LE 10 2/12 Introduction to Sound Properties

Amplitude perception:
- Perceived as loudness or intensity.
- Increasing amplitude increases loudness (e.g., adjusting stereo volume).
Sound definition:
- A wave in air.
- Physically characterized by pressure changes measured over time.
Unit of measurement:
- Decibels (dB) expresses changes in air pressure.
- Pressure waves modulated by vocal tract contribute to sound recognition.

Main parts:
- Outer ear
- Middle ear
- Inner ear
Outer ear composition:
- Pinna, the fleshy structure aiding in sound localization.
- Pinna functions:
- Funnels and redirects sound to the auditory canal, similar to a satellite dish.

Components:
- Tympanic membrane (eardrum)
- Ossicles (three bones)
- Malleus (hammer)
- Incus (anvil)
- Stapes (stirrup)
Eardrum role:
- Transmits pressure waves to the inner ear via oscillatory movement of ossicles.
- Movement induced by sound pressure changes.
Importance of Middle Ear:
- Prevents sound reflection by transitioning from low-density (air) to high-density (fluid in inner ear) mediums.

Structure:
- Curled tube (cochlea) with three main sections:
- Scala vestibuli (top)
- Scala tympani (bottom)
- Organ of Corti (middle)
Organ of Corti:
- Responsible for transduction of sound waves into electrical signals.
- Contains two membranes:
- Tectorial membrane (top)
- Basilar membrane (bottom)
- Hair cells:
- Inner hair cells (1 row) and outer hair cells (3 rows) with stereocilia (hairs).

Stereocilia arrangement:
- Contact with tectorial membrane leads to mechanical movement.
Shearing movement:
- Sliding of tectorial and basilar membranes causes stereocilia to bend, leading to ion channel activity.
  - when they bend in one direction, it opens ion channels. Conversely, bending in the opposite direction closes these channels, which ultimately leads to a burst of electrical signals resulting in firing of neurotransmitters.
Mechanically-gated ion channels:
- Opens and closes based on stereocilia movement, allowing ion flow and causing auditory nerve action potentials.

Type of ion channels:
- Mechanically-gated; respond to physical manipulation.
Ion concentration:
- High extracellular potassium near hair cells allows for rapid depolarization.

Sound processing sequence:
- Sound waves create pressure changes captured by the tympanic membrane.
- Ossicles amplify and transfer those changes to the oval window.
- Basilar membrane displacement occurs in response to sound waves.
Frequency mapping:
- Different regions of the basilar membrane are sensitive to varying frequencies (place theory).

Auditory system uses both phase locking and place coding to interpret sound frequency.
Place theory:
- Specific hair cells stimulate corresponding auditory nerve fibers regarding frequency recognition.
Volley theory:
- Patterns of neural firing rates correspond to sound frequency.

Loud sounds can damage hair cells leading to decreased auditory efficiency.
Chronic exposure to loud sounds may require assistive devices like hearing aids or cochlear implants to restore hearing.

Function:
- Amplify sound for individuals with hearing loss.
- Volume increase without addressing transmission failure.

Mechanism:
- Bypass damaged areas by directly stimulating remaining hair cells.
- External microphone captures sound, and internal electrodes stimulate the cochlea.
Effectiveness:
- Particularly beneficial for individuals with early language development before permanent deafness sets in.

Age-related hearing loss linked to chronic loud sound exposure.
Two primary theories for age-related hearing loss:
- Long-term structural change in cochlea due to pressure.
- Neural conduction changes affecting sound processing efficiency.

Overall hearing process involves intricate mechanical and neural components to translate sound into perceived auditory signals.
Understanding of the ear’s detailed anatomical structure and sound processing capabilities contribute to strategies for managing hearing loss effectively.