CBSE Class 8 Physics – 13. Sound (Notes)

Sound:- The forms of energy that produce sensation of hearing are called sound.

 

A sound is a form of energy that is 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.

Depending upon the vibrations, a sound is produced. Sound cannot be produced without any vibration.

Sound helps us communicate with each other.

Figure 1 Production of Sound through Vibrations

Figure 2 Different instruments produce different sounds

Different types of sound that we hear –

1. Ticking of clock                          (e) Children playing in the garden
2. Humming of mosquitoes             (f) A music player sound
3. Barking of dogs                          (g) A horn of a car
4. Speaking of man                        

Note: – Sound is a kind of wave. SI unit is Decibel (dB)

Wave – The periodic disturbance in a medium that transfer energy from one particle to another is called wave.

Note: – A wave is the motion of a disturbance in which particles of medium do not move from one place to another but they transfer the pattern of disturbance from one                    place to another.

Types of waves: –(a) Mechanical Waves    (b) Electromagnetic Weves

1. Mechanical wave: –           The wave that need medium (air,water) to travel is called mechanical wave.
2. Electromagnetic wave: –   The wave that can travel through medium as well as through vacuum is called electromagnetic wave.

Types of waves (depending upon how the particle moves).

1. Longitudinal
2. Transverse

1. LONGITUDINAL: –   The wave that particles are vibrate (move) in the form of to and fro is called longitudinal wave.

Longitudinal wave consist of Compression and Rarefaction.

Compression – It is a region where particles are closer to each other.

Rarefaction – It is a region where particles are far from each other.

2. Transverse: – The wave that particles are vibrate (move) in the form of up and down is called transverse wave.

Transverse wave consist of Crest and Trough.

Crest – An elevation in wavetrough – A depression in wave

NOTE: – 1. Sound is a kind of longitudinal wave and mechanical wave (medium)                        2. Light is an electromagnetic(medium and vacuum) & transverse wave.

TERM RELATED TO WAVE

Oscillation: – The complete movement of particle (up & down or to and fro) about its mean position is called oscillation.

Amplitude: – The maximum displacement of particles from its mean position in either direction (cm)  is called amplitude.

Frequency: –No of sound produced per secondare called frequency.(Hertz)

Time period: – The time taken to complete one oscillation (cycle) is called time period.(Sec)

      Frequency, f = 1Time period  =  1T   or f =  1T

  Or Time period, T= 1frequency  = 1f     or  T= 1f

      Also, Frequency = No of cycle time taken

Q1. A mass hung from a spring vibrates 15 times in 12 s. Calculate the frequency and the period        of vibration.

  Sol:- Given, number of cycle= 15 cycle, Total time = 12 s, frequency =?

            Now,  Frequency = No of cycle time taken  =1512  cycles/second  =1.2 Hz

             Now, f =  1T   or T= 1f  =11.2  = o.83s

Q2. The frequency of wave is 5.0 × 102 Hz. Calculate the time period.

 Sol:- f = 5.0 × 102 Hz, T = ?

        Now, T= 1f   = 15.0 × 102  = 0.002 s.

Wavelength: – The distance between two consecutive crest or trough is called wavelength.(cm)

Velocity of wave: – The speed with which sound travel is called velocity of wave (m/sec).

CONDITION NECESSARY TO PRODUCE SOUND

1.  A vibrating body
2. Medium to travel
3. A receiver to listen

1. A vibrating body: -A sound is produced only when source of sound vibrates(oscillates) at its mean position in to and fro direction.

Sometimes vibrations can be seen and sometime they are invisible. Ex- We can see vibration in ringing bell but not in case of playing harmonium.

The tightly stretched rubber string when plucked in middle it shows a fast to and fro motion.

When tuning fork is hit on rubber pad, a faint sound is heard but no vibration are seen. But if you bring that tuning fork near a hanging bell then bell shows an oscillatory motion.

When the source of sound vibrates it collides with the particles present in the air.

These particles after collision further collide with other particles and this continues in form of chain carrying the energy until the energy of particles becomes too feeble to collide further and propagate the sound.

As we move away from sound source the sound becomes feeble.

Medium to travel

To show that sound need medium to travel.

The apparatus is set as shown in figure.

When current is switched on bell starts ringing and we can easily hear the sound of ringing bell. But as we start removing air by means of vacuum pump the sound of ringing bell goes on becoming fainter and fainter and finally when there is no air left inside the sound of ringing bell stops.

RECEIVER: –Human ears.

CHARACTERSTICS OF SOUND

1.  Loudness             (B) Pitch          (C) Quality                (d) Intensity                Loudness: –It is the characteristic of sound and it is the degree of sensation of sound. It          depends up on amplitude.

More is the amplitude more louder is the sound.

It is measured in unit decibel.

Pitch:-It is the property that determines the shrillness or deepness of sound. It depends upon        the frequency.

More is the frequency more is the voice shrilled.

Less is the frequency deep is the sound.

Quality: – The property by which one can distinguish between two different sound of same         pitch and loudness is called quality.

Intensity: – The number of sound waves passing 1 meter square area in one second. It         depends upon square of amplitude.

Sound can travel through all three states of matter that is solid liquid and gases.

Speed of sound depends upon the density 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 occur faster.

Note: –Speed of sound in air is 340 metre per second in air.

Why speed of sound is more on humid day as compared to dry day?

Ò It is because on humid day particles are comparatively closer so transfer of energy occur faster.

Q. Compare speed of sound and speed of light in air?

Ò Light travel faster than sound that is 3×10⁸m\sec.

    Speed of sound is 340m\sec.

Why lighting is seen first and thunder is heard later on a stormy day?

Ò It is because speed of light is faster in air as compared to sound.

Sound produced by man

Ò In man voice box called larynx is present, it consists of two flap like ligaments called vocal cords with a narrow slit spacing them.

When we speak air is forced out. This exhaled air forces the vocal chords to vibrate due to which sound is produced

Length of vocal cord in            (a) Males: – 20mm             (b) Females: – 15mm

Sound produced by animals

The animals produce wide range of sounds. Many animals use lungs to blow air that make their vocal cords vibrate and sound is produced.

Lower organism use their wings to produce buzzing sound. Ex – Bee

Fish use airbladder to produce sound. Ex – Fish

Let us conduct an activity to show that sound can travel through liquids.

Ò Take a bucket or bathtub

Ò Fill it with water

Ò Take a small bell in hand

Ò Shake the bell inside water to produce sound

Ò Make sure it does not touch the body of bucket

Ò Now place your ear gently on water surface

Ò You will be able to hear the sound of ringing bell

Why astronomers fail to hear the sound of each other on surface of moon?

ÒIt is because there is no air and sound cannot travel through vacuum. Itneeds medium to travel.

How whales and dolphins are able to communicate?

Ò As sound can travel through liquids.

Whale                                 Dolphin

Audible & Inaudible Sound

Audible sound: Sound that can be heard by humans (audible sound)

 or   Audible: –   Human beings can hear sounds between frequencies 20 Hz and 20,000 Hz.         This range of frequencies is called the hearing range for humans or Audible sound.

Infrasound:-The sound with frequency below 20 Hz is called infrasound.

Ultrasound:-The sound with frequency above 20,000 Hz is called ultrasound.

Inaudible sound: That can’t be heard by humans      (inaudible sound)

or Inaudible Sounds:-The sound produce by frequencies below 20 Hz and above 20,000 Hz are not audible to our ear, such sound are called inaudible sounds.

What is Galton Whistle?

ÒThese are the special whistles (used for investigation purpose) that produce sound of frequency higher than 20,000 Hertz and dogs have the ability to hear it but humans cannot.

Human Ears

      It is divided into 3 parts:- (a) Outer         (b) Middle         (c) Inner

 

Outer part of Ears

It consists of

Ò Ear Pinna

Ò Auditory Canal

Ò Ear Drum

Middle part of Ears: – have 3 Interlocked Bones

1. Malleus          (b) Incus             (c) Stapes

Inner parts of Ears

Eustachian Tube (A coiled organ filled with fluid Cochlea).

Working of ears:-

The sound produced by vibrating body travel in the form of vibrations. It is collected by ear Pinna and it directs into ear canal where it travels and hit ear drum.

On receiving vibrations ear drum starts vibrating. Once the ear drum vibrates it passes vibrations to 3 interlocked bones of middle ear which further increase the amplitude of vibrations.

In inner ear vibrations are received by coiled organ.

On receiving the vibrations its fluid also starts vibrating and sensory cells present in them transforms the vibrations into electrical signals which are carried by auditory nerve to brain for interpretation and understanding.

Types of Sound:-

1. Music        2.Noise 

Music:-A sound which is pleasant to hear is called music.

Noise: -A sound which is unpleasant to hear is called noise.

Musical Instrument: – An instrument that produce a musical sound is called musical instrument.

A musical instrument uses following:-

1. Vibrating strings
2. Vibrating column of air
3.  Vibrations in wood, metal or stretched membranes

There are 3 main types of instruments:-

1. Stringed instruments      (b) Wind instruments         (c) Percussion musical instrument

Stringed instruments

These can be plucked, bowed or struck

The instruments with plucked strings of varied length include sitar, violin, guitar etc

Violin

And these all instruments are bowed to produce sound of different pitch.

Sitar                                                                      Guitar

Example: –        1. Violin produces the sound of highest pitch

2. Double bass produces the sound of lowest pitch

Violin                                                Double bass violin

Wind Instruments

The instruments are mainly made of brass and wood but sometimes they can be made of silver. They all differ in varied length of air column.

Examples: –   

Flute                          Shehnai                               Been

Percussion instruments: –

These instruments themselves as they include vibrating membranes as used in drums of various kinds.

Example:-

Tabla	Dholak

Noise

 The sound that is unpleasant and have irritating effect is called noice.

Noise pollution

 The contamination of air with unpleasant sound and irritable effect is called noise pollution.

 

Major causes of noise pollution

ÒSound of vehicles.

Ò Explosions including bursting of crackers.

Ò Sounds of machines.

Ò Sounds of loudspeakers.

Ò Playing transistors, radio at high volume.

Ò Sound of kitchen appliances.

Ò Sound of desert coolers.

Harms of Noise Pollution

Ò Lack of sleep

Ò Hypertension

Ò Anxiety

Ò More health problem

Ò Impairment of hearing

Measures to limit Noise Pollution

1. Silencing devices must be installed in aircraft engines, vehicles etc.
2. All industries should be located outside the city.
3. TV, music system should be operated at low volume.
4. Trees must be planted along roads and buildings.

Hearing Impairment:-Loss of hearing is called hearing impairment.

1. Total hearing.
2. Partial impairment

1. Total hearing: –Impairment is very rare, it is usually congenital, i.e. by birth.
2. Partial impairment: –Occur due to disease, injury or age.

A person with hearing impairment can learn sign language to communicate with others.

Que.1. Explain harmful effects of noise pollution.

Ans: –Harmful effects of noise pollution:

It causes deafness

It causes mental illness

It causes headache and high blood pressure

Que.2 What is the different between noise and music? Can music become noise sometimes?

Ans:- The sound which is unpleasant for our ears is 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.

 

Que.3 Lightning and thunder take place in the sky at the same time and the same distance from us. Lightning is seen earlier and thunder is heard later. Can you explain why?

Ans:-  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.