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Sound Synopsis



Production and Propagation of Sound

  • Sound is a form of energy which produces sensation in our ears.
  • Sound travels in the form of waves.

Production of Sound 

  • When you strike a tuning fork, it starts vibrating. When this tuning fork is brought near the ear, we hear some sound.
  • When this fork is brought near a hanging ball, the ball starts oscillating.

  • Thus, a vibrating source is necessary for the production of sound.
  • Vibrations in vocal cords produce the human voice. The buzzing sound when a bee flies is due to the vibration of air by the wings. The plucking of strings of a guitar produces sound.

    Thus, sound is produced by vibration of strings, air, membranes or plates, etc.


Propagation of Sound

  • The matter or substance through which sound is transmitted is called a medium.
  • When an object vibrates, the particles in the surrounding medium vibrate. These particles send the vibrations to the adjacent particles and the energy continues to transmit until it reaches our ears.
  • A wave is a disturbance moving through a medium when the particles of that medium set the adjacent particles into vibration.
  • The particles do not move forward, but the disturbance does.
  • Sound waves are characterised by motion of particles in a medium and are therefore called the mechanical waves.
  • When a vibrating object moves forward in air, it pushes and compresses the air around it, creating a high pressure in the surrounding region called compression. When the object moves backwards, it creates a low pressure region called rarefaction.

    When the prongs move forward, compression is formed.
    When the prongs move backward, rarefaction is formed.


Sound needs a Medium to Travel 

  • Sound is a mechanical wave and needs a material medium to propagate. It cannot travel through vacuum.

    • Two astronauts cannot speak in space (outside earth) because there is a vacuum present.
  • Take an electric bell and an airtight glass bell jar. The jar is connected to a vacuum pump.

  • The bell rings when the switch is pressed. If air is present inside the jar, then we are able to hear the sound. However, if the air is removed, then the sound is not audible even though the bell is ringing.
  • Sound can travel through solids, liquids and gases, but not through vacuum.


Sound waves are Longitudinal Waves

  • Sound propagates in a medium as a series of compressions and rarefactions.
  • A longitudinal wave is a wave where the individual particles vibrate parallel to the direction of the propagation of the wave.
  • Sound waves also propagate in the same manner, and hence, are called longitudinal waves.

Characteristics of Sound

Two sounds are distinguished by the following characteristics:
  • Loudness
  • Pitch or shrillness
  • Quality or timbre

  • Loudness is the property by which a loud sound can be distinguished from a faint one, both having the same pitch and quality.

  • The magnitude of the maximum disturbance in the medium on either side of the mean value is called the amplitude of the wave.
  • The loudness or softness of a sound is determined by the amplitude (or intensity) of the wave.
  • However, loudness is not the same as intensity. Intensity is a measurable quantity, while loudness is a sensation. Experimentally, Weber and Fechner established a relationship between the loudness L and intensity I which is given as:
    L = Klogl
    Where, K is a constant of proportionality. Obviously, loudness increases with the increase in intensity, but not in the same proportion.
  • The intensity at any point of the medium is measured as the amount of sound energy passing per second normally through unit area at that point. Its unit is microwatt per metre squared. Greater the energy carried by a sound wave, greater is the intensity of sound, and hence louder it seems to us.
  • The intensity of a sound wave in air is proportional to (i) the square of the amplitude of vibration, (ii) the square of the frequency of vibration, and (iii) the density of air.
Subjective nature of loudness and objective nature of intensity:
  • The loudness of a sound depends on the energy conveyed by the sound wave near the eardrum of the listener. Thus, the loudness of sound of a given intensity may differ from listener to listener.
  • Further, two sounds of the same intensity, but of different frequencies may differ in loudness even to the same listener because the sensitivity of the ears is different for different frequencies.
  • For normal ears, the sensitivity is maximum at frequency 1 kHz. Thus loudness is a subjective quantity, while intensity, being a measurable quantity, is an objective quantity for a sound wave. 
Factors affecting the loudness of sound:
The loudness of sound heard at a place depends on the following five factors:
  • Loudness is proportional to the square of the amplitude: When a body vibrates with greater amplitude, it sends forth a greater amount of energy, and hence the energy received by the eardrum is also large. Thus, the sound appears louder.
  • Loudness varies inversely as the square of the distance: If the listener is close to the source of sound, he will hear it quite louder, but if he is far away, the sound will become feeble.
  • Loudness depends on the surface area of the vibrating body: A large vibrating area sends forth a greater amount of energy. Hence larger the surface area of the vibrating body, the louder is the sound heard.
    When a tuning fork is sounded in air, the sound given by it is feeble, but when it is placed on a sound box, the sound becomes much louder. The reason is that the box provides comparatively a large area and forces a large volume of air to vibrate and thereby increases the sound energy reaching our ears. 
  • Loudness depends on the density of the medium: More the density of medium more is the loudness. 
  • Loudness depends on the presence of resonant bodies: The presence of resonant bodies near the vibrating body increases the loudness of sound. 

Units of loudness and sound level (phon and decibel): 
  • The unit of loudness is phon. The loudness of a sound in phon is the loudness in decibel (dB) of an equally loud pure sound of frequency 1 kHz. The sound level is usually expressed in decibel (dB). 
  • The loudness L is related to the intensity I as L = K log10 I. If at a given frequency, I1 and I0 are the intensities of two sounds of which loudness are L1 and L0 respectively, then
    L1 = Klog10l1
    L0 = Klog10l0  
    Therefore, we have the difference in loudness as
    L = L1 - L0
    = K(log10l- L1 = Klog10l0)

    begin mathsize 12px style equals Klog subscript 10 straight l subscript 1 over straight l subscript 0 end style
    The minimum intensity of audible sound intensity at 1 frequency 1 kHz is l0 = 10-12 W m-2. The loudness of sound is called sound level.

    begin mathsize 12px style straight L equals log subscript 10 straight l subscript 1 over straight l subscript 0 bel
equals 10 log subscript 10 fraction numerator straight l subscript 1 over denominator straight l subscript 0 cross times end fraction cross times 1 over 10
equals 10 log subscript 10 straight l subscript 1 over straight l subscript 0 decibel end style
  • Now, if L = 1 dB, then we have

    begin mathsize 12px style 10 log subscript 10 straight l subscript 1 over straight l subscript 0 equals 1
log subscript 10 straight l subscript 1 over straight l subscript 0 equals 1 over 10
straight l subscript 1 over straight l subscript 0 equals Antilog open parentheses 1 over 10 close parentheses
equals 1.26 end style
  • Thus, we can define 1 dB as the change in level of loudness when the intensity of sound changed by 26%.
Noise Pollution

The disturbance produced in the environment due to undesirable loud and harsh sound of level above 120 dB, from the various sources such as loudspeaker, siren, moving vehicles etc. is called noise pollution. 
A constant hearing of sound of level above 120 dB can cause headache and permanent damage to the ears of listener. The sound of level 30 dB to 10 dB has the soothing sensation, while the level 0 dB of loudness of sound represents the limit of hearing.
Pitch or Shrillness

  • Pitch is another characteristic of sound waves. It allows distinguishing between two sound waves travelling with same speed and arriving our ears at the same time.