Hearing & Audition

Fundamental Computations

1. Detection

2. Identification

3. Localization

Sound Waves

Air pressure variation over time

Air Pressure

Air molecule density

Frequency

• Pitch

• Cycles per second (Hz)

• Higher Hz → higher pitch

Amplitude

• Loudness

• Size of the pressure swing

• Larger → louder sound

Timbre

quality of the sound; how sounds are different even at the same frequency

Pinna & Canal

The visible outer part of the ear; collects sound waves from the environment and directs them down the auditory canal.

Eardrum (tympanic membrane)

Sound waves strike, causing it to vibrate;

membrane separating outer ear from middle ear

Ossicles

vibrations from the eardrum travel to malleus, incus, and stapes

Amplification

amplify sound waves and send them to the inner ear, through a membrane called the oval window

Cochlea

Snail-shaped, fluid-filled structure where sound is converted to action potentials

• Separates distinct sound frequencies

• Vibrations from middle ear cause fluid inside to move

Hair Cells

Within the cochlea, thousands of tiny cells detect fluid movement and bend

• Bending of cells converts physical vibrations into electrical impulses

Inner Ear

Sound Transmission and Conversion

Basilar Membrane

Witihin the cochlea; acts as a frequency analyzer

• vibrates at specific locations depending on the sound’s frequency

• stiff at the base (responds to high-frequency sounds)

• wide and flexible at the apex (responds to low-frequency sounds)

Tonotopic Map

Spatial representation of frequencies along the length of the cochlea

Inner Hair Cells

primary sensory transducers, responsible for converting vibrations into neural signals

• Release glutamate onto neurons (spiral ganglion neurons) which transmit information to the nervous system

• Not neurons, but have electrical properties

Outer Hair Cells

act as amplifiers, enhancing low-level sound to improve hearing sensitivity

Generation of Action Potential

K+ ions enter hair cell → depolarization → calcium channels open → further depolarization → releases neurotransmitter

Music and the Brain

Binaural Hearing

using both ears to perceive sound

Information available to you with just one ear:

• Frequency

• Intensity (loudness)

• Timing

Information available to you with both ears:

Location

Locating a Sound

• Sound must travel a slightly further distance to reach the ear farther away from the source

• This distance is enough for brain to determine where a sound is coming from

Binaural Cues

Location cues based on comparison of signals received by left and right ears

Interaural Time Difference (ITD)

Difference between the times that the sound reaches each ear

• no differences when same distance to each ear

Types of Hearing Loss

• Genetic impairment of the cochlea

• Infection-induced damage to cochlea

• Noise-induced hearing loss

Vestibular System

• Personal inertial guidance system: maintain postural balance, stabilize vision, and provide awareness of spatial position

• signals to control balance of body

• Dysfunction often causes vertigo, lightheadedness, dizziness, nausea, imbalance,