Audiology-Chapter 2

Overview of Chapter Two of Audiology for SLP Course- Discussing elements of sound and ear processing - Aiming for a unified understanding among students and providing a review of previously learned material

Definition of Sound: A physical phenomenon characterized by the movement or vibration of an elastic medium without permanent displacement of particles.
Longitudinal Waves: Sound travels as a longitudinal wave, meaning the molecules of the medium move back and forth in the same direction as the wave's propagation.
Understanding of sound as a physical phenomenon involves:
- Vibration of an elastic medium
- Temporary displacement of particles without creating permanent changes

Source of Energy: Necessary to generate sound (e.g., a strike, a pluck, or air flow).
Vibrating Object: This object produces an audible pressure wave through rapid back-and-forth movement.
Medium of Transmission: Commonly, air serves as this medium, though sound can travel through solids and liquids.

Physical properties of sound include:
- Mass: Quantity of matter present in an object ( $m$ ).
- Force: The push or pull on an object ( $F = ma$ ).
- Elasticity: Resistance of an object to deformation and its ability to return to rest; also known as stiffness.

Inertia: The tendency of an object to resist changes in its motion.
- Linked to Newton's First Law: An external force is necessary to overcome inertia.
- Example: Stopping a small pebble versus a heavy boulder.

Definition: A type of periodic motion where an object moves back-and-forth around a central equilibrium.
- Compression: The phase of a sound wave where molecules are pushed together, causing high pressure.
- Rarefaction: The phase where molecules are pulled apart, causing low pressure (partial vacuum).
Influenced by:
- Mass of the object: Heavier objects may swing wider and longer before settling.
- Frequency: The number of vibratory cycles per unit time (measured in Hertz, $\text{Hz}$ ).
- Amplitude: Measurement of distance from rest to maximal displacement.
- Period: Duration required to complete one cycle of vibration ( $T = 1/f$ ).
- Phase: Relationship between the timing of compressions and rarefactions in sound waves, measured in degrees ( $0^{\circ}$ to $360^{\circ}$ ).

Higher frequency sound waves have more cycles within the same period compared to lower frequency sound waves.
Amplitude correlates with loudness; higher displacement signifies louder sounds, while less displacement indicates quieter sounds.

Intensity Measurement: Typically measured on a logarithmic scale (decibels - $\text{dB}$ ).
Understanding Logarithmic Scales:
- Allows representation of a wide range of intensities using smaller numerical increments.
- Example: An increase from $30 \text{ dB}$ to $40 \text{ dB}$ indicates a sound intensity that is $10$ times greater.

Definition: Unit used to measure sound intensity, named after Alexander Graham Bell.
Logarithmic Base: The decibel is a ratio that compares a measured pressure to a reference pressure.
$0 \text{ dB}$ Reference: It should be noted that $0 \text{ dB}$ does not mean the absence of sound; it represents the threshold of human hearing for a healthy young adult.

Sound Pressure Level (SPL): Measures the physical sound intensity in the environment using pascals.
Hearing Level (HL): Used on audiograms; normalized to the average human hearing threshold at specific frequencies.
Sensation Level (SL): The number of decibels above a specific individual's hearing threshold.

Outer Ear: Functions to gather and funnel sound into the ear canal.
- Pinna (Auricle): Collects sound and helps in localization.
- External Auditory Meatus: The ear canal leading to the eardrum; it provides some natural resonance (approx. $2500 \text{ Hz}$ to $3000 \text{ Hz}$ ).
- Tympanic Membrane (Eardrum): Vibrated by incoming sound waves, converting acoustic energy into mechanical energy.
Middle Ear: Comprises the tympanic membrane and ossicles; it acts as an impedance-matching transformer.
- Impedance Matching: The middle ear overcomes the loss of energy as sound moves from air to the fluid-filled cochlea by concentrating pressure from the large eardrum onto the small stapes footplate.
- Ossicles: Three smallest bones in the human body:
- Malleus (Hammer): Attached to the eardrum.
- Incus (Anvil): Middle bone of the chain.
- Stapes (Stirrup): Connected to the oval window; its vibration moves the fluid in the cochlea.
- Eustachian Tube: Equalizes pressure in the middle ear and drains fluid.

Cochlea: Spiral-shaped, fluid-filled structure responsible for auditory transduction (mechanical to electrical energy).
- Scala Vestibuli: Upper chamber filled with perilymph.
- Scala Media: Middle chamber contains the organ of Corti and is filled with potassium-rich endolymph.
- Scala Tympani: Lower chamber containing perilymph.
- Basilar Membrane: The floor of the scala media; it facilitates the traveling wave.
- Organ of Corti: Contains hair cells responsible for detecting sound:
- Outer Hair Cells: Approximately $12,000$ cells; they act as an amplifier for low-intensity sounds.
- Inner Hair Cells: Approximately $3,500$ cells; they send the actual sound signal to the brain.

Traveling Wave: Movement of the stapes causes fluid waves that travel along the basilar membrane.
Tonotopic Organization: The cochlea is organized by frequency; the base (narrow and stiff) responds to high frequencies, while the apex (wide and floppy) responds to low frequencies.

Interpreting Sound: While the ear receives the physical signal, hearing is a cognitive process. The brain must interpret the electrical signals provided by the auditory nerve.

End of Chapter Two lecture. Students should review tonotopic organization and the decibel scale for the upcoming examination.