1. Trina Redford, Industrial Hygienist National Naval Medical Center
HEARING CONSERVATION
Trina Redford, Industrial Hygienist
National Naval Medical Center

2. 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

3. ANATOMY AND PHYSIOLOGY OF THE EAR

4. Main Components of the Hearing Mechanism:
Outer Ear
Middle Ear
Inner Ear
Central Auditory Nervous System

5. Structures of the Outer Ear
Auricle (Pinna)
Collects sound
Localization
Amplifies sound (approx. 5-6 dB)

6. 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

7. Mastoid Process Bony ridge behind the auricle
Provides support to the external ear and posterior wall of the middle ear cavity

8. Tympanic Membrane: Thin membrane
Forms boundary between outer and middle ear
Vibrates in response to sound
Changes acoustical energy into mechanical energy

9. The Ossicles: A: Malleus B: Incus C: Stapes Stapedius Muscle
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)

10. Eustachian Tube (AKA: “The Equalizer”)
Lined with mucous membrane
Connects middle ear to nasopharynx
“Equalizes” air pressure

11. Structures of the Inner Ear
Cochlea
Snail shaped organ with a series of fluid-filled tunnels
Converts mechanical energy to electrical energy

12. Organ Of Corti: The end organ of hearing
Contains stereocilia and hair cells.

13. Hair Cells: Frequency specific High pitches= base of cochlea
Low pitches= apex of cochlea

14. Vestibular System Consists of three semi-circular canals
Shares fluid with the cochlea
Controls balance

15. 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

16. 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!

17. QUESTIONS?

18. NOISE HAZARDS

19. NOISE HAZARD EVALUATION
Purpose
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 noise-induced hearing loss
To implement engineering controls

20. 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

21. Who can perform sound level surveys?
Audiologist
Industrial Hygienist
A suitably trained technician

22. Sound Level Meters Calibrated noise measuring device which meets the criteria of ANSI Standard S (R 1983)
Microphone (transducer)
Amplifier
Weighting network
Slow/fast averager
Display meter
Analog
Digital

23. Type I Precision SLM Tolerance of + or –1 dB accuracy
Individual octave band measurements
Required for booth certification
Cost: $2,000.00

24. Type II SLM Used routinely for surveys in the field
Has individual weightings networks:
A weighing
B weighing
C weighing
Cost: $ to $600.00

25. 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

26. Source of Error Reading (averaging) errors Wrong scale setting
Wrong microphone position

27. 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

28. 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

29. 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

30. 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

31. 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

32. Accuracy of Sound Level Readings – Affecting Factors
The body baffle effect
Shielding effect
Wind noise

33. Masking 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

34. CONCLUSION Questions Comments
Thank you for your attention & participation