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.