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.