Propagation of Sound Waves

• Sound is a form of energy that produces a sensation of hearing in our ears
• Sound reaches our ears in the form of waves due to vibration of particles
• The speed of sound is more in solids, less in liquids and least in gases because solids are more elastic than liquids and gases
• GR: If sound is produces at one end of a very long steel bar, two sounds are heard at the other end.

  One which reaches first is propagated through steel, and the one heard later, is thro air
• GR: A person hearing the sound thro steel rail gets an indication of the coming train well before sound is heard

  vibrations produced thro moving wheels of train travel much faster thro steel rail thanair
• Factors affecting speed of sound in gas

  CHANGE IN density, temperature, humidity & direction of wind
• Speed of sound is inversely proportional to square root of density of the gas
• A vibrating body is a source of sound
• A material medium is necessary for the propagation of sound from one place to another
• Requisites of the medium

  • The medium should be elastic so that its particles may come back to their initial positions after displacement of either side i.e. particles are capable of vibrating about their mean position
  • The medium should have inertia so that its particles may store mechanical energy
  • The medium should be frictionless so that there is no loss of energy in propagation of sound through it
• Give reasons: You cannot hear someone on the moon

  Sound does not travel in vacuum. There is no medium on the moon therefore one cannot hear the sound produces by others.
• Light does not require any material medium for its propagation and thus can propagate through vacuum as well.
• One complete to and fro movement of the strip produces one compression and one rarefaction which together constitutes as one wave
• Mechanical energy is required to start vibrations in the body
• Assertion: Sound waves are also called mechanical waves

  Reason: Sound waves are produced due to vibration of particles of the medium


  A and R are true and R is correct explanation for A
• Assertion: Sound waves cannot travel through vacuum

  Reason: There are no particles to exhibit the properties of elasticity and inertia in vacuum
• The frequency of sound is directly proportional to the velocity of sound in that medium. This means that if the velocity of sound doubles then the frequency will double too.
• Sound cannot travel at the same speed in different mediums. The speed of sound in a medium is affected by three things: • The density of the medium. For instance, speed of sound is the maximum through solids • The temperature of the medium. As the temperature increases, the sound propagates easily. • Humidity in the air also affects the travel of sound. As the humidity increases, so does the propagation of sound.
• The wave in which the particles of medium vibrate about their mean positions, in the direction of the propagation of sound is called a longitudinal wave
• Transverse wave


  The wave in which the particles of the medium vibrate about their mean positions in a direction perpendicular to the direction of propagation of the wave is called a transverse wave.
• the time taken by a particle of a medium to complete its one vibration is called the time period of a wave
• The number of vibrations made by a particle of a medium in one second is known as the frequency of the wave.

  It is the same as the number of waves passing through a point in one second.
• Wavelength (λ) = Distance travelled by the wave in one time period of vibrations of the medium is called its wavelength.

  It is denoted by lambda

  SI unit= metre (m)
• Frequency (f) =1/ time period (T)
• Amplitude: When a wave passes through a medium, the maximum displacement of the particle of the medium on either side of its mean position is called its amplitude.

  denoted by a

  SI unit: metre (m)
• Wavelength= Wave velocity x Time period

   λ= V x T

  or

  VT= λ

  but

  T= 1/f

  Vx 1/f= λ

  V=f λ

  therefore, Wave velocity= Frequency x Wavelength
• The distance travelled by wave in one second is called its wave velocity
• The speed of sound in gas increases with increase in temperature of the gas
• The speed of sound in air increases with increase in humidity
• If air is blowing in the direction of propagation of sound, the speed of sound increases, while if wind is blowing in opposite direction of sound it decreases
• Factors NOT affecting speed of sound in gas

  CHANGE IN pressure, amplitude of wave, wavelength or frequency of wave
• Sonic sound: Between 20 Hz to 20kHz
• Infrasonic sound: less than 20Hz, can't be heard by humans
• Ultrasound: Sound of frequency greater than 20 Hz
• Properties of Ultrasound

  • Energy carried by ultrasound is very high
  • The ultrasound can travel along a well defined straight path. it does not bend appreciably at the edges of an obstacle because of its small wavelength. ( It has high directivity)
• Applications of ultrasound:

  • Bats
  • drill holes in glass
  • cleaning minute objects
  • detection of defects in metals
  • Ultrasonography
  • Echo cardiography
  • Remove cataracts and kidney stones
  • SONAR ( sound navigation and ranging) to detect distance of objects unerwater
• Difference between ultrasonic and supersonic:

  ULTRASONIC: Used for ultrasound, frequency above 20 kHz)

  SUPERSONIC: Objects which travel with a speed greater than sound in air (330m/s or Mach 1) eg: concord jet planes and fighter planes