Hearing Conservation
TDI-Brooks International
Hearing Conservation
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Ear Anatomy
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Outer Ear
Middle Ear
Inner Ear
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Parts of the Ear
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External ear
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Pinna or auricle- portion attached to the outer surface of the
head
External ear canal
Middle ear
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Eustachian tube
Ear drum
Ossicular chain (hammer, anvil, stirrup)
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Oval window and round window
 Mastoid air cell system
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Inner ear
Cochlea
 Organ of Corti
 Vestibular system (semi-circular canals)
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External ear
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Pinna-visible part of the ear, functions as a
funnel to collect sound
External ear canal-separates the external ear
from the middle ear, tube through which
sound travels to the eardrum.
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Middle ear is between the ear drum and
the bony wall of the inner ear. The
ossicles (bones) are located here.
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Eustachian tube-connects middle ear to the
back of the nose. equalizes the pressure in
the middle ear with atmospheric pressure
(opens during yawning and swallowing).
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Ear drum-separates external ear canal from
the middle ear and is connected to the oval
window by the ossicles (conduct sound). This
is a thin membrane that vibrates when sound
waves hit it.
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Ossicular chain-hammer, anvil and stirrup.
These are three small bones. Sound is
transferred from the eardrum to the oval
window. The hammer is set into motion by
movement of the eardrum which then causes
the anvil and stirrup to move and vibrate the
oval window.
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Oval window and round window-located on
the inner wall of the middle ear. Move in
response to sound and transmit mechanical
motion into the fluid of the inner ear.
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Inner ear-contains the receptors for
hearing and position sense (balance).
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Vestibular receptive system-organ of
equilibrium. Three semi-circular canals filled
with fluid that respond to motion. Nerves then
conduct this information to the brain about
position.
Cochlea-looks like a snail shell, also house
the organ of corti (center of sense of hearing).
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Vibrations of the stirrup set the fluid of the
cochlea into motion which then stimulates hair
cells attached to nerve endings that sets up
electrical impulses that are transmitted to the
brain and interpreted as sound.
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Basically, outer ear collects sound waves
from the air and funnels them into the ear
canal where they are transported to the
ear drum. The collected sound waves
cause the eardrum to move back and forth
in a vibrating mechanical motion that is
passed on to the bones of the middle ear.
The stirrup is embedded in the oval
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window. As the stirrup rocks back and
forth it passes vibrations on to the inner
ear through the oval window and into the
fluid of the inner ear (cochlea). The
movement of the fluid interacts with the
hair cells in the cochlea which are
connected to nerves that transmit the
sound signals to the brain.
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As we age we steadily loose hearing
acuity, particularly high frequencies.
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A young ear can hear sounds from 20 Hz to
20,000 Hz. A person in their 60’s may be able
to hear up to 12,000 Hz.
Human speech ranges from 300 to 4000 Hz.
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Types of hearing loss
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Conductive-somehow the conductive
processes are damaged, the inner ear is
intact (damage to outer or middle ear)
Sensorineural-hearing loss due to damage to
the deep ear structures or auditory nerve.
(Sensori-organs of the inner ear, neural-nerve
fibers)
Mixed hearing loss-some combination of both
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Occupational hearing loss is defined as a
hearing impairment of one or both ears,
partial or complete that results form ones
employment.
Two categories of hearing loss
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Acoustic trauma
Noise-induced hearing loss
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Acoustic trauma is the injury to the
sensorineural elements of the elements of
the inner ear. Acoustic trauma is
produced by one or a few exposures to
sudden, intense noise from blasts,
explosions or by direct trauma to the ear.
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Noise induced hearing loss is the
cumulative, permanent of the
sensorineural type that develops over
month or years of hazardous noise
exposure.
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Sound is any pressure variation that the
ear can hear. The number of pressure
variations over time is frequency (Hz).
Noise-unwanted sound.
Measure sound in dB (decibels)
Loudness depends upon sound pressure
and impacted by frequency (pitch).
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Hearing loss risk factors
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Intensity of the noise (sound pressure level)
Type of noise (frequency)
Period of daily exposure
Total duration of exposure
Individual susceptibility
Age
Coexisting hearing loss and ear damage
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First 4 are most important and are called
noise exposure factors.
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Sound Pressure
(uPa)
20
Sound Pressure
Level (dB)
Example
0
Threshold of
hearing
2,000
40
Quiet office
200,000
80
Very noisy
restaurant
2,000,000
100
Loom in textile
mill, electric
furnace
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Sound Pressure
(uPa)
20,000,000
Sound Pressure
Level (dB)
120
Example
200,000,000
140
Threshold of pain,
jet plane
Hydraulic press
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Examples:
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20% of the population between 50 and 59
experience a hearing loss of 20% without any
industrial noise exposure..
However, this increases to 27% if exposed to
steady state noise at 90 dB, and is 36% if
exposed to 95 dB
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Current beliefs are that exposure of the
unprotected ear to sound levels about 115
dB is hazardous.
Exposure to sound levels below 70 dB is
safe and does not produce any permanent
hearing loss.
The majority of industrial noise exposure
fall within this range.
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Frequency also factors into hearing loss.
Noise about 500 Hz has a greater
potential for causing hearing loss.
The incidence of noise-induced hearing
loss is directly related to total exposure
time. Intermittent exposures are less
damaging to the ear.
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Classes of Noise Exposure
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Continuous (broadband)
Intermittent
Impact-sharp burst of sound
Permissible Noise Exposure
(OSHA)
Duration per Day (h)
Sound level (dB)
8
6
4
3
90
92
95
97
2
1.5
1
100
102
105
0.5
0.25
110
115
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Controls
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Engineering-add sound controls
Administrative-alter work schedules to reduce
worker exposure, alter machine operation
times to expose fewer workers.
PPE-hearing protection devices-acoustic
barriers that reduce the amount of sound
energy transmitted through the ear canal to
the inner ear.
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Classes of hearing protection
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Enclosures (entire head)-helmets worn by astronautsoffers protection of 35 dB at 250 Hz to about 50 dB at
higher frequencies.
Aural inserts (ear plugs)
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Formable
Custom molded
premolded
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Superaural protectors (canal caps)-seal
external openings of the ear. Soft material
held in place by a light weight headband.
Circumaural protectors (earmuffs)-consist of
two cup shaped devices that fit over the entire
external ear held in place by headband.
According to OHSA standard, hearing
protection should be worn in when exposed to
85 dB for a 8 hour time weight average.
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What type of PPE to use depends upon
work location, sound frequency and fit to
individual, how often exposed to noise.
Check the noise reduction rating of
hearing protection device.