Physics of Sound and Hearing

Overview

• This lecture covers the physics of sound, the mechanisms of hearing, and their clinical relevance.
• The focus: How sound wavers are generated, transmitted and perceived.

Learning Objectives

• Explain properties of sound waves.
• Describe physiological structures involved in hearing.
• Understanding the transmission of sound.
• Discuss the clinical relevance of sound physics in audiology and medicine.

Definition of Sound

• Sound is a mechanical wave that propagates through a medium (air, water, solids).
• Requires a source, a medium, and a receiver

Properties of Sound Waves

• Frequency (Hz): Determines the pitch of a sound.
  • Human hearing range is 20 Hz - 20,000 Hz.
  • PTA 500 Hz – 4000 Hz
• Amplitude (dB): Determines loudness; logarithmic scale used.
• Wavelength (λ) and Speed (v): v=fλ
  • The speed of a wave is directly proportional to both its frequency and its wavelength.
• Timbre: Distinguishes different sources.

Types of Sound Waves

• Longitudinal waves are when the particles move in the direction of the wave propagation.
  • Sound waves are an example of longitudinal waves.
• Transverse waves are when the particles move perpendicular to the direction of the wave propagation.
  • Light waves and ripples on the surface of water are traverse waves.
• Pure tones are a sound consisting of only one frequency.
  • Appears as simple sine wave pattern.
  • A tuning fork produces a pure tone.
• Complex sounds are made up of multiple frequencies combined
  • Complex waveforms
  • A violin playing a note makes a complex sound.

Outer Ear

• Pinna: Gathers and directs sound waves.
• External Auditory Canal: Amplifies certain frequencies.

Middle Ear

• Tympanic Membrane: Converts sound waves into mechanical vibrations.
• Ossicles (Malleus, Incus, Stapes): Amplify and transmit vibrations.
• Eustachian Tube: Equalizes pressure.

Inner Ear

• Cochlea: Converts mechanical vibrations into neural signals.
• Organ of Corti: Detects frequency-specific vibrations.
• Inner ear cochlear hair cells are sensory cells that detect sound and convert it into electrical signals for the brain.
• Basilar Membrane: Tonotopic organization (high frequencies at base, low at apex).
• How sound is transmitted:
  • Sound waves cause vibration in inner ear fluids
  • Vibrations cause the basilar membrane to vibrate.
  • Vibrations deflect the stereocilia
  • Stereocilia respond to the fluid motion and send receptor potentials to the brain.

Auditory Pathway

• Cochlear nerve → Brainstem → Thalamus → Auditory Cortex (Temporal Lobe).

Perception of Pitch and Loudness

• The place theory of hearing states that different frequencies of sound stimulate different locations along the basilar membrane within in the cochlea
  • Allowing the brain to perceive different pitches based on where the vibrations occurred on the membrane.
  • Different "places" on the cochlea correspond to different sound frequencies.
  • High frequencies stimulate the base, and low frequencies stimulate the apex.
• Temporal coding for low frequencies is how the auditory system encodes low-frequency sounds by the timing of neural firing.
  • Neurons effectively "lock" the neuron firing rate to the frequency of the sound wave.
  • Most effective for low frequencies due to neuron's ability to phase lock to the sound cycles at slower rates.
  • High frequencies are primarily encoded by the location of activation on the basilar membrane ("place coding")

Binaural Hearing and Sound Localization

• Interaural time difference (ITD) refers to the time it takes for a sound to reach each ear.
• Interaural level difference (ILD) is the difference in sound intensity between both ears.
  • Both ITD and ILD's are cues the brain uses to determine the direction of the sound source in space.

Hearing Loss and Disorders

• Conductive Hearing Loss: Outer/middle ear dysfunction.
• Sensorineural Hearing Loss: Inner ear or nerve damage.
• Presbycusis: Age-related high-frequency loss.
• Tinnitus: Perception of sound without external stimulus.

Medical Applications and Audiology

• Include hearing aids and cochlear implants
• Early diagnosis and treatment is very important.

Explores the physics of sound, hearing mechanisms, and clinical applications. It details sound wave properties like frequency and amplitude, the physiological structures involved in hearing, and sound transmission. It provides insights into audiology.