Physics of the Ear and Hearing

Physics of the Ear and Hearing

• The ear is the organ that detects sound, aiding in balance and body position.
• The ear is part of the auditory system.
• The ear converts weak mechanical waves into electrical pulses.
• The auditory system converts mechanical sound waves into electrical signals.
• The auditory system decodes and analyzes the electrical signals.
• The ear is divided into three parts: outer, middle, and inner ear.

Topics of the Lecture

• Hearing system
• Parts of the ear (outer, middle, inner)
• Hearing loss (conduction, nerve, central)
• Hearing test (audiometer)

Hearing System

• The ear system consists of:
  • Source of sound
  • Sound detector (ear): sensitive sound wave detection.
  • Receiver (brain): decodes and analyzes sound waves.
• Mechanical system (ear): catches and amplifies sound.
• Sensory system (electrical, auditory nerve): converts mechanical pulses to electrical signals.
• Auditory system (brain): decodes and analyzes electrical signals.

The Auditory System: Divisions

• Outer ear:
  • Pinna (collects and amplifies sound, directs sound to the ear canal)
  • Ear canal (protects eardrum, amplifies sound)
• Middle ear:
  • Tympanic membrane (eardrum): transfers sound from air to ossicles' vibrations
  • Ossicles (malleus, incus, stapes): amplify sound, transmit to inner ear
  • Eustachian tube: equalizes pressure in the middle ear
• Inner ear
  • Cochlea: fluid-filled snail-shaped structure, sound waves are converted to nerve impulses.
  • Basilar membrane: hair cells that translate sound vibrations into electrical signals in the cochlea.
  • Sensory cells (hair cells, outer and inner): detect different frequencies of sound, generate impulses.
  • Tectorial membrane: covers hair cells
  • Auditory nerve: transmits electrical signals to the brain.

Outer Ear Standing Waves

• A closed cylindrical air column produces resonant standing waves at fundamental frequency and odd harmonics.
• Resonance enhances sensitivity of the ear in higher frequency ranges (2000-10000 Hz) and optimal sensitivity is in the region of 2000-4000 Hz.

Tympanic Membrane (Eardrum)

• Cone-shaped piece of skin separating outer and middle ear.
• Highly sensitive to pressure variations, causing vibrations
• Transmits sound from air to ossicles.
• Reflection and transmission of sound waves at the eardrum—minimize reflection, maximize transmission. (e.g., I_ref/I_in ≈ 0.99 and I_trans/I_in ≈ 0.001 values relating reflection and transmission at eardrum)

Middle Ear

• Amplifies pressure on the oval window compared to that on the eardrum by a factor of about 22.
• Force on oval window is approximately 1.5 times the force at the eardrum.
• Area of the oval window is smaller than that of eardrum, about 15x smaller.
• Filters out body-generated noise.
• Protects the ear from high sound levels.

Inner Ear

• Transforms sound energy to nerve impulses.
• Cochlea, a snail-shaped structure, receives and performs sound processing.
• Stapes vibrating moves fluid in the cochlea, generating waves within the fluid, and activates hair cells.
• Inner ear fluid-filled chambers (vestibular, middle, tympanic).
• Reissner membrane between vestibular and middle chambers.
• Basilar membrane between middle and tympanic chambers separating the scala vestibuli from scala media,and scala media from scala tympani
• Hair cells located on the basilar membrane: translate sound vibrations into electrical signals.

Hearing Loss (Deafness)

• Conduction hearing loss: problems with sound transmission through outer or middle ear.
• Nerve hearing loss: damage to sensory cells or auditory nerve.
• Central hearing loss: damage to the brain's auditory pathways.

Hearing Loss (Types)

• Conduction hearing loss: problems with sounds transmitting through outer or middle ear (e.g., wax, hole in eardrum, fluid in middle ear)
• Nerve hearing loss: damage to sensory cells or auditory nerve. (e.g. damaged hair cells)
• Central hearing loss: problem with the brain's hearing pathways.

Hearing Test (Audiometer)

• Uses electronic instruments to evaluate hearing.
• Measures hearing ability for different frequencies and loudnesses, determining areas of hearing loss