Physical Aspects of Sound

Hearing: Chapter 11 Notes

Physical Aspects of Sound

The Auditory System

• Components include outer ear, middle ear, inner ear, auditory pathways, and auditory cortex.

Theories of Pitch Perception

• Place Theory: Pitch is determined based on which hair cells are activated in the cochlea.

• Frequency (Volley) Theory: Pitch is represented by the rate at which the nerve impulses fire.

Auditory Pathways

• Signal is processed from the cochlea to several nuclei before reaching the auditory cortex.

Hearing Loss

• Types: presbycusis (age-related), noise-induced hearing loss, and congenital hearing loss.

Infant Hearing

• Infants show preference for mother's voice and can recognize familiar stories heard in utero.

Why is Sound Important?

Communication

• Sound is essential for verbal interaction and social bonding.

Object Identification

• Enables identifying sources in the environment.

Object Location

• Sound helps determine the position of objects relative to the listener.

Definitions of “Sound”

Physical Definition

• Sound as pressure changes in the air or other medium.

Perceptual Definition

• Sound as the experience that arises when we perceive auditory stimuli.

Sound as Pressure Changes

Components of Sound Waves

• (a) Increase in pressure (compression)

• (b) Decrease in pressure (rarefaction)

Pure Tones

• Represented as sine waves where:

  • Amplitude: The difference in pressure between peak and trough of the wave, perceived as loudness.

  • Frequency: Number of cycles per second measured in Hertz (Hz).
    ( 1 ext{ Hz} = 1 ext{ cycle per second} )

Measurement of Loudness and Pitch

Loudness Scale

• Measured in Decibels (dB), with 0 dB being the threshold of hearing and common sounds like conversation measured at around 60 dB.

Frequency Range

• Frequencies range from 20 Hz to 20,000 Hz.

  • Infrasound: < 20 Hz

  • Ultrasound: > 20,000 Hz

Table of Relative Amplitudes and Decibels for Environmental Sounds

Complex Tones & Frequency Spectra

Understanding Waveforms and Harmonics

• Complex sounds are composed of a fundamental frequency and its harmonics.

  • Harmonics result from non-pure tones (e.g., musical instruments).

  • The waveform can show different frequencies using additive synthesis.

Audibility Curve

Representation of Hearing Thresholds Across Frequencies

• Displays how loud sounds must be at various frequencies for a listener to detect them, measured in dB across 20 Hz to 10,000 Hz.

Tonal Relationships and Pitch

Frequency Tones and Piano Keyboard Mapping

• Examples of specific frequencies that correspond to musical notes on a piano keyboard:

  • A0 (27.5 Hz), C1 (32.7 Hz), D4 (293.7 Hz), A4 (440 Hz), C6 (1046.5 Hz).

The Auditory System Overview

Components

• Outer Ear: Audiological structures collecting sound.

• Middle Ear: Includes tube structures and ossicles (malleus, incus, stapes).

• Inner Ear: Contains the cochlea and associated structures responsible for converting sound vibrations into neural signals.

The Cochlea Structure

• Components:

  • Scala vestibuli, scala tympani, and cochlear partition which contains auditory hair cells.

  • Transduction process occurs here.

Hair Cell Responses in Cochlea

• Inner and outer hair cells are responsible for detecting sound and transmitting these signals to the auditory nerve fibers.

• Tip links open ion channels allowing ion flow that creates action potentials in auditory nerve fibers when stimulated by sound waves.

Theories of Pitch Encoding

Place Theory

• The location on the basilar membrane where hair cells are stimulated determines perceived pitch.

Frequency Theory

• The rate at which auditory nerve fibers fire corresponds to the frequency of the sound wave. This is effective for frequencies below 4000 Hz while the volley principle is effective for those above.

Place Theory Mechanism

Békésy’s Contributions

• Investigated the basilar membrane's mechanics and frequency response. The base is stiffer and narrower than the apex of the cochlea, contributing to frequency discrimination.

Tonotopic Organization

• The cochlea has a tonotopic map where low frequencies stimulate the apex and high frequencies stimulate the base, facilitating frequency-specific processing.

Cochlear Implants

• Cochlear implants bypass damaged hair cells, allowing sound to stimulate auditory nerve fibers through electrodes inserted into the cochlea. The components include a microphone, sound processor, transmitter, and electrodes providing auditory signals to the brain.

The Central Auditory Pathways

Pathway Overview

• Involves structures such as the cochlear nucleus, inferior colliculus, medial geniculate nucleus, and primary auditory cortex. These areas communicate auditory information from both ears to enable sound localization and processing.

Patient Studies and Auditory Performance

• Studies have shown that damage to the primary auditory cortex (A1) worsens a patient's performance in sound duration, frequency discrimination, and more.

Auditory Pathway Functionality

What and Where Auditory Processing

• The “What” pathway correlates with the processing of sounds, typically located in the temporal lobe, while the “Where” pathway aids in sound localization, found in the frontal lobe.

Types of Hearing Loss

Presbycusis and Noise-Induced Hearing Loss

• Presbycusis: Gradual, age-related loss, primarily affecting high frequencies.

• Noise-Induced: Damage to cochlear structures from loud sounds, also affecting performance based on frequency exposure and excess sounds.

Hidden Hearing Loss

• Research shows underlying damage may not reflect behavioral thresholds, leading to discrepancies in auditory perception, even after noise exposure.

Infant Hearing Development

In Utero Hearing

• Fetuses have shown responsiveness to sounds including their mother’s voice and familiar stories read during pregnancy, indicating the early development of auditory perception.

Auditory Threshold Development in Infants

• Infants demonstrate gradually maturing auditory thresholds with time, aligning with changes in the ear and auditory cortex maturation.