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Ch 10

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Ch 10

The Auditory System Learning Outcomes

  • Key Questions

    • How can sound be described physically and perceptually?

    • What factors contribute to the timbre of a sound?

    • What does the equal loudness curve represent?

    • What are the structures and functions of the outer, middle, and inner ears?

    • What are the receptors for audition, and how does transduction occur?

    • What are the pathways for audition?

    • How do different theories explain pitch perception?

    • What are the symptoms and treatments for tinnitus? How can hearing loss be prevented?

Sound Definition

  • Sound: Waves in the air produced by a vibrating object, detected by the auditory system.

    • Distal stimulus: The vibrating object.

    • Proximal stimulus: Pattern of energy at the eardrum.

Physical Aspects of Sound

  • Phase: Point along the wave, measured in degrees (no direct perceptual counterpart).

  • Amplitude: Displacement of wave from peak to trough.

    • Range of audible amplitudes: threshold = 0.0002 dynes/cm², maximum = 200 dynes/cm²; difference of 1000×.

    • Decibels (dB) used for large amplitude ranges: dB = 20 log (p /p0) where p = sound pressure level, p0 = threshold SPL (20 µPa).

  • Frequency: Number of sound wave cycles per second; measured in Hertz (Hz).

    • Formula: Hz = # of cycles/time (s).

Perceptual Aspects of Sound

  • Loudness: Perceptual experience of sound intensity (not "volume"). Associated with amplitude & sound pressure.

    • To measure: compare to a standard sound (1000 Hz pure tone at 40 dB = 1 sone of loudness).

    • Doubling loudness: increase dB by ~10 dB (e.g., 60 dB is twice as loud as 50 dB).

  • Pitch: Quality of a sound; allows sounds to be ordered on a musical scale.

    • Range: 20 - 20,000 Hz.

    • Affected by intensity:

      • High-frequency sounds seem higher pitched as intensity increases.

      • Low-frequency sounds seem lower pitched as intensity increases.

    • Duration effects: sound < 10 ms long is heard as a “click.”

Sound Wave Dynamics

  • Sound waves: pressure variations in air due to surface vibration.

  • Waves may cause compression (high pressure) and rarefaction (low pressure).

    • Cycle: Time between two consecutive high peaks.

    • Sound energy weakens across time and space.

    • Speed of sound varies; e.g., travels faster in water vs. air.

  • Pure tones: Sound waves where pressure changes follow a sine wave format; most sounds are complex waves.

    • Waveform: Represents different frequencies heard at a particular pitch.

    • High amplitude = loud (high intensity).

Frequency Relationships in Music

  • Chromatic Scale: One note is an octave above another when its frequency is double. Subdivided into 12 intervals equally spaced logarithmically.

  • Fourier Analysis: Analyzes musical notes; lowest frequency in spectrum is fundamental.

    • Harmonics: Frequencies that are integer multiples of the fundamental.

  • Timbre: Character of sound; differences in sound quality for different instruments playing the same note.

Auditory Thresholds and Loudness

  • Each frequency has a different threshold.

  • Equal Loudness Curve: Describes absolute threshold for hearing different frequencies; auditory response area ranges from threshold to pain.

  • Phase Cancellation: Sound waves out of phase can cancel each other out (e.g., noise-canceling headphones).

Anatomy of the Ear

Outer Ear

  • Pinna: External “ear” channels sound waves into the external auditory meatus, amplifying frequencies between 2000 to 5000 Hz.

  • Tympanic Membrane (Eardrum): Vibrates in response to sound waves, transmits sound to middle ear structures.

Middle Ear

  • Ossicles: Malleus, incus, stapes (amplify sound).

    • Concentrate vibrations from the larger eardrum to the oval window.

    • Eustachian Tube: Equalizes pressure between middle ear and environmental pressure.

Inner Ear

  • Cochlea: Coiled structure with three fluid-filled chambers; responsible for sound transduction.

  • Hair Cells: Located in the Organ of Corti; transduce mechanical vibrations into neural signals.

Transduction and Auditory Pathways

  • Transduction Steps: Vibration of eardrum causes movement through ossicles to the oval window and cochlear partition, creating shear forces that bend hair cell cilia.

  • Auditory Pathways: Axons of spiral ganglion converge to form auditory nerve; superior olives send feedback to outer hair cells.

Theories of Pitch Perception

  • Place Code: Specific neurons encode different frequencies; activated hair cells indicate perceived pitches.

  • Frequency Theory: Entire basilar membrane vibrates synchronously with sound frequency.

  • Volleys: Neurons fire alternately to encode higher frequencies.

Tinnitus

  • Definition: Ringing sound in ears without external stimulus affecting 13 million North Americans.

  • Commonly caused by loud sounds, certain drugs, and ear infections.

  • Treatments: No cure, but therapies like tinnitus maskers may help.

Noise and Hearing Loss

  • Noise-Induced Hearing Loss (NIHL): Significant increase in auditory disorders; chronic exposure leads to irreversible damage.

  • Prevention: Limit exposure to loud noises, use hearing protection, be aware of occupational risks.