Physics of Hearing and the Ear

• The lecture discusses the physics of the ear and hearing.

Lecture Topics

• Hearing system components
• Parts of the ear (outer, middle, and inner)
• Hearing loss (deafness)
• Hearing tests using an audiometer

Hearing System Basics

• Any hearing system include a source of sound, a detector, and a receiver.
• The ear's role is to detect sound, aid in balance, and contribute to body position awareness.
• A clever design converts weak mechanical waves in the air into electrical pulses, which are then sent through the auditory nerve.
• The auditory system structures the body to catch and amplify sound, convert mechanical pulses into electrical signals, and decode these signals in the brain.
• The auditory cortex in the brain processes electrical nerve signals.

Auditory System Division

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

Outer Ear Components

• Includes the pinna, auditory canal, and tympanic membrane (eardrum).
• The pinna collects sound, acting as funnel, amplifies it, gives direction information of it.
• The auditory canal is a tube that runs from the outer ear into the middle ear.
• The auditory canal is 2.5cm in length and 0.7cm in diameter.
• The auditory canals functions: protect the eardrum, prevent harmful items, and amplify as a resonator.

Standing Waves in Outer Ear

• The ear canal acts as a closed cylindrical air column creating resonant standing waves.
• Resonant standing waves occur at a fundamental frequency with odd harmonics.
• Standing waves are determined by the formula L = n × (λ/4), where n = 1, 3, 5, 7,...
• This resonance enhances ear sensitivity in the 2000-10000 Hz range.
• The best sensitivity is in the 2000-4000 Hz region.
• Where (V) velocity of sound in air =330 m/s.
• The fundamental frequency: F1 = 1 * 330/4 * 2.5/100 = 3300 Hz.

Tympanic Membrane

• It is a cone-shaped piece of skin approximately 10 mm in width, separating the outer and middle ear.
• Sensitive to slight pressure variations, it vibrates in response.
• The main role is to transfer incoming sound to the ossicles in the middle ear.
• The acoustical signal travels along the ear canal, hits the eardrum, creating partial reflection and transmission.
• For optimal hearing, reflection is minimized, and transmission is maximized.
• The ratios for reflected and transmitted acoustical waves at the eardrum Zair = 430 kg/m².s and Z muscle= 1.48x106 kg/m².s: * Iref/Iin = 0.99.
• Transmitted ratio is Itrans/Iin = 0.01, indicated that the intensity is reflected and lost.

Middle Ear

• The middle ear is the central part of the ear behind the eardrum.
• Parts include the hammer, anvil, and stirrup.
• This transmits vibrations from the tympanic membrane to the oval window.
• It acts as a lever system, amplifying the pressure by a factor of approximately 22:
  • Fo (force on oval window) = 1.5 times Fm (force on eardrum)
  • Ao (area of oval window) = 15 times smaller than Am (area of eardrum)
  • Pressure on the oval window is increased = 1.5 × 15 = 22.
• Also work to filter noise and provide protection.
• Protects from excessive vibrations and adjusting for high levels by switching to a less efficient mode.

Inner Ear

• Transforms compressional wave energy by the inner ear fluid.
• Nerve impulses can be transmitted to the brain.
• Main organ is the cochlea is the hearing portion of the inner ear.
• The snail shaped structure that is connected to the stirrup (or stapes).
• The stapes movement creates fluid waves in the cochlea.
• Divided into three fluid chambers: the vestibular, middle, and tympanic.
• Reissner membrane separates the scala vestibule from the scala media.
• Basilar membrane separates the scala media from the scala tympani.
• sensory cells (hair cells) detect sound frequencies (pitch).
• Covered by the tectorial membrane.
• Incoming sound creates vibrations causing hair cells to shear against the tectorial membrane.
• Hair cells undergo electrical impulses to be converted into neural signals.
• High frequency sounds produce greatest basilar membrane motion near the oval window.
• Low frequency sounds produce greatest basilar membrane motion farthest from the apex.
• Results in various nerve cells activated, distributed the organ of corti for electrical pulses depending on frequency sound waves.

Hearing Loss (Deafness)

• Can be caused by changes at any part of the auditory system.
• Conduction hearing loss is caused by problems with sound transmission through the external or middle ear to the cochlea
• Nerve hearing loss occurs due to destruction of hair cells or auditory nerve damage.
• Central hearing loss results from brain damage.
• Specific types of conduction loss are caused by blockage of wax, fluid in the middle ear, and holes in the eardrum.
• Sensorineural (nerve) hearing loss id caused by damage to the sensory hair cells.
• Damage to the neural pathways responsible for hearing can also result in hearing loss.

Hearing Tests (Audiometer)

• Audiometers help check and track hearing status
• Measures the lowest threshold or points to start hearing
• Can identify if there are any issues that needs attention and what kind

This lecture explores the physics of hearing, covering the components of the hearing system and the anatomy of the ear, including the outer, middle, and inner ear. It further discusses hearing loss and methods for testing hearing using an audiometer. The lecture also examines how the ear detects sound, aids in balance, and contributes to body position awareness.