Audition

Airborne Sound Waves Relayed to the Inner Ear

1. Vibrating waves of air enter the outer ear and move to the eardrum.

   • Outer ear (pinna) funnels sound through the ear canal to the eardrum (tympanic membrane).

2. The eardrum transmits vibrations to ossicles in the middle ear.

   • Three tiny bones: hammer, anvil, and stirrup.

   • These bones pass vibrations to the cochlea in the inner ear.

3. The cochlea focuses vibrations on the basilar membrane.

   • The cochlea is a fluid-filled coiled tube, converting airborne waves to “seaborne” waves.

   • The stirrup vibrates against the oval window, setting cochlear fluid into wave motion.

   • This fluid wave causes vibration in the basilar membrane, a thin strip of hairy tissue.

4. The basilar membrane converts vibrations into neural messages.

   • Tiny hair cells on the basilar membrane sway, stimulating sensory nerve endings.

   • Excited neurons transform mechanical vibrations into neural activity.

5. Neural messages travel to the auditory cortex in the brain.

   • Signals leave the cochlea via the auditory nerve.

   • Neurons from both ears meet in the brain stem, passing information to both sides of the brain.

   • Signals arrive in the auditory cortex for higher-order processing.

Psychological Qualities of Sound: How We Distinguish One Sound from Another

Sound waves have two physical characteristics: frequency and amplitude, which the brain converts into three psychological sensations: pitch, loudness, and timbre.

Sensations of Pitch

• Frequency defines pitch: Higher frequencies produce high-pitched sounds; lower frequencies produce low-pitched sounds.

• Human Auditory Sensitivity: Ranges from $20 \text{ cps}$ to $20,000 \text{ cps}$.

• Pitch Perception Theories:

  • Place Theory: Different frequencies activate different locations on the basilar membrane. The pitch heard depends on which region receives the greatest stimulation.

    • Accounts for hearing high tones (above about $1,000 \text{ Hz}$).

  • Frequency Theory: Neurons on the basilar membrane respond with different firing rates to different sound wave frequencies.

    • Explains how the basilar membrane deals with frequencies below about $5,000 \text{ Hz}$.

  • Combined Action: Between $1,000$ and $5,000 \text{ Hz}$ (upper frequency range of human speech), hearing relies on both place and frequency.

Sensations of Loudness

• Amplitude defines loudness: More intense sound waves (larger amplitude) produce louder sounds; small amplitudes produce soft sounds.

• Measurement: Sound intensity is expressed in decibels (dB).

Sensations of Timbre

• Timbre: The complex quality of a sound wave that allows distinction between different sound sources (e.g., voices, instruments).

  • Most natural sound waves are mixtures rather than pure tones.

Hearing Loss

• Aging: Commonly involves loss of hearing acuity, especially for high-frequency sounds crucial for speech.

• Diseases: Can attack auditory nerves (e.g., mumps).

• Exposure to Loud Noises: Can damage hair cells in the cochlea (e.g., gunshots, jet engines, loud music).