HEARING CONSERVATION
Trina Redford, Industrial Hygienist
National Naval Medical Center
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NOISE EVALUATION
OBJECTIVE
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To familiarize ourselves with
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the anatomy and physiology of the ear
the purpose of a noise evaluation
types of sound level meters utilized to collect
measurements
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ANATOMY AND PHYSIOLOGY OF
THE EAR
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Main Components of the
Hearing Mechanism:
Outer Ear
 Middle Ear
 Inner Ear
 Central Auditory Nervous System
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Structures of the Outer Ear
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Auricle (Pinna)
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Collects sound
Localization
Amplifies sound
(approx. 5-6 dB)
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External Auditory Canal:
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Approx. 1 inch in
length
“S” shaped
Lined with cerumen
glands
Outer 1/3 surrounded
by cartilage
Inner 2/3’s surrounded
by mastoid bone
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Mastoid Process
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Bony ridge behind the
auricle
Provides support to
the external ear and
posterior wall of the
middle ear cavity
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Tympanic Membrane:
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Thin membrane
Forms boundary
between outer and
middle ear
Vibrates in response to
sound
Changes acoustical
energy into
mechanical energy
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The Ossicles:
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A: Malleus
B: Incus
C: Stapes
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Smallest bones in the body
Acts as a lever system
Footplate of stapes enters oval
window of the cochlea
Stapedius Muscle
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Connects stapes to wall of middle
ear
Contracts in response to loud
sounds (called the Acoustic Reflex)
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Eustachian Tube
(AKA: “The Equalizer”)
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Lined with mucous
membrane
Connects middle ear to
nasopharynx
“Equalizes” air
pressure
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Structures of the Inner Ear
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Cochlea
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Snail shaped organ with a
series of fluid-filled tunnels
Converts mechanical energy
to electrical energy
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Organ Of Corti:
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The end organ of
hearing
Contains stereocilia
and hair cells.
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Hair Cells:
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Frequency specific
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High pitches= base of
cochlea
Low pitches= apex of
cochlea
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Vestibular System
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Consists of three semicircular canals
Shares fluid with the
cochlea
Controls balance
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Central Auditory System
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VIIIth Cranial nerve or “Auditory Nerve”
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Carries signals from cochlea to brain
Auditory Cortex
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Temporal lobe of the brain where sound is
perceived and analyzed
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How Sound Travels Through
The Ear...
1. Acoustic energy, in the form of sound waves, is channeled into the ear canal by
the pinna
2. Sound waves hit the tympanic membrane and cause it to vibrate, like a drum,
changing it into mechanical energy
3. The malleus, which is attached to the tympanic membrane, starts the ossicles into
motion
4. The stapes moves in and out of the oval window of the cochlea creating a fluid
motion
5. The fluid movement causes membranes in the Organ of Corti to shear against
the hair cells
6. This creates an electrical signal which is sent up the Auditory Nerve to the brain
The brain interprets it as sound!
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QUESTIONS?
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NOISE HAZARDS
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NOISE HAZARD EVALUATION
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Purpose
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To identify noise hazardous areas
To document the magnitude of the noise hazard
To aid in the implementation of a hearing
conservation program
To protect personnel from developing a noiseinduced hearing loss
To implement engineering controls
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Survey Intervals
Annually-in all areas to identify potential
noise hazards AND to re-survey all
previously identified noise hazards
 Within 30 days of a procedural or
equipment change that affect ambient noise
 Whenever noise makes it difficult for two
people, with good hearing, to converse at
arm’s length
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Who can perform sound level
surveys?
Audiologist
 Industrial Hygienist
 A suitably trained technician
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Sound Level Meters
Calibrated noise measuring device which meets the criteria of
ANSI Standard S1.4-1971 (R 1983)
Microphone (transducer)
 Amplifier
 Weighting network
 Slow/fast averager
 Display meter
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Analog
Digital
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Type I Precision SLM
Tolerance of + or –1 dB accuracy
 Individual octave band measurements
 Required for booth certification
 Cost: $2,000.00
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Type II SLM
Used routinely for surveys in the field
 Has individual weightings networks:
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A weighing
B weighing
C weighing
Cost: $400.00 to $600.00
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Procedures for operation
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Check electroacoustic calibration date-must be within one year
Set meter response to appropriate weighing for calibrator, and
slow response
Check meter response with field calibrator –must be within 1 dB
of calibration source level
Set weighing to “A” and meter response to Slow
Adjust meter range until display is seen
Read display
Record results on NEHC form 5100/17Recheck field calibration
Maintain records for 40 years
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Source of Error
Reading (averaging) errors
 Wrong scale setting
 Wrong microphone position
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Distance from Source
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Inverse square law applies:
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Each time the distance from a source is
doubled, in a free field, the sound pressure level
drops by 6dB
This principal is used to define the noise hazard
radius
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Care of SLM
Things to Avoid
Excessive heat
 High humidity (>90%)
 Hard blows or shocks
 Handling of microphone
 Excessive dust
 Strong electromagnetic fields
 Pegging the needle
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Sound Levels and
Hearing Conservation
Noise measurements are essential
 HCP enrollment base on:
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Noise survey data-sound level surveys or
dosimetry
Job/task factors-length of exposure, level of
noise/vibration
Patient specific factors-pre-existing hearing loss
or ear disease
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Test Room Certification
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Principals of Sound Reduction
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Permit test signal to be heard at threshold levels
Reduce sound by producing an acoustical
barrier to sound transmission
Reduce high frequencies more than low
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Factors Affecting Performance
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Vibration
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Vibration isolators or shock mounts for control
Ventilation systems and muffler
 Door seals
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Test with the light test or paper test
Lighting
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Accuracy of Sound Level
Readings – Affecting Factors
The body baffle effect
 Shielding effect
 Wind noise
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Masking
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Masking Effect: when two sounds are
present at the same time
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One signal may mask or cover up the other
This can result in false threshold for hearing
during audiometric testing
Masking typically occurs when the
interfering noise is low in frequency
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CONCLUSION
Questions
 Comments
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Thank you for your attention & participation
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