Unit 1B Part 4 Hearing (Updated 2025)

Main ideas, concepts, and lessons

1) What sound is (and what its properties mean)

• Sound waves are vibrations in the air that travel as waves and are captured by the ears.

• Two key physical characteristics:

  • Wavelength

    • Definition: distance between two peaks of a sound wave
    • Perception: determines pitch
    • Short wavelength → higher pitch (e.g., whistle)
    • Long wavelength → lower pitch (e.g., drum)

  • Amplitude

    • Definition: height of the sound wave
    • Perception: determines loudness
    • Taller wave → louder (e.g., shout)
    • Shorter wave → quieter (e.g., whisper)

• Core lesson: The ear uses wavelength (pitch) and amplitude (loudness) to perceive tones and volume.

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2) The auditory (hearing) system as a whole

• The auditory sensory system (aka audition) detects and processes environmental sounds.

• General processing chain (sound → brain):

  1. Sound wave enters the outer ear
  2. Sound causes vibrations of the eardrum
  3. Vibrations are transmitted through the middle ear
  4. Vibrations are converted into electrical signals in the inner ear
  5. Electrical signals go to the thalamus
  6. Signals are processed in the temporal lobe (auditory cortex) and recognized as sounds

• Core purpose: detection, differentiation, and response to sounds (communication, alerting to danger, enjoying music).

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3) College Board emphasis (memorization vs understanding)

• Students do not necessarily need to memorize exact ear anatomy.
• Instead, students should:
  • Explain the general process
  • Understand structures and functions broadly
  • Avoid focusing on specific labeled parts

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4) Detailed step-by-step: how hearing works (auditory pathway)

A. Outer ear and middle ear (mechanical processing)

• Sound waves enter from a sound source (speech, music).

• Pinna / outer part of ear (shaped like a funnel)

  • Captures and guides sound waves into the ear canal
  • Helps direct and amplify sound waves

• Ear canal

  • Tube that concentrates and guides sound further

• Eardrum (aka tympanic membrane)

  • Thin, flexible membrane sealing the outer ear from the middle ear
  • Vibrates when struck by sound waves

• Middle ear includes three tiny bones (“ossicles”):

  • Malleus (hammer)
  • Incus (anvil)
  • Stapes (stirrup)

• Transfer sequence:

  • Eardrum vibrates → hammer (malleus) moves
  • Hammer vibrations → anvil (incus) passes vibrations onward
  • Stapes transfers vibrations to the oval window of the cochlea

B. Inner ear (transduction into neural signals)

• Cochlea (inner ear)

  • Spiral-shaped, fluid-filled, snail-shell-like structure
  • Contains an opening called the oval window (sealed by a small membrane)

• Fluid and membrane vibrations:

  • Stapes vibrates the oval window → creates fluid waves in the cochlea
  • Fluid waves travel along the basilar membrane
  • The basilar membrane vibrates in response

• Hair cells and transduction:

  • Hair cells sit on the basilar membrane
  • Basilar membrane vibration bends hair cells
  • This bending generates electrical signals
  • This conversion is transduction: vibration → electrical signals for the nervous system

• Neural processing:

  • Electrical signals travel via the auditory nerve → thalamus
  • Processed in the auditory cortex in the temporal lobe as recognizable sounds

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5) Pitch theories (what explains pitch perception)

College Board requirement: students should explain three pitch theories.

1) Place theory

• Idea: Pitch depends on where the cochlea vibrates most.
• Mechanism:
  • Imagine uncoiling/stretching the cochlea like a piano keyboard
  • High-pitched sounds → base vibrates more
  • Low-pitched sounds → tip vibrates more
• Summary: Pitch is determined by the location of vibration in the cochlea.

2) Frequency theory

• Idea: Pitch depends on the rate of neural firing matching sound frequency.
• Mechanism:
  • Low-frequency sounds → the basilar membrane and neurons vibrate in sync
  • Neurons fire at a rate matching the sound’s frequency
  • Example: a 200 Hz sound → auditory nerve fibers fire 200 times per second
• Analogy: neurons as drummers
  • low pitch → slow beat
  • high pitch → fast beat
• Summary: Neural firing rate matches sound frequency.

3) Volley theory

• Idea: For higher frequencies, groups of neurons alternate (“take turns”) firing.
• Mechanism:
  • A single neuron may not fire fast enough for high-pitch signals
  • Multiple neurons work together like a relay race
  • High-pitched sound represented by alternating firing across multiple neurons
• Summary: Groups of neurons send higher-frequency information.

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6) Sound localization (finding where a sound comes from)

• Defined: ability to identify the location of a sound.

• Main cues (when both ears function normally):

  1. Time difference between ears (interaural timing)

     • Example: a clap from the right reaches the right ear sooner than the left
     • The brain uses the time lag to infer direction

  2. Loudness difference between ears (interaural intensity)

     • Example: sound is louder in the ear closer to the source because the head blocks sound to the farther ear

• Additional cue:

  • The outer ear (pinna) shape modifies sound depending on whether the source is above/below/front/behind.
  • The brain uses these modifications to help determine position.

• Key takeaway: The brain uses multiple cues, especially time difference and loudness difference.

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7) Types of hearing loss/deafness (categorized by where the problem occurs)

A) Conductive deafness (conductive hearing loss)

• Cause location: outer and middle ear
  • These parts normally conduct sound vibrations into the ear.
• Example causes:
  • Ear infections
  • Blockages (e.g., something stuck)
  • Earwax buildup
  • Perforation of the eardrum
  • Malformation/structural problems in outer/middle ear parts

B) Sensory-neural deafness (sensory neural hearing loss)

• Cause location: inner ear / auditory pathway
• Hearing loss occurs when:
  • Hair cells in the cochlea are damaged, or
  • Auditory nerve fibers are defective/damaged
• Common causes:
  • Aging and long-term exposure to loud sounds (hair cell damage)
  • Genetics
  • Disease
  • Head trauma
• Assistive technologies:
  • Hearing aids: amplify sound into the ear
  • Cochlear implants: detect sound outside the ear and transmit electrical signals directly into the cochlea

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8) Review questions (as presented)

1. Yaroslav listens to music: sound waves cause which option to vibrate first?
2. Low notes on a tuba, played quite loudly: which option best explains the physical properties of the sound waves?
3. Dr. Cisme study on sensory-neural hearing loss: which option is an appropriate operational definition for the variable of interest?

Instruction: pause the video to answer; correct answers are displayed at the end (not included in the provided subtitles).

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Speakers / sources featured

• Course instructor / presenter (unnamed; leads the lesson and review questions)
• College Board (referenced as the source of AP Psychology objectives/expectations)
• Miracle-Ear (hearing aid company advertisement used as the visual depiction of the auditory system)