Noise Reduction Rating

By Graphic Products Editorial Staff

hearing protection sign on factory equipment

The Noise Reduction Rating (NRR) of hearing protection is a way for measuring the reduction in sound levels a hearing protection device can provide. The Noise Reduction Rating is expressed in decibels (dB), which is the normal way of measuring sound levels. The higher the Noise Reduction Rating means the more effective the hearing protection device is.

Noise Reduction Rating – Why Hearing Protection is Important

Unlike many other injuries, an injury to your hearing from loud noise is a permanent injury. Hearing loss can never be recovered. There is no surgery, nor hearing aid, that can repair the damage caused by loud noise.

Not all loud noise results in a permanent hearing lose. Hearing loss is dependent on both the loudness of the noise and the duration of the exposure. Short-term exposure may only have a temporary effect on hearing, such as causing a decreased ability to hear or causing a ringing in the ear. A short-term hearing problem may clear up within a few minutes or hours after the exposure to the noise ends. However, hearing damage is cumulative. Repeat exposures can lead to:

  • Limited ability to hear high frequency sounds.
  • Limited ability to understand speech.
  • Problems understanding and communicating with others.
  • Permanent hearing loss.

Hearing damage from exposure to noise can affect us in other ways. These include:

  • Creating both physical and psychological stress.
  • Reduced productivity.
  • Interference with communication can:
    • Reduce your ability to socialize with friends.
    • Result in difficulty playing with your children or grandchildren.
    • Lead to psychological and social isolation.
  • Reduced ability to concentrate.
  • Difficulty hearing warning signals (thus increasing other types of injuries).

Logarithmic Scale – What Level of Noise is Harmful?

Sound is measured using units called decibels. A logarithmic decibel scale called “A-weighted sound levels” (dBA) was designed to match the human perception of loudness. If a sound is logarithmic that means that a small change in the number of decibels is associated with a large change in the magnitude of the actual sound pressure waves. If the logarithmic increases, so does the potential for damage to hearing.

The limits OSHA has set on noise exposure in the workplace are based on the time weighted average (TWA) noise level measured throughout an eight hour day. This means that noise levels are measured at various times throughout the day, and then calculations are performed to determine the time weighted average.

The permissible exposure limit originally set by OSHA was 90 dBA for an eight hour day. Since hearing damage is related to both the sound level and the duration of the exposure, if the sound level was above 90 dBA, the allowable duration of exposure was decreased.  For every increase of 5 dBA, the maximum exposure time was cut in half. This is called a 5 dBA exchange rate. What it means is that, if the exposure was measured to be 95 dBA, then workers may only be exposed to that noise for four hours. If the sound level was 100 dBA, the maximum exposure time allowed by OSHA is two hours.

Although the OSHA standard was 90 dBA with a 5 dBA exchange rate, current research by the National Institute for Occupational Safety and Health (NIOSH) has shown that the limits should be lower. NIOSH recommends a noise exposure limit of 85 dBA for an eight hour period, with an exchange rate of 3 dBA. This means that if the sound level is 100 dBA, NIOSH recommends that the exposure time be less than 15 minutes.

As a result OSHA has changed their requirements for general industry (but not construction) to be a time weighted average of 85 dBA for an eight hour period. If noise levels in the workplace exceed 85 dBA, OSHA then requires employers to implement a Hearing Conservation Program. This requires employers to:

  • Measure noise levels
  • Provide free annual hearing exams
  • Provide free hearing protection
  • Provide hearing protection training
  • Conduct evaluations of the adequacy of the hearing protectors, which requires familiarity with Noise Reduction Ratings.

Noise Reduction Rating – Hearing Protection Devises

The manufacturers of hearing protection are required to evaluate their devices and label them with a Noise Reduction Rating based on ANSI S3.19-1974.  However, OSHA has found that the Noise Reduction Rating determined in a laboratory is not realistic when the hearing protection device is used in a real-life situation. This means the employer must apply a correction factor to determine the actual amount of protection (sound level reduction) that is provided by each device.

OSHA provides the following procedure for estimating the sound level attenuation provided in a work environment by ear muffs, plugs, or a combination of both.

Single Hearing Protection Device

Determine the laboratory-based noise attenuation provided by the hearing protection device (HPD). This is the Noise Reduction Rating (NRR) and is listed on the HPD packaging.
Subtract the NRR from the dBC time weighted average workplace noise level, as follows:

  • Estimated Exposure (dBA) = TWA (dBC) - NRR

If the C-weighted noise level data is not available, the A-weighted data can be used by subtracting a 7 dB correction factor from the Noise Reduction Rating, as follows:

  • Estimated Exposure (dBA) = TWA (dBA) - (NRR - 7)

The following example is provided by OSHA:

  • TWA=100 dBA, and the ear muff NRR=19 dB
  • Estimated Exposure When Wearing the Ear Muff = 100 - (19-7) = 88 dBA

This exposure exceeds the 85 dBA maximum OSHA allows for an 8 hour period. This means that either the exposure time must be decreased, or the level of ear protection increased.

Dual protection (ear muffs and plugs are used simultaneously):

Using a combination of ear muff and ear plugs will provide additional protection. For dual protection  use the following calculation method:

  1. Determine the laboratory-based NRR for the higher rated protector (NRRh).
  2. Subtract 7 dB from NRRh if using A-weighted sound level data.
  3. Add 5 dB to the field-adjusted NRR to account for the use of the second hearing protector.
  4. Subtract the remainder from the TWA as follows:
    • Estimated Exposure (dBA) = TWA (dBC) - (NRRh + 5)
    • or
    • Estimated Exposure (dBA) = TWA (dBA) - [(NRRh- 7) + 5]

OSHA provides the following Example:

  • TWA=110 dBA, plug NRR=29, and muff NRR=25 dB
  • Estimated Exposure = 110 - [(29 - 7) + 5] = 83 dBA

Noise Reduction Rating – Evaluating Ear Protection When Used With Engineering Controls

The first step in protecting employees from hazards is always the use of engineering controls. For example, one goal of engineering controls is to eliminate the source of the hazard. If the hazard cannot be eliminated, engineering controls are used to guard the hazard, or in the case of noise, reduce the level of the noise.  This may be done by redesigning noisy equipment, or through the use of enclosures or barriers.

However, engineering controls on their own may not always be affordable or practical, and that leads to the use of ear protection.  This means that, based on the Noise Reduction Rating of ear protection, calculations must be done to determine the amount of noise reduction, if any, that must be achieved through the use of engineering controls such that ear protection can safely be used.

As noted above OSHA has determined that the laboratory determined Noise Reduction Rating for ear protection can seldom be achieved in reality. When determining the level of noise reduction that needs to be achieved through engineering controls, OSHA strongly recommends that a 50% correction factor be used.  The above equations need to be modified as follows:

  • Single Protection:
    • Estimated Exposure (dBA) = TWA (dBC) - [NRR x 50%]
    • Estimated Exposure (dBA) = TWA (dBA) - [(NRR - 7) x 50%]
  • Dual Protection:
    • Estimated Exposure (dBA) = TWA (dBC) - [(NRRh x 50%) + 5]
    • Estimated Exposure (dBA) = TWA (dBA) - {[(NRRh - 7) x 50%] + 5}

Noise Reduction Rating and the Importance of Labels and Signs 

Labels and signs provide information and warning messages about hazardous areas. They are used to direct pedestrian traffic around areas with high noise levels. They inform people when ear protection is required before entering a high noise area.  And they warn people to keep out of areas with dangerously high levels of noise.

Warning and informational signs are only effective if they remain in place and legible. A failed sign opens the door to workers being injured. That's why many major corporations are using DuraLabel custom label printers and tough-tested supplies. DuraLabel assures you of long-lasting, superior quality. That assurance comes from DuraLabel backing their quality claims with the industry's only warranty on self-printed signs and labels that applies after they have been installed. That's just one reason why nothing beats a DuraLabel. Call 888.326.9244 to find out more about DuraLabel printers and supplies.

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