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HEARING CONSERVATION. Trina Redford, Industrial Hygienist National Naval Medical Center. NOISE EVALUATION OBJECTIVE. To familiarize ourselves with 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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hearing conservation


Trina Redford, Industrial Hygienist

National Naval Medical Center

noise evaluation objective
  • To familiarize ourselves with
    • the anatomy and physiology of the ear
    • the purpose of a noise evaluation
    • types of sound level meters utilized to collect measurements
main components of the hearing mechanism
Main Components of the Hearing Mechanism:
  • Outer Ear
  • Middle Ear
  • Inner Ear
  • Central Auditory Nervous System
structures of the outer ear
Structures of the Outer Ear
  • Auricle (Pinna)
    • Collects sound
    • Localization
    • Amplifies sound (approx. 5-6 dB)
external auditory canal
External Auditory Canal:
  • 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
mastoid process
Mastoid Process
  • Bony ridge behind the auricle
  • Provides support to the external ear and posterior wall of the middle ear cavity
tympanic membrane
Tympanic Membrane:
  • Thin membrane
  • Forms boundary between outer and middle ear
  • Vibrates in response to sound
  • Changes acoustical energy into mechanical energy
the ossicles
The Ossicles:
  • A: Malleus
  • B: Incus
  • C: Stapes
    • Smallest bones in the body
    • Acts as a lever system
    • Footplate of stapes enters oval window of the cochlea
  • Stapedius Muscle
    • Connects stapes to wall of middle ear
    • Contracts in response to loud sounds (called the Acoustic Reflex)
eustachian tube aka the equalizer
Eustachian Tube (AKA: “The Equalizer”)
  • Lined with mucous membrane
  • Connects middle ear to nasopharynx
  • “Equalizes” air pressure
structures of the inner ear
Structures of the Inner Ear
  • Cochlea
    • Snail shaped organ with a series of fluid-filled tunnels
    • Converts mechanical energy to electrical energy
organ of corti
The end organ of hearing

Contains stereocilia and hair cells.

Organ Of Corti:
hair cells
Hair Cells:
  • Frequency specific
    • High pitches= base of cochlea
    • Low pitches= apex of cochlea
vestibular system
Vestibular System
  • Consists of three semi-circular canals
  • Shares fluid with the cochlea
  • Controls balance
central auditory system
Central Auditory System
  • VIIIth Cranial nerve or “Auditory Nerve”
    • Carries signals from cochlea to brain
  • Auditory Cortex
    • Temporal lobe of the brain where sound is perceived and analyzed
how sound travels through the ear
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!

noise hazard evaluation
  • Purpose
    • To identifynoise 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 noise-induced hearing loss
    • To implementengineering controls
survey intervals
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
who can perform sound level surveys
Who can perform sound level surveys?
  • Audiologist
  • Industrial Hygienist
  • A suitably trained technician
Sound Level MetersCalibrated 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
    • Analog
    • Digital
type i precision slm
Type I Precision SLM
  • Tolerance of + or –1 dB accuracy
  • Individual octave band measurements
  • Required for booth certification
  • Cost: $2,000.00
type ii slm
  • Used routinely for surveys in the field
  • Has individual weightings networks:
    • A weighing
    • B weighing
    • C weighing
  • Cost: $400.00 to $600.00
procedures for operation
Procedures for operation
  • 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
source of error
Source of Error
  • Reading (averaging) errors
  • Wrong scale setting
  • Wrong microphone position
distance from source
Distance from Source
  • Inverse square law applies:
    • 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
care of slm things to avoid
Care of SLMThings to Avoid
  • Excessive heat
  • High humidity (>90%)
  • Hard blows or shocks
  • Handling of microphone
  • Excessive dust
  • Strong electromagnetic fields
  • Pegging the needle
sound levels and hearing conservation
Sound Levels and Hearing Conservation
  • Noise measurements are essential
  • HCP enrollment base on:
    • 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
test room certification
Test Room Certification
  • Principals of Sound Reduction
    • 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
factors affecting performance
Factors Affecting Performance
  • Vibration
    • Vibration isolators or shock mounts for control
  • Ventilation systems and muffler
  • Door seals
    • Test with the light test or paper test
  • Lighting
accuracy of sound level readings affecting factors
Accuracy of Sound Level Readings – Affecting Factors
  • The body baffle effect
  • Shielding effect
  • Wind noise
  • Masking Effect: when two sounds are present at the same time
    • 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
  • Questions
  • Comments

Thank you for your attention & participation