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Employees exposed to excessive in-plant industrial noise may be at risk to suffer a variety of physiological and psychological consequences. Other intangible effects have also been hypothesized to be caused by stress associated with noise exposure. Prevention of hearing loss is the focus of hearing   conservation programs. It is the only physiological effect that has an undisputed, well documented association to noise exposure in humans. The table below summarizes some of the possible effects that are linked to noise exposure. Total employee exposure includes recreational sources.

Effects Linked To Noise Exposure


Noise Induced Hearing Loss

The basic mechanism of hearing involves converting sound waves hitting the ear drum to structure-borne vibrations transmitting through bones in the middle ear. From there, vibrations are changed into nerve impulses in the cochlea of the inner ear. The fluid filled cochlea contains 40,000 tiny hair cells like the one shown at right (magnified) that initiate the nerve impulse which is transmitted to the brain. With repeated exposure to excessive noise, these hair cells lose some of their resilience and may even break off resulting in sensorineural or noise induced hearing loss. Hearing loss is permanent because once damaged, the hair cells can never be repaired or replaced.


The Federal Noise Standard For Employee Exposure As Developed By OSHA

 (Occupational Safety & Health Administration)



Key Points

  • 8 hours at 90 dBA equals the permissible exposure level (100% dose)
  • 8 hours at 85 dBA equals 50% of the permissible exposure level (PEL)
  • The OSHA Standard was formulated to minimize not eliminate the risk of hearing loss
  • Dosimeters, not sound level meters, are used to establish employee exposure/dose
  • Feasible engineering controls MUST be implemented when the equivalent dose for 8 hours exceeds 90 dBA or where continuous noise is over 115 dBA

OSHA Derating Instructions For All Hearing Protectors Using NIOSH Method #2


  1. Take NRR from package     29 NRR
  2. Subtract (7) dB                – 7dB 
  3. Divide by (2) or 50%        ÷2
  4. OSHA adjusted NRR          11

A comprehensive hearing conservation program guideline was added to the original OSHA standard. The hearing conservation   amendment outlines requirements for annual audiometric testing, training and documentation/ record keeping.

OSHA Compliance Strategy Summary







Action Level (AL)

The time weighted average (TWA) exposure which requires program inclusion, hearing tests, training and optional hearing protection.

AL=85 dBA TWA. AL is exceeded when TWA≥85 dBA, integrating all sounds from 80 to at least 130 dBA.

Permissible Exposure Limit (PEL)

The TWA, which when exceeded, requires feasible engineering and MSHA administrative controls, and mandatory hearing protection.

PEL=90 dBA TWA. PEL is exceeded when TWA > 90 dBA, integrating all sounds from 90 to at least 140 dBA (see 62.101, Definitions).

Exchange Rate

The rate at which exposure accumulates; the change in dB TWA for halving/doubling of allowable exposure time.

5 dB

Ceiling Level

The limiting sound level above which employees cannot be exposed.

P code violation issued for any protected or unprotected exposures >115 dBA SPL.


Assessment of noise exposure.

Mine operator must establish system to evaluate each miner's exposure sufficiently to determine continuing compliance with rule.

Noise Control

Investigation and implementation of feasible engineering and administrative control measures.

Feasible engineering and administrative controls required for TWA>90dBA; even if controls do not reduce exposure to the PEL, they are required if feasible (i.e. ≥ 3 dBA reduction). No credit for HPD allowed. Administrative controls must be provided to the miner in writing and posted.

Hearing Protection

Exposure requirements and conditions for use of hearing protection devices (HPDs).

Optional for ≥ 85 dBA TWA; mandatory for > 90 dBA TWA, or for ≥ 85 dBA TWA for workers with STS. Amount of protection no specified; choices must include 2 plugs and 2 muffs. Double hearing protection (muff plus plug) required at exposures > 105 dBA TWA.

Supervisor of Audiometric Testing

The person responsible for conducting audiometric tests, and is the reviewer of audiograms.

Licensed or certified audiologist, or physician.

STS Standard Threshold Shift

A change in hearing compared to an earlier (baseline) hearing test that requires follow-up action.

≥ 10 dB average shift from baseline hearing levels at 2000, 3000 and 4000 Hz in either ear.

STS Retests

Follow-up audiogram that is permitted or required when initial STS is detected.

May obtain retest within 30 days and substitute for annual audiogram.

STS Follow-up

Required actions when an STS is detected.

Notify worker within 10 days; unless STS is not work-related, must retrain the miner, provide miner an HPD or a different HPD, and review effectiveness of any engineering and administrative controls to correct deficiencies.





Noise Reduction

Ratings (NRR)

May Over



A recent study by Dennis A. Giardino and George Durkt, Jr. of the Mine Safety and Health Administration published in the American Industrial Hygiene Association Journal compared field performance to the EPA noise reduction rating (NRR). The results from some 1,265   hearing protective device (HPD) evaluations showed that field performance is significantly less than that specified by the NRR, especially for low frequency noise. Results also showed that the NRR is not a good indicator for comparing relative performance of HPD models.


Conducting An Industrial Noise Control Survey

Listed below are several of many possible questions that need to be researched as part of the noise control survey to determine feasible and practical treatments/solutions. Meeting the acoustical requirements alone is not sufficient to solve the problem in most cases. Identifying all the needs of the noise control treatment can best be accomplished by soliciting input from all disciplines that might be affected.   Forming a noise control team with representatives from maintenance, operations, quality control, production, safety, engineering, management, etc. will assure the best possible design to meet the needs of all involved. Preliminary input from the parties involved leads to “ownership” and “buy–in” creating a sense of accomplishment for the team.