Is a Form of Energy, That Gives the Sensation of Hearing

Is a Form of Energy, That Gives the Sensation of Hearing


Definition: -

Vibrations transmitted through an solid or a liquid or gas, with frequencies in the approximate range of20 to 20,000 hertz, capable of being detected by human organs of hearing.


Is a form of energy, that gives the sensation of hearing.

Basic concepts concerning about sound


Cause of generation of sound


Reasons for different sound from different sources.


Propagation of sound from one place to other place.


Mechanism of hearing of sound by human ear.

Sounds and Vibrations

Rapid Back and forth movement of molecules of matter is calledvibrations.

Sound is produced because of vibrations


If the vibrations are very small,it may not be possible to detectthe sound. Vibrations are easilyseen to be produced by thefollowing instruments:-

Sound produced by Humans

In the human body Voicebox or larynx help us toproduce sound.

The voice box has twovocal chords stretchedacross it. These vocalchords help the air to passthrough it.

The lungs force airthrough the narrow gappresent between the twovocal chords and makethem vibrate to producesound.

The muscles attached tothe vocal chords can makethem tighter or loosened/thinner or thicker. Thesechanges in the vocalchords help to change thepitch of the sound.

Sound Produced By animals

Different types of birds and animals have different mechanism in their bodies for producing their characteristicssounds.


Most of the animals use their vocal chords to produce sound as the human beings produces the sound by usingvocal chords. e.g.-Dog-Bark/Cats-Mew/Horse-Neigh/Lions-Roar and Cows -Moo.


Birds have a ring of cartilage called SYRINX in their voice box to produce their characteristic

s sound.


Insects use their wings to produce sound. They vibrate their wings rapidly so that they can produce SOUND.More is the vibration higher will be the pitch of the sound.


Frogs use their vocal sacs to produce sound. The BULGING of their throat helps them to make their croakinglouder.


Sound needs a medium for its propagation.

It cannot propagate through vacuum but it can propagate through allthe three states of matter i.e. solid liquid and

Ex- When a person speaks, the air closer to his mouth ispushed in the direction of sound. The air molecules get distributedand start vibrating about their mean position. Such moleculeswhich vibrate will disturb the nearest molecules for vibratingthem. Continue this method until the molecules starts vibrationwhich is nearer to listener. This causes the vibrations in thediaphragm of the ear.The sound is always produced by vibrating body. When agong of a bell is struck with a hammer, sound isproduced. The gong is set into the vibrations and soundwaves are propagated in the air. When these waves reachthe ear, the ear drum is set into vibrations. Thesevibrations are communicated to brain and we getsensation through the hearing.

Experiment to show that sound require a material medium for its propagation


:To show that sound can travel through air but cannot travelthrough vacuum

Material required

: An electric bell , a big glass jar, a vacuum pump.


: Hang the bell inside the glass jar connected to the vacuumpump. When the bell is switched on, the sound is heard. Now, switchon the vacuum pump. We will observe that as the air is pumped out, thesound becomes faint and finally disappears


Result: sound cannot travel through vacuum.











Light propagates faster than sound

The following examples show that sound takes an appreciable time to travel from one place to another then light..

Though lightning and thunder are produced simultaneously, the flash of the lightning is seen much beforethe sound of the thunder.

When a gun is fired at some distance, the flash is seen before the sound is heard.

The steam coming out from the whistle of an engine is seen before the sound is heard.

In a cricket match, the striking of the ball by the batsman is seen before hearing the sound.

Reason: The speed of sound is 330m/s in air where as the speed of light is 3 x 10


m/s , which is muchfaster than the speed of sound. Hence light reaches earlier than the sound.Test yourself

Q What is the other name of the vocal cordsQ Why cant two astronauts hear each other on the surface of the moon?Q During a thunder storm, the sound of thunder is heard after lightning is seen. Give reason.Q Explain in brief, how is sound produced?Q You can easily hear the sound of an approaching train by putting your ear on the railway track well before the soundis heard in the air. Why?Q With the help of a diagram, Explain that sound cannot propagate through vacuumORWith the help of a diagram show that sound needs a material medium for propagation

Audible and Inaudible sound:-

The sound which can be heard by a human being is called audible sound.RANGE OF AUDIBLE SOUND:

20 Hz to 20,000 Hz .Sound frequencyType of sound

Below 20 HzInfrasonic sound(Inaudiblesound)Between 20 Hz to 20,000 Hz Audible sound( SONICSOUND)Above 20,000 Hz Ultrasonic sound(Inaudiblesound)TEST FOR YOURSELFQ Name the sound having frequency a) less than 20Hz b) between 20 and 20000Hz c) Greater than 20,000HzQ To which frequency range is the human ear sensitive?Q Name two animals, birds that can hear ultrasonic sounds.

Ultrasonic sound are produced by dolphins,bats and porpoises.

Infrasonic sound are produced by whales andelephants and they can observe theearthquake before the main shock waves.

Bat is able to locate any obstacle inits path due to the reflection of theultrasonic waves from the object.

Persistence of Hearing:-

The impression of sound heard by our ears persists in our mind for 1/10th of a second.

Sound takes only 1/10


of a second to reach our ear when the sound producing body is 33 meter away from theear.

Sound is always reflected back when it fall on the objects around them, these reflected sound reach to our earin the time less than 1/10


of a second.

If the reflected sound and the direct sound reach to our ear in mingled form then it is slightly difficult todistinguish between them.

Reflected sound can only be hear when the reflecting surface is 17 meter away from the ear.

Structure of the Ear

The outer ear:

The outer ear consists of the pinna.

Pinna collects the sound vibrations from the air.

These sound vibrations reach to the ear drum through ear canal/auditory canal.

Middle ear:

It is air filled cavities which lies just beyond the ear drum and is surrounded by a bone.

There are three bones in the cavity. The




(incus) &



These small bones help to amplify the vibrations.

Inner ear:

The inner ear is embedded in the bone of the skull and is filled with a liquid.

Consists of main organ of hearing called COCHLEA, that carries the vibrations in the form pf electricalimpulses from the AUDITORY NERVE to the BRAIN

1. The sound makes theear drum vibrate.3. The bones make thefluid and hair move4. The auditory nerve takesthe message to the brain2

The ear drum carriesthe vibrations to thethree bones




JUNE 2013

  • Sound needs a medium (like solid, liquid and gas) for propagation. It can not travel through vacuum.
  • Sound can travel through liquids, solids and gases.
  • Sound is a form of energy, that gives us the sensation of hearing.


  • Rapid to and fro movement is called vibration or oscillation.
  • Sound is produced because of vibrations or oscillations. It is produced by vibrating bodies.
  • Sound travels in the form of waves and is characterized by three basic quantities :
  • Amplitude
  • Frequency
  • Time period


  • The maximum displacement of a vibrating body, from itsmean position, on either side of the mean position is called amplitude.


  • The number of vibrations or ocsillations completed by a vibrating body in one second is called frequency.
  • The SI unit of frequency is hertz (Hz)


  • The time taken by a vibrating body to complete one vibration or oscillation is called timeperiod
  • It is measured in seconds.


There are three characteristics that make us differentiate between sounds:

  • Loudness – Degree os sensation of sound produced in the ear. It depends on 4 factors:
  • Sensitivity of the ear of the listener
  • Amplitude - greater amplitude, louder is the sound produced
  • Area of the vibrating body – larger the vibrating area, louder is the sound
  • Distance of the listener from the source
  • Pitch
  • The frequency determines the shrillness or pitch of the sound. When the frequency of the vibration is higher, then the sound has a higher pitch. When the frequency of vibration is lower, then the sound has a low pitch.
  • The frequency of the voice of a child is higher than that of an adult. The voice of a woman has higher frequency and is shriller than that of a man.
  • Pitch also depends on the relative motion between the source and the listener.
  • Quality or Timber
  • Timbre is the characteristic that enables us to distinguish between two sounds having same pitch and same loudness, produced by two different sources.


  • The sounds of the frequencies less than about 20 vibrations per second (20 Hz) are called infrasonic frequencies.
  • The sound of frequencies higher than about 20,000 vibrations per second (20000Hz or 20 kHz) are called ultrasonic frequencies.
  • The sound which can be heard by a human being is called audible sound.

For human ear, the range of audible frequencies is from 20Hz to 20,000 Hz

  • Ultrasonic sounds are produced by dolphins,bats and porpoises.
  • Infrasonic sound are produced by whales andelephants and they can sense an earthquake before the main shock waves.
  • A bat is able to locate any obstacle in its path due to the reflection of the ultrasonic waves from the object.


  • In the human body voicebox or larynx help us toproduce sound.
  • The voice box has two vocal chords stretchedacross it. These vocalchords help the air to pass through it.
  • The lungs force airthrough the narrow gappresent between the twovocal chords and makethem vibrate to producesound.
  • The muscles attached tothe vocal chords can makethem tighter or loosened/thinner or thicker. These changes in the vocalchords help to change thepitch of the sound.


  • Sound waves enter the external ear andstrike on the eardrum causing it to vibrate.
  • These vibrations are amplified many timesby three bones: (i) hammer (ii) anvil (iii)stirrup, in the middle ear and transmitted tothe inner ear by the hollow bones of the middle ear.
  • The inner ear has a coiled tube called cochlea. The tiny hairs present in it picks up the vibrations and sends the signals to the brain through auditory nerve to the brain, which interprets and make us hear sounds.


  • Music is sound that has a pleasant sensation on the ears.
  • Musical instruments are of 4 types:
  • Stringed instruments – guitar, sitar, violin
  • Wind or Reed instruments – flute, shehnai, bugle
  • Percussion or membrane instruments – drums, tabla, mridangam, dholak
  • Ghana Vadya or rhythm instruments – cymbals, jal tarang, ghatam


  • The disturbance produced in the environment by undesirable, loud and harsh sounds from various sources is called noise pollution. Noise pollution at a particular place is determined by-
  • Loudness of the sound
  • Duration of noise at that place
  • Hearing loss
  • Fatigue
  • Loss of concentration
  • Extreme emotional behaviour – anger, tension, irritability
  • Setting up industries away from residential areas
  • Planting more trees as they absorb sound
  • Restricted use of loudspeakers and auto horns
  • Use of carpets , curtains at homes
  • Use of improved silencers in automobiles, generators

Q1. Fill in the blanks:

  1. Only vibrating bodies produce ______.
  2. The range of infrasonic vibrations is less than ______.
  3. The unit of frequency is ______.
  4. The range of ______vibrations is above 20,000 Hertz.
  5. The to and fro motion of a vibrating body about its mean position is called ______.

Q2. Statements given below are incorrect . Write the correct statements :

  1. All kind of vibrations in the material bodies produces sensation of sound.
  2. The time taken to complete 1 vibration is called frequency
  3. Material medium is not necessary for the propagation of sound.

d The louder the sound , lesser is the amplitude of the vibrating body.

Q3. Match the statement in column A with those in column B :

Column A Column B

  1. Maximum displacement of a vibrating body from its mean position 1. Time period
  2. Time in which a vibrating body completes one vibration 2. Amplitude
  3. Number of vibrations produced by a vibrating body in one second 3. Depth of a sea
  4. Sonar is used to find 4. Frequency
  5. A repetition of sound produced due to reflection 5. Vacuum
  6. Sound cannot travel through this 6. Echo
  7. The sound vibrations between frequency 20 Hertz to 20,000 Hertz 7. Ultrasonic vibration

h. A sound produced by a whistle which is only heard by dogs but 8. Audible frequencies

not human beings

Q4. Define the terms a) frequency b) amplitude c) time period d) noise e) noise pollution

Q6. Name two animals that can hear ultrasonic sounds.

Q7. Name two animals that can hear infrasonic sounds.

Q8. Which sound has a higher pitch? A) low frequency B) high Frequency

Q9. With the help of a neat, labelled diagram describe an activity that proves that - sound can travel through a) liquids b) solids c) gases

Q10. With the help of a neat, labelled diagram describe an activity that proves that sound cannot travel in vacuum.


Questions: Sound; June 2013

Some Extra Reading!

Speed of Sound

Light travels very, very fast. Light travels through air at

3 x 108m/s i.e. 300000000 m/s ! Sound travels much slower. The speed of sound in air is 330 m/s. In the time it takes for light to travel the length of a football pitch, sound would travel 0.1 mm !

Storms make both light, as lightning, and sound, as thunder. If a storm is 10 km away, the light from the lightning would reach you in 0.00003s, while the sound from the thunderclap would take 30s to reach you. The longer the gap between the lightning and the thunder, the further away the storm.

Measuring the speed of sound

Janet and Luisa are measuring the speed of sound. Janet goes to the other end of the playing field, measuring the distance with a measuring wheel. She ends up 300 m away from Luisa. Luisa has a large wooden clapper. Janet starts her stopwatch when she sees Luisa close the clapper, and stops her stopwatch when the sound reaches her. Janet measured 0.95 s. Calculate the speed of sound.

Remember: Speed = Distance/Time





Class 8, Sound, June 2013 Page 1 of 5 Uploaded by: Ms. Mou Bera, June 2013