Physics of Hearing and the Ear

• The lecture covers the physics of the ear and hearing.
• Topics include the hearing system, parts of the ear (outer, middle, and inner), hearing loss (deafness), and hearing tests using audiometers.

Hearing System

• A hearing system consists of a sound source, a detector, and a receiver.
• The ear is the organ that detects sound, aids in balance, and is part of the auditory system.
• The ear converts weak mechanical waves in the air into electrical pulses in the auditory nerve.
• The auditory system has a mechanical component for catching and amplifying sound by the ear.
• A sensory (electrical) system converts mechanical pulses into electrical signals passed to the brain.
• The brain decodes and analyzes electrical nerve signals in the auditory cortex.

Auditory System Structure

• The ear is divided into the outer, middle, and inner ear.

Outer Ear

• It collects and directs sound waves to the tympanic membrane via a canal.
• Consists of the pinna, auditory canal, and tympanic membrane (eardrum).
• The pinna collects sound, acting as a funnel to amplify it and provide directional information.
• The auditory canal is a tube from the outer to the middle ear, about 2.5cm long and 0.7cm in diameter.
• Functions of the auditory canal include:
  • Protection of the eardrum from shocks.
  • Prevention of harmful items from entering with hair and wax.
  • Sound amplification by acting as a resonator, producing standing waves.

Outer Ear and Standing Waves

• A closed cylindrical air column produces resonant standing waves at a fundamental frequency and odd harmonics.

Standing Waves in the Ear Canal

• Resonance will enhance ear sensitivity in the 2000-10000 Hz range, with best sensitivity in the 2000-4000 Hz region.

Tympanic Membrane

• A cone-shaped piece of skin about 10 mm in width, separating the outer ear from the middle ear.
• Very sensitive, it vibrates with slight pressure variations.
• The primary function is to transfer sound from the air to the ossicles in the middle ear.
• Acoustic signals travel along the ear canal and strike the eardrum, resulting in reflection and transmission.
• Hearing sensitivity reflection should be minimized, and transmission maximized.
• Incoming wave intensity mostly reflects indicating bad impedance matching and hearing loss.

Middle Ear

• Central part of the ear behind the eardrum.
• Contains the hammer, anvil, and stirrup, which transmit vibrations to the oval window of the inner ear.
• Functions of the middle ear bones:
  • Acts as a lever system, amplifying pressure on the oval window by approximately 22 times.
    • The force on the oval window (fo) is about 1.5 times the force on the eardrum (fm).
    • The area of the oval window (Ao) is approximately 15 times smaller than the area of the eardrum (Am).
  • Filters out noise generated in the body.
  • Protects from excessive vibrations via a less-efficient vibration mode at high sound levels.

Inner Ear

• Transforms the energy of compressional waves into nerve impulses for transmission to the brain.
• The hearing portion of the inner ear is the cochlea, connected to the stirrup (stapes).
• The Cochlea is a snail-shaped structure in which fluid waves produces.
• Divided into three fluid-filled chambers: vestibular, middle, and tympanic.
• Reissner membrane separates the scala vestibule from the scala media.
• Basilar membrane separates the scala media from the scala tympani.
• Two types of sensory hair cells (inner and outer) along the basilar membrane.
• The sensory hair cells respond to frequency (pitch) and are covered by the tectorial membrane.
• Sound energy causes basilar membrane vibration, shearing hair cells on the tectorial membrane.
• Inner ear contains electrical charges that result in neural impulses which travel along the auditory nerve to the brain.
• High frequency sounds produce the greatest motion of the basilar membrane near the oval window.
• Low frequency sounds produce the greatest motion of the basilar membrane farthest from the apex.
• This results in electrical pulse production and transmission along the organ of Corti.

Hearing Loss (Deafness)

• May be caused by changes at any auditory system level.
  • Conduction hearing loss involves impairments in sound transmission through the external or middle ear to the cochlea.
  • Nerve hearing loss occurs with the destruction of hair cells or damage to the auditory nerve.
  • Central hearing loss happens due to brain damage.
• Conductive hearing loss affects the passage of sound between the eardrum and the inner ear.
• Nerve hearing loss is where there is damage to the hair cells in the cochlea.

Hearing Test (Audiometer)

• Electronic instrument used to measure hearing.

Lecture on the physics of hearing and the ear. It covers the hearing system, the outer, middle, and inner ear, and the different parts of the ear, including their functions. Also discussed are hearing loss (deafness) and hearing tests using audiometers.