Sound

Sound

1. Requisites of medium

   • Elasticity for particle vibration
   • Inertia for particles to store mechanical energy
   • Frictionless to prevent loss of energy during propagation of sound

2. Sound propagation requires a material medium

   • Bell jar experiment - The hammer of an electric bell suspended in an air tight bell jar strikes the gong repeatedly producing sound when the circuit is complete by pressing the key. Keeping the key pressed, air is gradually withdrawn from the jar by starting the vacuum pump, the loudness of sound goes on decreasing. When the entire air has been sucked out, no sound is heard even when the hammer is striking the gong.
   • Astronauts cannot hear each other in outer space

3. Sound is a form of energy that produces a sensation of hearing.

   • Mechanical energy starts vibrations in a body producing sound.
   • Vibrations transmit in a medium in the form of waves
   • These waves on reaching our ears, ear drums produce vibrations which are perceived as sound now

4. A vibrating body is a source of sound

   • A sound is heard on plucking the strings pivoted at both ends.
   • A rider placed near the middle of a thin wire stretched between two nails, flies off on plucking the wire near the rider, producing sound and vibrations.
   • When a tuning fork is brought near a tennis ball suspended by a string after striking one of its arms against a rubber pad, the ball jumps to and fro and sound of the vibrating tuning fork is heard.
   • On beating a drum, its vibrating membrane produces sound.
   • On plucking the strings of a sitar, the string vibrates and sound is heard.

5. Terms related to wave motion

   • Amplitude (a) is the maximum displacement of the particle of a medium on either side of its mean position when a wave passes through the medium. S.I unit - metre (m)
   • The time taken (T) by a particle of a medium to complete its one vibration is called the time period of wave. S.I. unit : second (s)
   • The number of vibrations made by a particle of a medium in one second is called the frequency of wave (f). S.I. unit : second^-1 (s^-1) or hertz (Hz)
     • f = 1/T
     • produces disturbance
     • not dependent on amplitude of vibration
     • does not depend on nature of medium
   • The distance travelled by the wave in one time period of vibration of a particle of the medium is called its wavelength (λ : lambda). SI unit is metre (m)
   • The distance travelled by a wave in 1 second is called its wave velocity (V). SI unit is metre per second (ms^-1).
     • Depends on elasticity & density of medium

6. Waves

   • Longitudinal
     • compressions and rarefaction
     • sound wave
       • travel in vacuum
       • speed in air - 330 m/s at 0°C
         • In thunder, light is seen earlier than the sound of thunder is heard, although they are produced simultaneously.
         • When the starter in an athletic event fires a gun, a spectator sitting at a distance hears the sound of fire a little later while the smoke is instantaneously seen.
         • The spectators watching a cricket game hear the sound of strike a little later than the batsman seen making it.
       • speed decreases in optically denser medium
   • Transverse wave
     • crest and trough
     • light wave
       • cannot travel in vacuum
       • speed in air - 3 x 10^8 m/s
       • speed increases in denser medium

7. λ = V T

   • Since T = 1/f , V x (1/f) = λ
     • V = f λ

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