Sound                          Sound is Vibrations that travel through a medium and can be heard.

Vibration                      A vibration is rapid back and forth movement of body about its mean position. This is also known as oscillatory motion.

Amplitude                    The maximum displacement of any object from its mean position during oscillations is known as amplitude. It is expressed in metres.

Frequency.                  The number of oscillations per second is known as frequency of oscillations. It is expressed in Hertz.                                    

Wavelength                   Wavelength is defined as the distance between two successive crests or two successive troughs.

Time Period                 The time taken by wave to complete one oscillation is known as time period of wave.

Pitch                            Pitch of sound is determined by frequency of wave, more the frequency higher is the pitch.

Loudness                     Loudness of sound is determined by amplitude of wave, more the amplitude more is the loudness of sound. It is expressed in Decibel(db).

Ear Drum                     Ear drum is like a stretched rubber sheet. Ear drum sends vibrations to inner ear.

Noise                           Sounds which are unpleasant to ear are known as noise.

Music                          Sounds which are pleasant to ear are known as music.

Noise Pollution             Presence of excessive or unwanted sounds in environment is known as noise pollution.



A sound is produced due to vibration. A vibration can be defined as the rapid back and forth movement of body about its mean position. So we can say that sound is a vibration that can be heard.

When a tightly stretched band is plucked it produces sound; when it stops vibrating, it does not produce sound. It means that vibration produces sound. But in most case like these, we cannot see the vibrations since the amplitude of the vibration is very small.                                                                                    

The musical instruments produces sound by their vibrating parts. Different instruments have different vibrating parts. Some musical instruments and their vibrating parts are given below.

   Sr. No.

    Musical Instrument

Vibrating parts producing sound



        Stretched string  



        Stretched membrane





When we pluck the string of an instrument, like a sitar, the sound we hear is not only that of string but also the whole instrument that is put to vibrations.


The organ in human beings that is involved in the production of sound is called Larynx. It is also known as the Voice Box. The Voice Box is situated at the upper end of Wind Pipe. The sound produced is controlled by Vocal cords, which are thin membrane stretched across Voice Box in such a way that it leaves a narrow slit between them for passage of air.

When the lungs force air through the slit, the vocal cords vibrate and produce sound. Muscles attached to vocal cords make the cord tight or loose. When the vocal cords are tight and thin the sound produced is different from that when vocal cords are loose and thick,

Vocal cords in women are tight and thin so they produce sound of high frequency, whereas in case of male, the vocal cords are loose and thick and hence produce sound of low frequency.

Vocal cords in men are about 20mm long. In women the vocal cords are 5mm shorter. Children have very short vocal cords. That is why voices of men, women and children are different.


When we throw pebbles in a pool of water, then pebbles create ripples of waves, which spread far in the pool. Similarly vibrations cause waves in air and we are able to hear when these waves reach our ear.

Sound wave travels in the form of regions of Compressions and Rarefactions.

Compressions are the regions where vibrating particles are at minimum distance to each other i.e. they lie closer to each other.

Rarefactions are the regions where vibrating particles are at maximum distance to each other i.e. they lie far away from each other.

Crest is the point on wave where the displacement of the medium is at a maximum.

Trough is the point on wave where the displacement of particles is at minimum.

Sound needs a material medium like solids, liquids or gases to travel. Sound cannot travel through vacuum because it travels by producing a vibration in the molecules of a medium, whereas vacuum has no molecules for vibrations.

As the sound wave travel, the molecules themselves do not travel from one point to another. Particles only vibrates about their mean position, it is the ENERGY that transfers from one particle to another, hence sound travel through the medium.

Speed of sound is different in different medium. Sound travel fastest in solids and slowest in gases. This is because solids have more number of molecules per unit volume as compared to liquids and gases.

When we turn on the switch for an electric bell placed inside an empty glass jar, we are able to hear sound because of air particles (medium) present in the jar.

However, if air is removed from jar with the help of vacuum pump and then the switch is turned on, then the bell would ring but we would not be able to hear sound. This is because, there is no medium inside the jar and sound cannot travel through a vacuum.


1. Amplitude of wave is defined as the maximum displacement of vibrating body about its mean position. Amplitude of wave is measured in metres.

Amplitude determines the loudness of sound, which is proportional to square of amplitude. If the amplitude becomes twice then the loudness is increased by the factor of four.

Loudness = Square of Amplitude

Loudness is measured in unit called Decibel (dB).

Loudness of sound from various sources is listed below.


Normal Breathing

10 dB

Soft Whispering

30 dB

Normal Conversation

60 dB

Busy Traffic

70 dB

Average Factory

80 dB


Sound above 80 dB is physically painful to human beings.

2. Frequency of wave is defined as number of oscillations completed by the wave in one second. SI unit to measure frequency is Hertz. Frequency also determines the pitch or shrillness of wave. If the frequency of sound wave is high then the sound is shrill and the pitch is high, whereas if the frequency is low then the sound have a lower pitch.

Examples of higher pitch sound are sound produced by whistle and sound produced by bird.

Examples of low pitch sound are roar of lion and men’s voice.


3. Time Period of a wave is defined as the time taken by wave to complete one oscillation. Time Period is measured in seconds.

Relation between Time period and Frequency is inverse.

Time period= 1/ Frequency

4. Wavelength of the wave is the length between two successive crests or two successive troughs. It is measured in metres. Shriller (High Pitch) sound has shorter wavelength and heavy (Low Pitch) sound has longer wavelength. It is denoted by Lambda.

5. Speed of wave is distance travelled by sound wave in per unit time. It is expressed in m/s.




Velocity is nothing but Speed with respect to Direction.




The ear structure can be broadly classified into three parts: The outer ear, the middle ear, the inner ear.


Outer Ear- The part of outer ear that is visible to us is called Pinna. The pinna collects sound waves and directs them to Ear Tube. At the end of ear tube is the ear drum. Ear Drum is also known as Tympanic membrane. The ear drum vibrates when the sound waves strike it and then transmits the sound to middle ear.

Middle Ear- The middle ear consists of three bones named Hammer, Anvil and Pinna. These bones are placed in such a way that they move whenever ear drum vibrates and then they transmit the vibrations to the inner ear.

Inner Ear- The inner ear is connected to middle ear through a small opening. The inner ear is filled with a Fluid. When this fluid vibrates, it excites Tiny Hair in the inner ear. These hair transforms the vibrations into electrical impulses, which are then transferred to brain through Auditory Nerve. Hence we hear the sound.



Sound waves travel at different speed in different medium. The speed of sound depends on various factors such as temperature, nature of medium and humidity.


Speed of substance


343 m/s


1498 m/s


1452 m/s


5000 m/s


5000 m/s


5000 m/s


12,000 m/s



Sound are of different types - Soft, Loud, Pleasant, Unpleasant, Musical, Audible, Inaudible etc.


Audible Sound is the sound which can be heard by human beings. Human ear can hear sound wave of a frequency ranging between 20 Hz to 20,000 Hz.

Inaudible Sound is the sound which cannot be heard by human beings. Sound wave of a frequency below 20 Hz or above 20,000 Hz is not audible to human ear. Sound wave of frequency below 20 Hz is known as Infrasonic waves and sound waves of frequency above 20,000 Hz are known as Ultrasonic waves.

Some animals can hear sounds of frequency higher than 20,000 Hz; Dogs have this ability. Some animals produce sounds that we do not hear because the frequency of the sound is either too low or too high. For example, Elephant communicate with each other in a sort of rumbling sound, which is in the frequency range of 12-22 Hz

Dolphins use lower frequency waves (Higher Wavelength) for distant Echolocation. To echolocate objects nearby, dolphins produce high frequency waves (Lower Wavelength).


Sound which is pleasant to ear is called Music. We enjoy the sounds from musical instrument and this is known as a musical sound.


Unpleasant, discomfort-causing sound from any source is called Noise. Sound coming from a construction site, sound produced by vehicles on road and sound produced by loudspeaker are unpleasant and called as Noise.

NOISE POLLUTION- Presence of harmful, unwanted and annoying noise is called noise pollution.


  1. Noise pollution can cause irritation and loss of concentration.
  2. Noise pollution can cause anxiety, sleep disturbance, stress and high blood pressure.
  3. Noise pollution can also cause ear damage and loss of hearing if one has continuous exposure to noise over a long period of time.


Minimising noise pollution requires a certain degree of discipline from all of us. Some of measures we should adopt to keep noise pollution under control are:-

  1. The use of loudspeakers should be avoided.
  2. Television or music should not be loud.
  3. While driving, people should not play loud music and not use the horn unnecessarily.
  4. Trees should be planted along roadside, as they absorb noise.


Total hearing impairment is usually from the birth. Partial disability may result from any disease, injury or age. Children with impaired hearing need special care. By learning sign language, such children can communicate effectively. Speech develops as the direct result of hearing; a child with hearing impairment may have defective speech.


  • The speed of sound is around 1,230 km per hour, that means sound takes 12.9 days to reach the moon while light reach in 1.2 seconds.
  • Sound has a “shadow zones” which are based on temperature. In these shadow zones, one cannot hear any sound. This is because sound waves change direction and bend upward based on temperature changes.
  • In Africa, a tribe of people called Maabans live in such quiet that they can hear a whisper from across a baseball field.
  • Our ears pick up sound while we sleep, but our brain does not hear it.
  • The hammer, anvil and stirrup are the three smallest bones of body present in our ear, responsible for transmission of sound. Together they are about the size of a pea.
  • Whales in the ocean “sing” to each other. The sound of their song can travel a distance of 800km and emitted at particular frequencies that is receivable by other whales.
  • Flies are DEAF!!