Physics

• Necessary condition for production of sound

  Vibration of a body, travels in a medium whose particles can vibrate, frequency between 20Hz and 20,000Hz
• Effect of medium on speed of sound

  Sound travels 3 times faster in solids than liquids, 5 times faster in liquids than gases, elastic property and density of medium affect speed
• Proving mechanical nature of sound

  Ringing bell in evacuated jar, no sound heard as no particles to transport disturbance through vacuum
• Longitudinal waves


  Particles of medium move back and forth along direction of propagation
• Sound as a wave

  • Carries energy, obeys reflection, refraction and diffraction, obeys interference
• Characteristics of sound waves

  • Reflection (echo), diffraction (listening in other room), refraction (change in speed and direction)
• Loudness


  Characteristic of sound by which loud and faint sounds can be distinguished
• Intensity of sound


  Sound energy passing per second through a unit area held perpendicular to the direction of propagation
• Relationship between loudness and intensity

  Loudness is directly proportional to logarithm of intensity
• Factors affecting loudness


  • Amplitude of vibrating body
  • Area of vibrating body
  • Distance from vibrating body
  • Physical condition of listener's ears
• Intensity level of sound


  Difference between loudness of unknown sound and loudness of faintest audible sound
• Unit of intensity level

  Bel (large unit), decibel (dB, 1 bel = 10 dB)
• Decibel


  Logarithmic unit which measures intensity level of sound
• Reason for using logarithmic scale for sound intensities
• Frequency


  Number of sound waves passing through a point in one second
• Pitch


  Characteristic of sound by which we can distinguish between shrill and grave sounds
• Pitch and frequency


  Pitch is directly proportional to frequency
• Amplitude and loudness


  Increasing amplitude increases loudness, and vice versa
• Frequency and pitch


  Increasing frequency increases pitch, and vice versa
• Increasing pitch


  Frequency increases, wavelength decreases, wave velocity remains same, amplitude unchanged
• Echo


  Reflection of sound, occurs when distance between person and reflecting surface is at least 17m and time difference between original and reflected sound is at least 0.1s
• Audible frequency range


  20Hz to 20,000Hz, decreases with age
• Noise


  Sound with jarring and unpleasant effect, causes negative health effects and accidents
• Acoustic protection


  Balancing reverberation and absorption in design of halls and auditoriums
• Uses of ultrasound in medicine

  • Removing blood clots
  • Imaging thyroid gland
• Stretched string better medium for sound transmission than air
• Voice quality allows recognition of speakers
• Sound waves can diffract around corners, light waves cannot
• Carpeted floor reduces sound reverberation, requiring higher volume
• Speed and frequency are different wave properties
• Hearing sensitivity differences cause loudness disagreement
• Difference between echo and reflection

  Echo is repetition of sound after reflection under specific conditions
• Two 50dB sounds do not make 100dB, dB is exponential scale
• Ultrasound useful in medicine due to shorter wavelengths
• Elastic property


  Refers to how easily a medium can be compressed and expanded. Solids have the highest elastic property, followed by liquids, and then gases.
• Density


  Refers to how closely packed the particles are in a medium. Gases have the lowest density, followed by liquids, and then solids.
• Speed of sound in solids vs liquids

  Sound travels 3 times faster in solids than liquids.
• Speed of sound in liquids vs gases

  Sound travels 5 times faster in liquids than gases.