Sounds like it's time for reality check
September 1st 2009

When specifying hearing protection many people give a great deal of credence to the SNR published on the packaging.David Leonard of Howard Leight by Sperian points out that there are weaknesses to the SNR system and advocates a field-verified Personal Attenuation Rating

The SNR was adopted by ISO (International Organisation for Standardisation) in 1994 as a simple method for estimating the attenuation of hearing protectors. But, the SNR is simply an estimate of the protection a Hearing Protection Device can offer based upon population averages, not a measurement of actual protection. It is an estimate based upon 16 subjects in an acoustical laboratory.

SNR is therefore a theoretical number and how much protection an individual worker will achieve with a particular earplug in the field remains unknown, especially as the correct fitting of HPDs is the key to obtaining comfort and maximum protection levels.

Recognising the inherent weaknesses of the system, many countries have devised their own HPD de-rating recommendations for applying to attenuation ratings, but again these apply to a population not a person. In Germany, for example, the BG Technical Committee recommends subtracting 3dB from custom moulded earplugs, 5dB from earmuffs and 9dB from all other earplugs. Some experts in the UK advise 4dB should be subtracted from the SNR while in the USA 7dB is a common de-rating.

It is possible to eliminate the variability which SNR and de-rating introduce, as a recent field attenuation study conducted by Howard Leight Acoustical Laboratory showed.Workers were tested with a field verification system during their standard working shift. VeriPRO, the system used, is able to ascertain each worker's Personal Attenuation Rating (PAR) which can then be compared against such factors as the published SNR and the noise hazard.VeriPRO uses a loudness balancing technique to determine the effectiveness of an employee's earplug over a range of frequencies. The resultant PAR identifies the actual protection an employee receives from his or her earplug in each ear. This allows safety managers to determine whether employees are receiving optimal protection, require additional training on how to fit their earplugs, or need to try a different model.

VeriPRO can be used with any earplug, and tests performed in virtually any setting.

VeriPRO offers two test protocols: the Complete Check runs employees through five frequencies to determine PAR and is recommended for annual use; the Quick Check is a simplified test of earplug fit at one frequency, used as a fast and easy pass/fail check for more frequent use.

A key feature of VeriPRO, especially in the light of recent compensation claim payouts for hearing loss, is that employee test results are captured and stored and can be analysed to determine trends, to generate reports and to provide evidence of real world attenuation.

The system also includes short training videos on how to properly fit earplugs, a feature which, in many cases, overcomes problems in achieving the expected protection.

The field attenuation study referred to above involved 104 workers at eight facilities.

It showed that one-third of workers achieve attenuation higher than the published attenuation ratings for their earplugs; one third achieved attenuation within 5dB of those ratings and the remaining third had attenuation that was more than 5dB below published attenuation. Therefore some were over-protected, some were protected and some were not being protected adequately for the environment within which they work.

Armed with such exact individual measurements, those who are at risk from noise-induced hearing loss can be targeted for training.

The study above is valuable as, for the first time, a field-deployable test is bridging the gap between lab estimates of attenuation and real world attenuation achieved by workers as they normally wear their HPDs. This point about bridging the gap between the lab and the field was highlighted in the recent HSE study Real World Use and Performance of Hearing Protection as the measurements used in the HSE study were made using an artificial ear simulator in a laboratory. As the study concedes, 'the attenuation of an earplug in an ear simulator often exceeds the performance achieved by human subjects as the earplug is fitted into a smooth cylindrical ear canal simulator'.

Within the working world the gap between laboratory and field has been bridged, allowing those with health and safety responsibility to discard the theoretical data they have had to work with in the past and to base their hearing conservation initiatives firmly in reality.

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