i.e., the milk is agitated with ultrasonic vibrators. These vibrations break down the larger particles of the fat present in milk to smaller particles.
  2. Now a days Ultrasonic vibrations can used in dish washing machines. In such machines,ultrasonic vibrators create vibration in water and detergent. The vibrating detergent particles rub against the dirty utensils and thus clean them.
  3. Ultrasonic vibrations produce a sort of depression in rats and cockroaches. It can used to drive rats and cockroaches from godowns.
  4. Ultrasonic vibrations used for imaging internal organs of human body. In fact they even used to study the growth of fetus in mother’s womb.

What is noise and music?

Noise is a sound which does not have a pleasing sensation on the ears. Scientifically, a noise mainly produced by irregular vibrations. Conversely, a sound which has a pleasant sensation on the ears is called music. Scientifically, a regular vibrations produce musical sound. For example, the prong of a tuning fork or the string of a sitar vibrates at regular intervals and hence, they produce musical sound.

What is noise pollution?

As per the syllabus of Sound for class 8 CBSE, the disturbance produced in the environment by undesirable, loud and harsh sound from various sources is a cause of noise pollution.

Noise pollution is a recent phenomenon of twentieth century. Increasing dependence of the man on various kinds of machines at home, or work place or factories, etc., has contributed a lot to the noise pollution. We can determine the noise pollution by following factors :

 (i) Loudness of the sound

(ii) Duration of noise.

What are the effects of noise pollution?

  1. Noise in the surroundings interferes with conversation with another person.
  2. A long exposure, to noise pollution may result in the loss of hearing.
  3. Noise pollution reduces concentration and results in the loss of work efficiency.
  4. It causes anger, tension and interferes with the sleep pattern of individuals.
  5. Excessive noise produces headaches, irritability and nervous tension.
  6. It can cause loss of night vision as well as cause color blindness.

In the modem society we cannot eliminate noise, but can lower its level to bearable limits by taking following measures :

  1. The machines should designed in such a way that they produce minimum noise.
  2. All automobiles, electric generators, etc., should be provided with improved silencers.
  3. The heavy vehicles should restricted in residential areas.
  4. The use of loudspeakers for various social or religious functions should banned.
  5. The factories should relocated far away from the residential areas.
  6. At homes, T.V, radio, power music system, should be played at low volume.

Let see how you learn Sound for Class 8 CBSE ?

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Hi myself Subham Dutta, having 15+ years experience in filed of teaching. I love to teach and try to build foundation of students. Try to make them imagine what they learn.

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