chapter 11 - hearing

physical definition of sound

pressure changes in the air or other medium

perceptual definition of sound

the experience we have when we hear

amplitude

difference in pressure between high and low peaks of wave

frequency

number of cycles within a given time period, measured in Hz

pitch

perceptual quality we describe as high and low

pure tone

• a tone with a sinusoidal wave form

• sounds like a beep

complex tone

• the summation of many pure tones

• most sounds in the word

fundamental frequency/first harmonic

repetition rate

additive synthesis

process of adding harmonics to create complex sounds

frequency spectrum

display of harmonics of a complex sound

human hearing range

20 to 20,000 Hz, most sensitive to 2,000 to 4,000 Hz

audibility curve

shows the threshold of hearing in relation to frequency

auditory response area

falls between the audibility curve and the threshold for feeling, shows range of response for human audition

timbre

• all other perceptual aspects of a sound besides loudness, pitch, and duration

• related harmonics, attack, and decay of a tone

periodicity pitch

removal of the first harmonic results in a sound with the same perceived pitch, but with a different timbre

attack of tones

buildup of sound at the beginning of a tone

decay of tones

decrease in sound at end of tone

structures of the outer ear

pinna and auditory canal

pinna

helps with sound location

auditory canal

• tube-like 3 cm long structure

• protects the tympanic membrane at the end of the canal

• resonant frequency of the canal amplifies frequencies between 1,000 and 5,000 Hz

middle ear structures

• tympanic membrane (barrier between outer and middle ear)

• oval window (barrier between middle and inner ear)

• ossicles (malleus, incus, and stapes)

tympanic membrane

eardrum

oval window

leads to cochlea

malleus

• hammer

• moves due to the vibration of the tympanic membrane

incus

• anvil

• transmits vibrations of malleus

stapes

• stirrup

• transmits vibrations of incus to inner ear via the oval window of the cochlea

ossicles bones

• malleus, incus, and stapes

• can amplify vibrations by ~20x

• protects against strong sounds (tensing during swallowing, talking, general body movement, tensor tympani and stapedius muscles)

inner ear structures

cochlea - fluid filled, snail-like structure that vibrates

inner hair cells

• convey almost all information about sound waves to brain

• 3,500 total

outer hair cells

• convey information from brain (efferent fibers)

• elaborate feedback system

• 10,500 total

• stiffen to suppress noise, less stuff can tune to specific frequency

bakery’s place theory of hearing

• frequency of sound is indicated by the place on the organ of corti that has the highest firing rate

• high pitch: proximal end of cochlea (base)

• low pitch: distal end of cochlea (center of coil)

phase locking

firing of a single neuron at one distinct point in the period (cycle) of a sound wave at a given frequency

volley principle

multiple neurons can provide a temporal code for frequency if each neuron fires at a distinct point in the period of a sound wave but does not fire on every period

auditory nerve

• CN VIII or vestibulocochlear nerve

pathway to brain

1. auditory nerve

2. cochlear nucleus

3. superior olive

4. inferior colliculus

5. medial geniculate nucleus

6. primary auditory cortex

tinnitus

ringing in the ears

presbycusis (hearing loss)

• greatest loss at high frequencies

• affects males more severely

• appears to be caused by exposure to damaging noises or drugs

noise-induced hearing loss

• loud noises can severely damage the organ of corti

• leisure noise can also cause hearing loss

conduction loss

• bone problems, wax build-up, infection (otitis media), otosclerosis

• treated with hearing aids

sensorineural hearing loss

• most common, and most serious auditory impairment

• due to defects in cochlea or auditory nerve, when hair cells are injured