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Chapter 10: Sound and the Ears (The Ear (Cochlea (Organ of Corti (inner…
Chapter 10: Sound and the Ears
Sound
Sources of Sound
usually initiated by movement that disturbs air molecules, causing them to collide with other air molecules, resulting in changes in air pressure that propagate outward from source
when pushed, molecules are compressed; when moves back, process called rarefaction
air molecules themselves don’t travel far as they oscillate back and forth, but waves of pressure change
sound waves:
waves of pressure change in air caused by the vibrations of a source
cycle:
in a sound wave, a repeating segment of air pressure changes
as gets further from source of sound, covers more space so sound energy at any given point on wave front decreases with distance from the source
example of inverse square law
Physical and Perceptual Dimensions of Sound
Frequency and Pitch
frequency:
the physical dimension of a sound that is related to the perceptual dimension of pitch; expressed in hertz, the number of cycles per second of a periodic sound wave
pitch:
the perceptual dimension of sound that corresponds to the physical dimension of frequency; the perceived highness of lowness of a sound
hertz (Hz):
the number of cycles per second of a sound wave; the physical unit used to measure a frequency
usually range from about 20 to 20,000 Hz
Amplitude and Loudness
Audibility Curve: The Absolute Threshold of Hearing
absolute threshold for sound depends on sound’s frequency
much more sensitive to 1,000 Hz tones than to 100Hz tones
audibility curve:
a curve showing the minimum amplitude at which sounds can be detected at each frequency
threshold at lower and higher frequencies much greater than it is at frequencies near middle, around 500-50,000Hz
auditory sensitivity is maximal in middle range, which happens to be range of frequencies present in most human speech sounds
Equal Loudness Contours
audibility curve doesn’t tell us how to quantify loudness of such sounds or how the loudness varies with frequency
ask participants to make loudness-matching judgments, say when two different tones seem equally loud
can make judgments despite frequency or pitch
match loudness of all frequencies to loudness of 1,000 Hz tone at any given amplitude
experiment performed for all frequencies, to determine what amplitude each must have in order to sound just like as loud as a 1,000 Hz tone set at 50dB SPL
equal loudness contour:
a curve showing the amplitude of tones at different frequencies that sound about equally loud
phon:
a unit of loudness; the loudness of a tone in phons is numerically equal to the amplitude of a 1,000 Hz tone that sounds equally loud
although we are most sensitive to frequencies in the middle of the frequency range for low-amplitude sounds, this difference in sensitivity tends to decrease as amplitude increases
amplitude:
the difference between the maximum and minimum sound pressure in a sound wave; the physical dimension of sound that is related to the perceptual dimension of loudness
the vertical distance from a peak to a trough in the waves
loudness:
the perpetual dimension of sound that is related to the physical dimension of amplitude; how intense or quiet a sound seems
perceived loudness also depends on frequency of the sound and other factors
periodic sounds that aren’t pure tones have peaks and troughs that vary in height; the peak amplitude within some time interval defined as the largest peak-to-trough difference during that interval
decibels (dB):
a physical unit used to measure sound amplitude; logarithmically related to sound pressure measured in micropascals
because amplitude measured in terms of sound pressure, decibel must also be based on sound pressure, but relationship between decibels and sound pressure is logarithmic, not linear
express amplitude of sound in decibels as dB SPL = 20 log(p/p0) where p is measured sound pressure
as physical amplitude of a sound doubles, the perceived loudness increases approximately by a constant amount
JND is intensity of sound about 1 dB
Waveform and Timbre
very few commonly heard sounds approximate pure tones
most consist of multiple pure tones added together
all waveforms can be decomposed into a collection of sine waves with various frequencies and amplitudes (called the frequency components of the complex wave) by applying the mathematical procedure
Fourier analysis:
a mathematical procedure for decomposing a complex waveform into a collection of sine waves with various frequencies and amplitudes
Fourier spectrum:
a depiction of the amplitudes at all frequencies that make up a complex waveform
fundamental frequency:
the frequency of the lowest-frequency component of a complex waveform; determines the perceived pitch of the sound
harmonic:
a component frequency of a complex waveform that is an integer multiple of the fundamental frequency; the first harmonic is the fundamental frequency; the second harmonic is twice the fundamental frequency, and so on
second and higher harmonics can be referred to as overtones
quality of compels periodic sounds depends on frequency and amplitude of fundamental frequency and of each overtone
timbre:
the difference in sound quality between two sounds with the same pitch and loudness; for complex periodic sounds, timbre is mainly due to differences in the relative amplitudes of the sounds’ overtimes; the perpetual dimension of sound that is related to the physical dimension of waveform
very low amplitude overtones contribute less to timbre than do higher-amplitude overtones
differences in relative amplitudes of harmonics are what give each sounds its distinctive timbre
when remove only the fundamental frequency and keep harmonics, there is a difference in the sound quality, pitch of sound seems to be about the same even though fundamental frequency (which determines pitch) was removed
called illusion of the missing fundamental
shows that auditory system uses pattern of frequencies in sound’s harmonics as part of the perception of pitch
perception of timbre doesn’t depend just on harmonic structure of periodic sound; also manner of its onset and offset (attack and decay)
may quickly attain maximum amplitude and then quickly fade
much more difficult to tell different timbres apart when first and last part of sound are trimmed off
many nonperiodic sounds differ in ways that we typically attribute to timbre, even though can’t easily be characterized in terms of fundamental frequencies and harmonics or attack and decay
three important physical dimensions of sound- frequency, amplitude, waveform; related to pitch, loudness, timbre
periodic sound waves:
waves in which the cycles of compression and rarefaction repeat in a regular, or periodic, fashion
different from aperiodic sound waves, the sound waves associated with an abrupt/turbulent event
pure tone:
a sound wave in which air pressure changes over time according to a mathematical formula called a sine wave, or sinusoid
simplest periodic wave
The Ear
Pinna, Auditory Canal, and Tympanic Membrane
pinna:
the outermost portion of the ear
consists of fat and cartilage
shape of pinnae can modify the incoming sound in a way that contributes to sound localization
auditory canal:
a narrow channel that funnels sound waves gathered by the pinna onto the tympanic membrane and that amplifies certain frequencies in those waves
about 25mm long and 6 mm in diameter
also amplifies frequencies in range of 2,000-5,000 Hz, which contributes to the high sensitivity to sound at these frequencies
tympanic membrane (or eardrum):
a thin, elastic diaphragm at the inner end of the auditory canal that vibrates in response to the sound waves that strike it; forms an airtight seal between the outer ear and the middle ear
Ossicles and Sound Amplification
ossicles:
three small bones (the malleus, incus, and stapes) in the middle ear that transmit sound energy from the tympanic membrane to the inner ear
malleus (or hammer):
a small bone in the inner ear; one of the ossicles; transmits sound energy from the tympanic membrane to the incus
stapes (or stirrup):
a small bone in the inner ear; one of the ossicles; transmits sound energy from the incus to the oval window
incus (or anvil):
a small bone in the inner ear; one of the ossicles; transmits sound energy from the malleus to the stapes
when tympanic membrane vibrates, it moves the malleus, the malleus causes the incus to move, which in turn displaces the stapes, which then transmits vibrations to the oval window
oval window:
a membrane-covered opening at the base of the cochlea; vibrations of the membrane transmit sound energy from the ossicles into the cochlea
cochlea is filled with fluid and sound vibrations from air have insufficient energy to cause significant vibrations in that fluid, so need bones to transmit sound from tympanic membrane into cochlea
if sound waves in the air were transmitted directly to the fluid-filled cochlea, there would be about 30 dB loss in the sound energy
two characteristics of ear anatomy help compensate for loss of sound energy
tympanic membrane about 15-20 times larger in area than oval window; all sound energy collected by the tympanic membrane is concentrated on much smaller area, amplifying its effect
the physical arrangement of the ossicles produces a sort of lever action, with the result that the relatively small force applied to the malleus by the vibration of the tympanic membrane is amplifies into a relatively large force applied to the oval window by the stapes; action of ossicles magnifies the vibrations of the tympanic membrane
these two anatomical characteristics of the ear amplify the sounds in the air by 20-30 dB, just about completely offsetting the loss in the transmission from air to fluid
Eustachian Tube
in order for tympanic membrane to vibrate effectively in response to incoming sound, the air pressure on the outer side of the membrane must be approximately equal to the air pressure on the inner side
greater pressure on inner side of membrane makes membrane stretch, which dampens its vibrations and causes a muffling sound
Eustachian tube:
a tube connecting the middle ear and the top part of the throat; normally closed but can be briefly opened (e.g., by swallowing or yawning) to equalize the air pressure in the middle ear with the air pressure outside
returns tympanic membrane to its ‘neutral’ state
Cochlea
Basilar Membranes
basilar membrane:
a tapered membrane suspended between the walls of the cochlea; thicker, narrower, and stiffer at the base than at the apex
pressure waves in perilymph set up traveling waves in basilar membrane, displacing the membrane to various degrees along its length, depending on the frequencies in the traveling waves
stiffness of membrane at each location along its length that is the main determinant of how much the membrane moves at that location in response to different frequencies
the stiff base responds most readily to high frequencies and floppy apex responds most readily to low frequencies
characteristic frequency:
the frequency to which each location on the basilar membrane responds most readily
traveling wave displaces the basilar membrane by some amount at each location, depending on the amplitudes of the various frequencies composing the incoming sound wave and on the characteristic frequency at each location
basilar membrane separates out the frequencies of the sinusoidal components of the complex wave, basically performing a Fourier analysis
Organ of Corti
Inner Hair Cells and Outer Hair Cells
auditory transduction occurs when inner hair cels release neurotransmitters as result of depolarization of cell membrane
neurotransmitter release initiates action potentials in Type I auditory nerve fibers, which carry information to brain about frequency/amplitudes of incoming sound waves
auditory information enhanced by action of outer hair cells
in outer hair cells, depolarization of cell membrane results in change in shape of protein called prestin in membrane
shape change causes cell body and stereocilia to execute physical movements similar to stretching/contracting
motile response:
a response by outer hair cells that magnifies the movements of the basilar membrane, amplifying sounds and sharpening the response to particular frequencies
changes cell length by as much as 2-3%
except for motile response, little known about function of outer hair cells, partially because Type II nerve fibers only 5% of auditory nerve
study found Type II nerve fibers respond only to very intense sounds, speculated that Type II fibers may play role in perception of sounds that are loud enough to be painful
organ of Corti:
a structure in the cochlea situated on the basilar membrane; consists of three critical components—inner hair cells, outer hair cells, and the tectorial membrane
is within cochlear duct and rests on/extends length of basilar membrane
responsible for auditory transduction
inner hair cells:
neurons in the organ of Corti; responsible for auditory transmission
one row of about 3,500 inner hair cells
outer hair cells:
neurons in the organ of Corti; serve to amplify and sharpen the responses of inner hair cells
three rows of about 12,000 outer hair cells total
tectorial membrane:
a membrane that lies above the hair cells in the organ of Corti
stereocilia:
small hairlike projections on the tops of inner and outer hair cells
each hair cell has 50-150 stereocilia
inner and outer hair cells differ in a number of ways
outer hair cells cylindrical, inner pear-shaped
tips of outer hair cell stereocilia are attached to tectorial membrane while tips of inner hair cell stereocilia aren’t, just float free in endolymph
only inner hair cells are responsible for transfusing sound into neural signals
outer hair cells serve to amplify/sharpen responses of inner hair cells
auditory nerve:
the nerve that conveys signals from the hair cells in the organ of Corti to the brain; made up of Type I and Type II auditory nerve fibers bundled together
inner hair cells connected to Type I auditory nerve fibers
each Type I connects to just one or two inner hair cells; each inner hair cell connects to multiple fibers
Type I are thick and myelinated, promotes rapid conduction of action potentials
outer hair cells connected to Type II auditory nerve fibers
each Type II connects to 30-60 outer hair cells
Type II thinner and unmyelinated, results in relatively slower conduction of action potentials
of 30,000 nerve fibers that bundle to make auditory nerve of each ear, 95% Type I and 5% Type II
Stereocilia Bending and Tip Links
when basilar membrane moves upward, stereocilia at tips of outer hair cells, which are in contact with overarching tectorial membrane, bent by a shearing force due to different motions of the two membranes, which have separate anchor points
stereocilia of inner hair cells bent in same direction by resistance of surrounding endolymph
for sound near auditory threshold, stereocilia move less than 1 nanometer
tips of adjacent stereocilia are connected to one another with tiny fibers called tip links
tip links:
tiny fibers connecting the tips of adjacent stereocilia on hair cells; increased tension on tip links pulls open ion channels in the membranes of the stereocilia
when stereocilia bend, distance between tip links increases, which pulls open ion channels in membranes of stereocilia to which they’re attached
positively charged potassium and calcium ions enter hair cell through open channels, causing cell membrane potential to depolarize; leads to critical reactions in both types of hair cells
cochlea:
a coiled, tapered tube within the temporal bone of the head, partitioned along its length into three chambers; contains the structures involved in auditory transduction
about 33mm long, with a diameter of about 5mm wide at base and 2mm at apex
vestibular canal:
one of the three chambers in the cochlea; separated from the cochlear duct by Reissner’s membrane; filled with perilymph
cochlear duct:
one of the three chambers in the cochlea; separated from the tympanic canal by the basilar membrane; contains the organ of Corti; filled with endolymph
tympanic canal:
one of the three chambers in the cochlea; separated from the cochleae duct by the basilar membrane; filled with perilymph
resting on basilar membrane within cochlear duct is structure responsible for auditory transduction, the organ of Corti
helicotrema:
an opening in the partitioning membranes at thee apes of the cochlea; provides an open pathway for the perilymph to carry vibrations through the cochlea
when stapes presses in on oval window at base of vestibular canal, initiates wave of pressure in perilymph that ultimately presses out on the membrane-covered round window
round window:
a membrane-covered opening at the base of the tympanic canal in the cochlea; serves as a king of ‘relief valve’ for the pressure waves traveling through the perilymph
vestibular and tympanic canals sealed off from cochlear duct by partitioning membranes
ear is the peripheral part of the auditory system; the structure that transduces sound into neural signals that are sent to the brain
conventionally divided into three parts- the outer ear, middle ear, inner ear
outer ear consists of three parts- the pinna, the auditory canal, and the outer surface of the tympanic membrane
principle function is to funnel sound from the environment onto the tympanic membrane, which vibrates in response to the sound waves
vibrations transmitted into the middle ear, a tiny air-fulled chamber containing three small bones, the ossicles, that pick up and amplify the vibrations of the tympanic membrane and transmit them into the inner ear
specifically, transmit them to the cochlea, the specialized structure containing neurons that transduce the vibrations into neural signals that are then sent to the brain via the auditory nerve
Neural Representation of Frequency and Amplitude
Frequency Representation
Place Code Frequency
Physiological Frequency Tuning Curves
measurements of basilar membrane movements made using microscope and strobe light
more precise measurements since then that have been compared with responses of Type I auditory nerve fibers to same frequencies
nerve fibers have characteristic frequencies like locations on basilar membrane do
frequency tuning curves of Type I fibers and basilar membrane nearly identical, ,shows that frequency tuning of Type I auditory nerve fibers can be almost entirely accounted for by frequency tuning of basilar membrane, a purely mechanical factor
frequency tuning curves of several different Type I auditory nerve fibers become progressively narrower as characteristic frequency increases, meaning that nerve fibers with higher characteristic frequencies are more sensitive to frequency differences between incoming sounds near characteristic frequency than are nerve fibers with lower characteristic frequencies
place code provides relatively better frequency representation of high-frequency sounds than of low-frequency sounds
Type I auditory nerve fibers with different characteristic frequencies project to correspondingly different positions along the basilar membrane
provides clear evidence for a place code for frequency
Psychophysical Frequency Tuning Curves
psychophysical methods can also be used to generate frequency tuning curves based on the perceptual judgment of human listeners
present a target tone (pure tone with given frequency and fairly low amplitude) against silent background then narrowband white noise presented simultaneously with target tone
narrowband white noise refers to sound waves with equal amplitude at all frequencies within a narrow band of frequencies
narrowband white noise presented with target tone called noise masker and is characterized by its center frequency and its level
level of masker then adjusted so it just barely prevents the listener from detecting the target tone, level called the masked threshold
center frequency of noise masker then changed in increments across range of frequencies surrounding the frequency of target tone, and masked threshold determined for each center frequency
lowest-level masked threshold typically associated with a masker that has a center frequency equal or very nearly equal to frequency of target tone and masked threshold generally increases as the center frequency of the masker increasingly departs from the frequency of the target tone
across all target tones, these patterns generate center frequency tuning curves that closely resemble the frequency tuning curves of auditory nerve fibers, presumably because listeners’ perceptual judgments are based on the responses of Type I auditory nerve fibers
when noise masker has center frequency at or near frequency of target tone, masker activated Type I auditory nerve fibers in the region of the basilar membrane with a characteristic frequency corresponding to the frequency of the target tone
since those nerve fibers already responding to the noise masker, can’t also indicate the presence of the target tone
as center frequency of masker moves away from target tone frequency, auditory nerve fibers in that region are freed up to respond to target tone
when masker made louder, begins to activate nearby auditory nerve fibers, inducing those most sensitive to target tone
psychophysical tuning curves provide evidence supporting place code for frequency
Temporal Code for Frequency
based on a match between the frequencies in incoming sound waves and the firing rates of Type I auditory nerve fibers
possibility arises from fact that movements in inner hair cell stereocilia are caused by and time locked to displacements in basilar membrane, which are themselves time locked to the changes in air pressure of an incoming sound wave
not necessarily a 1:1 ratio of frequency:action potentials per second because can hear sounds at frequencies approaching 20,000 Hz but neurons cannot produce action potentials at a rate above about 1,000 spikes per second
however, time locking mechanism can work as long as each nerve fiber in population of fibers produces action potentials in phase with the incoming sound stimulus
as long as action potentials are produced at the same time as the peaks in the incoming sound wave, even if not at every peak
volley principle:
the idea that each nerve fiber in a population of auditory nerve fibers produces action potentials in phase with the peaks in the incoming sound wave, even if not at every peak; explains how a temporal code could represent frequencies much higher than the maximum firing rate of any individual fiber
in experiments with squirrel monkeys, found that this type of phase synchronization breaks down for frequencies above 4,000-5,000 Hz because precision of phase locking is not great enough
a temporal code, with phase locking, can precisely represent frequencies up to about 5,000 Hz, while place code provides the sharpest representation for frequencies above 5,000 Hz
uses two different mechanisms to encode frequency in neural signals sent to brain
represented by the displacement of the basilar membrane at different locations, with different degrees of displacement resulting in correspondingly different rates of action potentials being sent along Type I auditory nerve fibers at those locations
place code:
frequency representation based on the displacement of the basilar membrane at different locations
represented by a match between the frequencies in incoming sound waves and the timing of action potentials sent by Type I auditory nerve fibers in brain
temporal code:
frequency representation based on a match between the frequencies in incoming sound waves and the firing rates of auditory nerve fibers
Amplitude Representation
generally, rate of action potentials produced by auditory nerve fibers increases with amplitude of incoming sound wave
however, not that simply because range of amplitudes we can hear/discrimination (dynamic range) is much greater than range of firing rates of any one auditory nerve fiber
dynamic range:
the range of amplitudes that can be heard and discriminated; when applied to an individual auditory nerve fiber, the range of amplitudes over which the firing rate of the fiber changes
means that firing rate of any one nerve fiber is not sufficient to represent al the different amplitudes we can discriminate
one way auditory system copes with this is based on the increase in the number of nerve fibers that respond to a tone of a given frequency as its amplitude increases
some of the Type I auditory nerve fibers respond to tones of different frequencies as the amplitude of the tone increases
for a 500Hz tone, fiber B begins firing when the tone reaches an amplitude of about 37 dB SPL, fiber A at about 55 dB SPL, and fiber C at about 58 dB SPL
increase in number of nerve fibers responding to a given tone as the tone’s amplitude increases is based on the fact that an individual nerve fiber doesn’t respond just to tones with the fiber’s characteristic frequency; each fiber responds to range of frequencies
as tone increasingly departs from fiber’s characteristic frequency, typically must have higher amplitude to produce a response
such patterns include the responses of hundreds or thousands of auditory nerve fibers that fire in response to a given tone
number of possible patterns of different fibers firing at different rates enables the auditory system to discriminate very small differences in amplitude
fibers can also differ in hw they respond to tones that match their characteristic frequency
for any given nerve fiber, there is a range of low-amplitude (quiet) sounds that evoke no increase in firing rate
after the tone’s amplitude passes the fiber’s threshold, the firing rate increases steadily as the amplitude increases, until the firing rate reaches the fiber’s saturation level
range of amplitudes over which the nerve fiber’s firing rate increases from baseline to saturation is called that fiber’s dynamic range
when firing rate is plotted against amplitude across a large number of fibers, the resulting curves tend to be S-shaped, although individual fibers can generate curves that depart somewhat from that idealized shape
different auditory nerve fibers have different thresholds and different dynamic ranges
just as auditory system can use patterns of response of nerve fibers with different characteristic frequencies to gauge the amplitude of an incoming sound, so it can also use the patterns of thresholds and dynamic ranges to gauge amplitude
Disorders of Audition
Hearing Tests and Audiograms
audiometer:
an instrument that presents pure tones with known frequency and amplitude to the right or left ear; used in estimating the listener’s absolute threshold for specific frequencies and to construct an audiogram
essentially use staircase method to estimate person’s absolute threshold for each of 6-8 frequencies between 250 and 8,000 Hz
audiogram:
a graphical depiction of auditory sensitivity to specific frequencies, compared to the sensitivity of a standard listener; used to characterize possible hearing loss
hearing level (auditory sensitivity) measured relative to the audibility curve of the standard listener
standard listener would have hearing level of 0dB SPL at every frequency
if person can detect a tone of 4,000 Hz at about the same amplitude required for detection by the standard listener, person is said to have hearing in the normal range at that frequency
if can detect tone only if it’s 50 dB louder than level required by standard listener, said to have a 50 dB hearing loss in tested ear at 4,000 Hz
hearing impairments, whatever the degree of hearing loss, can be broadly categorized into two general categories depending on how they arise- conductive hearing impairments and sensorineural hearing impairments
Conductive Hearing Impairments
conductive hearing impairments:
hearing impairments characterized by a loss of sound conduction to the cochlea, as a result of problems in the outer or middle ear
could be blockage of auditory canal, perforated/torn tympanic membrane, or damage to ossicles
can also be caused by otosclerosis, growth of bone in the middle ear that interferes with movement of ossicles
temporary conductive hearing loss can accompany inflammation in middle ear, called otitis media or an earache
could also be caused by buildup of cerumen (earwax) that blocks auditory canal
conductive hearing loss usually not profound (loss of 90 dB or more) because eve if ear canal is blocked or ossicles not working properly, sound is still conducted to cochlea via vibrations in bones in which cochlea is embedded
usually just have to amplify sounds with hearing aid
Sensorineural Hearing Impairments
Age-Related Hearing Impairment
presbycusis:
age-related hearing impairment
many environmental/biological factors can contribute to this
could be lifelong exposure to noise or certain industrial chemicals, smoking/alcohol abuse, diabetes/cardiovascular disease, head trauma, poor nutrition
may be a genetic component
loss is most pronounced at higher frequencies and greater for men than women
Noise-Induced Hearing Impairments
prolonged exposure to sounds with an amplitude greater than about 85 dB SPL likely to cause noise-induced hearing loss
when sounds reach about 120 dB SPL, even short-term exposure is likely to cause hearing loss
can be temporary/reversible or permanent, depending on noise level and its duration
noise-induced hearing loss often maximal at 4,000 Hz and less severe at 8,000 Hz, leading to a V-shaped audiogram
different from age-related hearing loss, which tends to become progressively more severe as the frequency increases
several ways in which delicate cells and membranes of cochlea can be damaged by loud noise
one is mechanical damage due to very high amplitude pressure waves pulsing through the cochlea
can tear basilar membrane, separating it from the walls of the cochlear, or can damage outer hair cells by pulling their stereocilia out of their insertion points in the temporal membrane or by breaking the tip links between stereocilia
another is hair cell death, which can take hours or days to occur following noise exposure; can be caused by
excitotoxicity, in which excessive amounts of neurotransmitter glutamate are released, causing swelling in and damage to auditory nerve fibers, with death of the hair cells to which the fibers connect
reduced cochlear blood flow due to mechanical damage to the wall of the cochlea, which can kill hair cells
production of oxygen-based free radicals, molecules that can destabilize other molecules, damaging tissues and causing hair cells to die
when outer hair cells die, critical amplification function is lost, reducing auditory sensitivity at the frequencies corresponding to the positions of the dead hair cells along the basilar membrane
when inner hair cells also die, even more dramatic loss
some factors contributing to hearing loss can’t be avoided, like loud noises
earplugs are usually most effective in protecting against low-frequency noises while earmuffs are most effective in protecting against high-frequency noises
sensorineural hearing impairments:
hearing impairments caused by damage to the cochlea, the auditory nerve, or the auditory areas or pathways of the brain
can be congenital or acquired and can range from minor deficits in hearing some frequencies to total deafness
congenital deafness occurs in about 1 of 1,000 births and is usually due to a recessive gene inherited from both parents
acquired sensorineural hearing impairments are much more common and have many causes, most common are effects of normal aging and effects of exposure to loud noise
Tinnitus
more than 50 million Americans experience some degree of tinnitus, and more than 5% of adults over 50 have it so badly it can cause troubles falling asleep
can be loud/quit, in one/both ears, intermittent/continuous
causes variable and poorly understood
include damage to cochlea, irritation of/pressure on auditory nerve by blood vessel/tumor, changes in neural circuits within auditory cortex
often associated with noise-induced hearing loss, but no clear correlation between severity of tinnitus and magnitude of hearing loss
no single treatment works in all cases
some drugs aim at reducing neural activity in the auditory nerve/cortex
hearing aids enhance desirable sounds
electrical stimulation of auditory nerve/magnetic stimulation of auditory cortex aimed at reducing excitability of these parts of auditory system
hearing impairment:
a decrease in a person’s ability to detect or discriminate sounds, compared to the ability of a healthy young adult