Sound, Hearing, and Ears

Sound and Physics

• Sound Waves:
  • Result from waves of changes in density (pressure) of air molecules.
• Key Terms in Sound:
  • Frequency: Related to pitch
  • Amplitude: Related to loudness
  • Wavelength: The distance between repetitions of a wave.
  • For sound in air: speed = frequency × wavelength
  • Example: Sound speed = 200 cycles/sec × 5.5 ft/cycle = 1100 ft/sec.

Human Hearing Range

• Audible Frequency Range: 20 Hz to 20,000 Hz.
• Comparison to Light:
  • Sound: 20 - 20,000 Hz (audible sounds)
  • Light: 400 - 700 nm (visible light)

Timbre and Sound Complexity

• Timbre: The perceptual property of sound that distinguishes different types of sound creation, like instrument or voice.
  • Complex sound waveforms consist of a combination of frequencies.
  • Fourier Analysis: Any waveform can be represented as a sum of sinusoidal components of various frequencies and amplitudes.

Anatomy of the Ear

• External Ear:

  • Pinna: Outer part of the ear.
  • Eardrum: Air vibrates the eardrum; this vibration transfers to the ossicles.

• Middle Ear:

  • Ossicles: Small bones (Malleus, Incus, Stapes) that amplify sound vibrations.
  • Tympanic Membrane: Eardrum, responsible for converting sound waves into mechanical waves.

• Inner Ear:

  • Cochlea: Fluid-filled structure that performs frequency analysis of sound.
  • Hair Cells: Located within the cochlea, convert mechanical energy into electrical signals to the brain.
  • Movement of tips opens ion channels for potassium, leading to depolarization and signal transmission.

Processing and Perception of Sound

• Tonotopic Organization: Different frequencies activate different regions of the basilar membrane.
• Cochlear Nucleus to Auditory Cortex Pathway:
  1. Hair cells produce signals
  2. Signals travel via the cochlear nerve to the cochlear nuclei in the brainstem.
  3. Further processing occurs at the superior olive, lateral lemniscus, and inferior colliculus toward the auditory cortex (A1).

Sound Localization

• Interaural Time Difference (ITD): Time difference in sound arrival between the ears helps determine direction.
• Interaural Level Difference (ILD): Difference in sound intensity at each ear provides location information.
  • Low frequencies utilize ITD; high frequencies use ILD due to the acoustic shadow created by the head.

Hearing Loss

• Causes of Hearing Loss:
  • Infection-induced damage, genetic impairment (e.g., Connexin gene mutations), noise exposure (acute or chronic).
• Hearing Aids: Amplify sound through microphones and speakers.
• Cochlear Implants: Bypass damaged hair cells; external processors convert sound to digital signals sent to electrodes in the cochlea which stimulate the auditory nerve directly.

Vestibular System

• Components:
  • Semicircular canals, utricle, saccule.
  • Function: Balance and spatial orientation through detection of head position and movement.
• Hair Cells: Involved in detecting changes in motion, contributing to balance.

Vertigo

• Definition: A sensation of spinning or dizziness, associated with issues in the vestibular system.