sound wave physics

Sound of Class 8

Sound is a form of energy which produces the sensation of hearing. This sensation is produced by longitudinal waves in an elastic medium.

A wave can be described as a disturbance that travels through a medium, transporting energy from one location to another location. The medium is simply the material through which the disturbance is moving. It can be thought of as a series of interacting particles.


Sound is a mechanical wave that results from the back and forth vibration of the particles of the medium through which the sound wave is moving. If a sound wave is moving from left to right through air, then particles of air will be displaced both rightward and leftward as the energy of the sound wave passes through it. The motion of the particles is parallel (and anti-parallel) to the direction of the energy transport. This is what characterizes sound waves in air as longitudinal waves.

In laboratory, sound is produced by a tuning fork when beaten with rubber pad, by plucking a stretched string of violin, by striking tabla, by blowing flute etc.

sound wave physics


  • Musical instruments and their vibrating parts :



Musical instrument

Vibrating part producing sound



Stretched string



Stretched membrane





  • Sound produced by humans :

In humans, the sound is produced by the voice box or the larynx. Put your fingers on the throat and find a hard bump that seems to move when you swallow. This part of the body is known as the 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 the passage of air.

When the lungs 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.


The vibrating string of a musical instrument or the vibrating prongs of the tuning fork, set the molecules of the air surrounding them into vibration. These vibrating air molecules, in turn pass on their vibrations to the neighbouring air molecules and in this manner, sound travels to the listener in the form of waves. A wave is a disturbance which travels through a medium due to the vibratory motion of the particles of the medium. Sound cannot travel in vacuum. It always need a medium to travel.


An electric bell is enclosed inside an inverted bell jar by hanging from the rubber cork. The jar is closed at the bottom by an airtight plate with a hole in the centre. A pipe through the hole leads out to a vacuum pump (pump which draws the air out of a vessel).

The bell is connected to a battery through a key.

The bell is started by closing the key. Initially when jar has normal air inside it, sound waves produced by the ringing bell heard outside the jar.

The vacuum pump is started and the air from inside the jar is gradually drawn out. With decrease air inside the jar, sound heard becomes weaker and weaker. After sometime no sound is heard, though the bell hammer is seen in vibration.


In the absence of medium (air) around the source, sound is not being propagated.

A natural fact : Moon has no atmosphere. The space above the atmosphere is also vacuum. If some explosion takes place on moon, sound of the explosion will not be propagated to the earth. So the sound waves never reach the earth.


Sound waves are of longitudinal type because these make the particles of the medium vibrate in the direction in which wave travels. A moving sound wave produces compressions (regions of more density and pressure) and rarefactions (regions of less density and pressure) alternately at regular time intervals. This wave can be graphically represented as:

     sound wave physics

Compression :

The region of maximum density of medium in a longitudinal wave, is called compression.

Rarefaction :

The region of minimum density of medium in a longitudinal wave, is called rarefaction.

sound wave physics

sound wave physics


  • Amplitude : The maximum displacement of vibrating particles from its mean position is known as amplitude. It is denoted by `a' or A. The amplitude of a sound is represented by the height of the wave. When there is a loud sound, the wave is high and the amplitude is large. Conversely, a smaller amplitude represents a softer sound. A decibel is a scientific unit that measures the intensity of sounds. The softest sound that a human can hear is the zero point. When the sound is twice as loud, the decibel level goes up by six. Humans speak normally at 60 decibels.

sound wave physics


  • Wavelength : The wavelength is the horizontal length of one cycle of the wave. The period of a wave is the time required for one complete cycle of the wave to pass by a point. So, the period is the amount of time it takes for a wave to travel a distance of one wavelength.

Wavelength is defined in following different ways

  •  It is the distance travelled by a wave during one complete vibration of the vibrating particle. 
  •  It is the distance between two nearest particles in the same phase.
  •  It is the distance between two consecutive crests or troughs of a transverse wave.
  •  It is the distance between two consecutive rarefactions or compressions of a longitudinal wave. 
  •  It is the length of one complete wave. Wavelength is denoted by `λ' (lambda). Its unit is metre (m).

sound wave physics

The wavelength of a wave is merely the distance that a disturbance travels along the medium in one complete wave cycle. Since a wave repeats its pattern once every wave cycle, the wavelength is sometimes referred to as the length of the repeating patterns - the length of one complete wave. For a transverse wave, this length is commonly measured from one wave crest to the next adjacent wave crest or from one wave trough to the next adjacent wave trough. Since a longitudinal wave does not contain crests and troughs, its wavelength must be measured differently. A longitudinal wave consists of a repeating pattern of compressions and rarefactions. Thus, the wavelength is commonly measured as the distance from one compression to the next adjacent compression or the distance from one rarefaction to the next adjacent rarefaction.

Since a sound wave consists of a repeating pattern of high-pressure and low-pressure regions moving through a medium, it is sometimes referred to as a pressure wave. If a detector, whether it is the human ear or a man-made instrument, were used to detect a sound wave, it would detect fluctuations in pressure as the sound wave impinges upon the detecting device. At one instant in time, the detector would detect a high pressure; this would correspond to the arrival of a compression at the detector site. At the next instant in time, the detector might detect normal pressure. And then finally a low pressure would be detected, corresponding to the arrival of a rarefaction at the detector site. The fluctuations in pressure as detected by the detector occur at periodic and regular time intervals. In fact, a plot of pressure versus time would appear as a sine curve. The peak points of the sine curve correspond to compressions; the low points correspond to rarefactions; and the "zero points" correspond to the pressure that the air would have if there were no disturbance moving through it. The diagram below depicts the correspondence between the longitudinal nature of a sound wave in air and the pressure-time fluctuations that it creates at a fixed detector location.

sound wave physics



Do not conclude  from the above diagram that sound is a transverse wave that has crests and troughs. Sound waves traveling through air are indeed longitudinal waves with compressions and rarefactions. As sound passes through air (or any fluid medium), the particles of air do not vibrate in a transverse manner. Do not be misled - sound waves traveling through air are longitudinal waves.


  • Time-period : Time taken by a vibrating particle to make one complete vibration is called its time-period. Also, time taken by a wave to move a distance equal to its wavelength λ, is called the time-period of the wave. It is denoted by T. Its unit is second (s).
  • Frequency : The number of vibrations completed by a particle in one second is called its frequency. Frequency is also defined as the number of waves (or crests) passing a given point in one second. It is denoted by ‘n' or N . Its unit is hertz (Hz). Every cycle of sound has one condensation, a region of increased pressure, and one rarefaction, a region where air pressure is slightly less than normal. The frequency of a sound wave is measured in hertz. Hertz (Hz) indicate the number of cycles per second that pass a given location. If a speaker's diaphragm is vibrating back and forth at a frequency of 900 Hz, then 900 condensations are generated every second, each followed by a rarefaction, forming a sound wave whose frequency is 900 Hz.
  •  Pitch : Pitch is the sensation (brain interpretation) of the frequency of an emitted sound. Faster the vibration of the source, higher is the frequency and higher is the pitch. Similarly low pitch sound corresponds to low frequency.

A high pitch sound is called a shrill sound (e.g. humming of a bee, sound of guitar etc). A low pitch sound is called a hoarse sound (e.g. roar of a lion, car horn etc).

How the brain interprets the frequency of an emitted sound is called the pitch. We already know that the number of sound waves passing a point per second is the frequency. The faster the vibrations the emitted sound makes (or the higher the frequency), the higher the pitch. Therefore, when the frequency is low, the sound is lower.

Loudness or softness :

Loudness or softness of a sound wave is the sensation that depends upon its amplitude. When we strike a table top with more force, it vibrates and produces loud sound waves which have more amplitude. When struck with smaller force, vibrating table top produces soft sound waves which have less amplitude. A loud sound wave carries more energy and can be heard at large distance. Reduction in amplitude at large distance, makes the sound soft.

  •  Timbre or quality : Quality or timbre is a characteristic of a sound which enables us to distinguish between the sounds of same loudness and pitch. This characteristic of sound helps us to recognise our friend from his voice without seeing him. The quality of two sounds of same loudness and pitch produced by two different sources are distinguishable because of different wave forms produced by them. e.g. The violin and flute (Bansuri).
  •  Intensity :Intensity of a sound is defined as the sound energy transferred per unit time per unit area placed perpendicular to the direction of the propagation of sound. i.e. intensity of sound = (Sound Energy/Time x Area)

Intensity of a sound is an objective physical quantity. It does not depend on the response of our ears.

The S.I. unit of intensity of sound is joule s-1 m-2 or watt m-2( ⇒1 Js-1 = 1W)

The intensity level of sound is measured in decibels (dB). Decibels are units of intensity that are based upon a logarithmic scale. This means that a sound with an intensity of 20 dB is ten times as loud as one with an intensity of 10 dB, 30 dB is ten times as intense as 20 dB, and so on. The reason for this logarithmic scale is that humans hear intensity on a similar logarithmic scale. So, while a 20 dB sound is ten times as intense as a 10 dB sound, we perceive it as only twice as loud. The hearing threshold (level at which humans begin to perceive sound) is 0 dB. When a sound reaches upwards of 120 dB, it is above the threshold of pain (point at which most people begin feeling pain). Everything in between can be heard by a human with normal hearing.

A Table of Decibel levels


Let frequency = n

∴ Time taken to complete n vibrations = 1 second

Or Time taken to complete 1 vibration = 1/n second 

But the time taken to complete one vibration is called time-period T, hence

T= 1/n or n × T = 1

∴ Frequency × Time period = 1





Intensity of a sound


Loudness is a subjective quantity. it depends upon the sensitivity of the human ear. A sound may be loud for a person but the same sound may be feeble for another who is hard of hearing.

Intensity of a sound is an objective physical quantity. It does not depend on the sensitivity of a human ear.


Loudness cannot be measured as a physical quantity because it is just sensation which can be felt only.

Intensity of a sound can be measured as a physical quantity.


sound wave physics

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