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: 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.

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).

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 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).

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.

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).

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.

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.

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 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.

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.

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-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.