Auditory Perception and Music

Compressions (sound)

Areas of high density and pressure where particles are pushed together

Rarefactions (sound)

Areas of low density and pressure where particles are pulled apart

Sine wave (“pure tones'“)

Simplest sound wave, only has one frequency (the fundamental)

Frequency

Measured in Hz. Doubling —> octave up

Related to perceived pitch

Amplitude

Measured in dB

Related to the perceived loudness (intensity)

Harmonics

Higher multiples of a fundamental frequency —> gives sound “timbre” (unique qualities that distinguishes a voice from an instrument for ex)

Changes sound without changing its pitch

Waveform

Intensity over time

Spectrogram

Frequency and intensity over time

Pure tones = one frequency, one line

Complex tones = multiple frequencies at once, many lines

External auditory canal

Basilar membrane

Located within the cochlea; forms bulges due to sound —> pushes hair cells up

Hair cells

Specialized neurons

Auditory equivalent of photoreceptors (but they detect mechanical energy (pressures on the hair) instead of light)

How do hair cells work

Converts mechanical sound vibrations into electrical signals for the brain. Sound causes fluid in the cochlea to vibrate, bending hair-like stereocilia atop these cells, which opens ion channels, depolarizes the cell, and releases neurotransmitters to the auditory nerve

Too loud sounds → break tips of hair sounds → permanent damage because hair cells cannot repair themselves

Cochlea (“acoustic prism”)

Physical structure mirrors spectrogram

High frequencies stimulate hair cells near base, low frequencies stimulate hair cells near apex

Transmits electrical impulses to auditory complex

Cochlear implant

Artificially produces electrical impulses

Can stimulate much less variations of sound —> much lower sound quality

Conductive hearing loss

Vibrations inhibited due to ear wax buildup, infection, otosclerosis (degeneration of ossicles)

Hearing loss due to physical obstructions to ear

Sensorineuron hearing loss

Caused by damage to the inner ear’s hair cells or nerve pathway to the brain

Metabolic - can be caused by certain drugs (ototoxicity)

Sensory - caused by exposure to loud noises over long periods of time

Auditory cortex

Primary auditory cortex (A1) in the temporal lobe

After A1, splits into dorsal (where) and ventral (when) stream, like vision!

Tonotopic organization

Tonotopic organization

Different neurons react differently to different pitches, organized spatially

Binaural cues

Sound localization technique using auditory signals from both ears

ITD & ILD

Interaural Time Different (ITD)

Sound reaching opposite ear from source takes longer

Interaural Level Difference (ILD)

Sound reaching opposite ear from source is quieter

Monaural cues

Sound localization technique using auditory signals from one single ear

Pinna folds

Pinna folds

Shape of ear; used for sound localization as their shape filters incoming sound waves differently depending on their source

Cone of confusion

Region where you can’t discriminate the location of a sound

ITD and ILD are ambiguous

Best way to resolve = moving the head around

Localizing distance (sound)

The best for sound is 1 meter

Inverse square law —> we underestimate long distances

We are good at telling of things are approaching or receding

Reverberations

Sound bounces off surfaces

Sound localizing technique; if someone is far in a room, much of the sound will be bouncing off the surface. If someone is close, much of the sound will be direct

Auditory Stream Segmentation

We need to segment one “stream” (one source) of sound from others in an environment where they’re all mixed together

Use auditory grouping principles

Auditory Grouping Principles

Proximity (in time) - sounds occurring close together in time are likely to be perceived as one stream

Size and pitch - bigger things (ex: animal vocal tracts) vibrate slower —> lower pitch

Timbre

Continuity

Cocktail effect

Ability to focus attention on one speaker alone

Acoustic startle response

Very rapid motor response to a loud unexpected noise

Amusia

Inability to perceive / reproduce tone

“tone deafness”

Music agnosia

Inability to hear music holistically

Can be selective to music; cannot recognize familiar songs