Appendix D:Hearing Protector (HPD) Selection

• ISBN: 978-1-4249-4249-7
• Issued: May 2007
• Content last reviewed: June 2009
• PDF Version [ 97 kb/ 28 pages | Download Adobe Reader ]

Methods of selecting Hearing Protection Devices (HPDs) are based on the measurement and statistical treatment of octave band sound attenuation data for test subjects. Until 1997, the standard for testing was ANSI Standard S3.19-1974, which tested subjects under "laboratory conditions." ANSI Standard S12.6-1997, Method B, tests sound attenuation under "real world" conditions, where the protection provided is much less than under the laboratory conditions previously used. However, much of the sound attenuation data in use today is still ANSI S3.19-1974 data, including the NRRs (Noise Reduction Ratings) that manufacturers print on their packages of HPDs. Consequently, National Institute for Occupational Safety and Health (NIOSH) recommends derating NRRs by a multiplicative factor of 75% for earmuffs, 50% for foam earplugs and custom plugs, and 30% for all other ear plugs (i.e. NRR × 0.75 for muffs, NRR × 0.5 for foam plugs and custom plugs, and NRR × 0.3 for other ear plugs).

There are various methods of selecting HPDs. The most accurate, but most complicated, are octave band procedures. Octave band computation, as described in s.9.8.6.1 and in Appendix B of CSA Standard Z94.2-02, requires measurement of the unweighted workplace sound levels in the 125, 250, 500, 1000, 2000, 4000, and 8000 Hz octave bands and the octave band attenuation data for the HPD being assessed. Below is an example of how to estimate the effective noise level by this method when wearing a particular HPD :

The estimated protected exposure level is calculated using the following equation:

Overall level=10 log (10^5.97 + 10^6.74 + … + 10^5.39) = 76.0

CSA Standard Z94.2-02 requires use of this method for exposures above a L_ex,8 of 110 dBA.

The National Institute for Occupational Safety and Health (NIOSH) provides an online octave band selection method using ANSI S12.6-1997, Method B data in their Hearing Protector Device Compendium Search web site

To simplify HPD selection, NIOSH developed a single number rating system, the Noise Reduction Rating (NRR). The system was designed for use with C-weighted sound level measurements, as follows:

Protected A-weighted exposure = Unprotected C-weighted exposure − NRR

NIOSH also suggested that unprotected A-weighted data may be used in place of C-weighted data by adding a +7 dB correction factor to the A-weighted sound level to estimate the C-weighted sound level, as follows:

Protected A-weighted exposure = Unprotected A-weighted exposure + 7 − NRR

As previously stated however, NRRs are based on laboratory data rather than real world data, so NIOSH now recommends that NRRs be derated for use in the above equations. For example, given a muff type of HPD with an NRR of 32 and an unprotected exposure level of 94 dBA, the derated NRR would be 32 × 0.75 = 24 and the protected A-weighted exposure would be estimated as:

Protected A-weighted exposure = 94 dBA + 7 − 24 =77 dBA

NIOSH also provides this selection method on their HPD compendium search web site.

CSA Standard Z94.2-02 provides a similar Single Number Rating System, the SNR (SF84). It is also designed for use with C-weighted exposure measurements, but it is based on "real world" test data (ANSI S12.6-1997, Method B). It is the value that would be achieved by 84% of the using population in a well run hearing conservation program. The SNR (SF84) is used by subtracting it from the unprotected C-weighted sound level or exposure (L_ex,8) to calculate the effective A-weighted level or exposure. For example, if the measured L_eq is 98 dBC and a particular HPD has a SNR (SF84) rating of 22 dB, the predicted A-weighted effective L_eq when wearing this device is:

98 − 22 = 76 dBA

CSA Standard Z94.2-02 also provides a method for HPD selection based on either a grading or classification system in conjunction with sound exposure data (i.e. L_ex,8). Classes are the "old" CSA system in which HPDs are assigned to Class A, B, or C, based on laboratory attenuation data. In the "new" CSA system, grades are 0, 1, 2, 3, or 4, based on real world attenuation data, the SNR (SF84). A table (Table 4) is provided in the standard that specifies the recommended CSA Grade or Class for given values of the L_ex,8 exposure.

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