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Sound and Hearing

How is Sound Produced?

  • Sound is made by vibrations.
  • Sound waves are longitudinal waves, meaning the particles vibrate parallel to the direction of the wave.
  • Sound is a mechanical wave, requiring a medium to travel through (e.g., air, solids, liquids).
  • Examples:
    • Hearing someone because sound travels through the air.
    • Hearing vibrations through a string with a hanger.
    • Hearing sounds underwater, even motors in a lake.

Sources of Sound

  • Every sound results from a vibration.
  • Vibration is a rapid back-and-forth motion in solids, liquids, or gases.
  • Energy is carried by sound waves caused by these vibrations.
  • Example: Plucking a guitar string transfers energy, causing it to vibrate and collide with air particles, creating a sound wave.

Human Voice

  • Vocal folds vibrate as air moves, creating sound.
  • The sinuses and other structures distinguish individual voices.
  • People perceive their own voice differently than recordings because of how sound travels through the body.

Musical Instruments

  • Vibration is key to sound production in instruments.
    • Woodwind Instruments:
      • Flutes use air passing over an edge to create sound.
      • Saxophones and bassoons use reeds that vibrate.
    • String Instruments:
      • Guitars and pianos use vibrating strings (pianos use keys to strike the strings).
    • Percussion Instruments:
      • Drums and cymbals produce sound when hit.
    • Harmonicas:
      • Use reeds that vibrate when air is blown through them.
    • Brass Instruments:
      • Trumpets: Lips vibrate in a cup-shaped mouthpiece; keys change the note by altering air column length.

Sound Waves: Compressions and Rarefactions

  • Sound waves are transmitted through the air as longitudinal waves.
  • Longitudinal waves consist of:
    • Compressions: Areas where air particles are squished together.
    • Rarefactions: Areas where air particles are spread apart.
  • Sound radiates out in all directions from the source.

Energy and Sound Waves

  • Particles of a medium vibrate back and forth, carrying energy away from the source, similar to a line of people bumping into each other.

Speed of Sound

  • Sound travels faster in water than in air.
  • The speed of sound varies in different materials (see page 567).
  • Speed of sound in air:
    • 331 \, m/s at 0 degrees Celsius.
    • 343 \, m/s at 20 degrees Celsius.
  • Speed of sound in water is around 1500 \, m/s.
  • Sound travels fastest in solids, slower in liquids, and slowest in gases.

Factors Affecting Speed of Sound

  • Density: How closely packed the particles of a medium are.
    • Gases have particles far apart, so energy transfers more slowly.
    • Solids have closely packed particles, allowing for quick energy transfer.
    • Seawater, with dissolved salts, has a higher density than freshwater, so sound travels faster.
  • Stiffness: Rigid solids facilitate faster energy transfer.
  • Temperature:
    • In gases: As temperature increases, particles move faster and collide more often, increasing energy transfer.
    • In liquids and solids: Higher temperatures usually decrease the speed of sound because particles are already close together.

How We Detect Sound: The Human Ear

  • Ears can detect the direction of sounds.
  • Outer Ear (Pinna):
    • The outer part (pinna) cups air and directs it into the ear canal.
  • Ear Canal:
    • Leads to the eardrum.
  • Eardrum (Tympanic Membrane):
    • A tight membrane that vibrates when sound waves reach it.
  • Middle Ear:
    • Contains the three smallest bones in the body: malleus (hammer), incus (anvil), and stapes (stirrup).
    • These bones vibrate and transfer the vibrations to the cochlea.
  • Inner Ear:
    • Cochlea: A coiled, fluid-filled structure that converts vibrations into nerve signals.
  • The brain interprets these signals as sound.

Additional Structures

  • Semicircular Canals:
    • Fluid-filled loops above the cochlea that help with balance.
    • Ear infections can affect these, impacting balance.

Hearing Loss

  • Damage to the eardrum can cause hearing loss. A tear can allow bacteria into the ear, causing infection which may heal with thick scar tissue that reduces sensitivity to sound.
  • Ruptured Eardrum:
    • Can result from sudden loud sounds or pressure changes.
  • Eustachian Tube:
    • Connects the middle ear to the throat, helping to equalize pressure.
    • Problems can lead to pressure imbalances and infections.

Preventing Hearing Loss

  • Listening to loud music over extended periods can cause damage.
  • Loud noises can fatigue or fuse hair cells in the cochlea, and damaged hair cells do not regrow.
  • Ear protection is important in loud environments.
  • Headphones can be dangerous if the volume is too high, as they trap pressure and cause damage.