Chapter-7 Waves & Sound

• Mechanical waves such as sound wave need material medium to propagate and cannot pass through vacuum
• Waves don't carry matter; they carry energy
• Mechanical waves


  Require a material medium to propagate
• Transverse waves


  Displacement of particles in the medium is perpendicular to the direction of the wave
• Longitudinal waves


  Displacement of particles in the medium is parallel to the direction of the wave
• Combinational waves


  Exhibit a combination of transverse and longitudinal waves
• Combinational waves


  • Water waves
• Stationary (standing) waves


  Formed by the superposition of two progressive waves of the same frequency and amplitude travelling in opposite directions
• Waves have linear property - when two or more waves pass the same point, the resultant wave is the sum of the individual waves
• Constructive interference


  Resultant wave has a larger amplitude than individual waves
• Destructive interference


  Resultant wave has a smaller amplitude than individual waves
• Completely destructive interference


  Resultant wave has zero amplitude
• Crest


  Highest point of a wave
• Trough


  Lowest point of a wave
• Wavelength


  Distance between any two consecutive wave crests or troughs
• Amplitude


  Distance from the equilibrium to the crest or trough
• Frequency (f)


  Number of completed waves passing a point per second
• Period (T)


  Time taken by the wave to travel one complete wavelength
• Intensity of a wave


  Power or energy per unit time transported per unit cross-sectional area
• Sound is a form of energy that is transferred from one place to another in a certain medium
• Sound wave is produced by a vibrating object placed in a medium
• Pressure changes occur alternately in the medium by vibrating object
• Audible range


  Frequency range that an average person can hear, between 20 Hz and 20 kHz
• Frequency (f)


  The number of completed waves passing a point per second
• Unit of frequency


  Hertz (Hz)
• Period (T)


  The time taken by the wave to travel one complete wavelength
• Transverse Wave


  • Wavelength
  • Crest
  • Equilibrium
  • Trough
• Intensity of a wave


  The power or energy per unit time transported per unit cross-sectional area
• Sound Wave


  1. Vibrating object placed in a medium

  2. Pressure changes occur alternately in the medium

  3. Sound waves are produced and transferred from one place to another
• Audible Range


  • Frequencies higher than 20 Hz and lower than 20kHz
• Sound Waves


  • Infrasound (lower than 16 Hz)
  • Audible frequencies (16 Hz to 20,000 Hz)
  • Ultrasound (higher than 20,000 Hz)
• Speed of Sound


  Depends on the density of the medium, the denser the medium the greater the speed
• Sound waves are longitudinal waves
• There is no sound in a vacuum
• Harmonics


  Sound waves with frequencies that are integer multiples of a fundamental tone
• Fundamental tone frequency


  The lowest frequency sound that can be produced on a tube
• Doppler Effect


  When a sound object moves towards you, the frequency increases (higher pitch)

  When a sound object moves away from you, the frequency decreases (lower pitch)
• Decibel Scale


  • 0 dB (Faint)
  • 40 dB (Moderate to Quiet)
  • 50 dB (Moderate)
  • 70 dB (Loud)
  • 80 dB (Very Loud)
  • 110 dB (Extremely Loud)
  • 140 dB (Threshold of Pain)