Hearing protection - does it fit?

In the past, when a safety professional wanted to select or assign a hearing protection device (HPD) for an employee, the data they have historically had available was based on fitting scenarios in a controlled laboratory environment. This information bore little resemblance to the conditions under which workers had to wear HPDs on a daily basis.

Attenuation testing to the Australian standard AS/NZS 1270 is done under specified conditions using a panel of 16 (muffs) or 20 (earplugs) untrained subjects with normal healthy hearing.

There are a number of problems with this approach, one of which is that the attenuation results for each of these individuals will likely be different but the results are averaged when the SLC80 (sound level conversion) is calculated. The averaged (mean) values for the test panel are used to predict the performance for an individual wearer in an occupational setting.

In fact, some of those tested will get a higher level of protection than the calculated SLC80 and others will get a lower level of protection. For an individual, this can be significant in terms of the protection actually supplied in the workplace. Even if the laboratory data were representative of the actual group using the device, the individual variability is large enough that attempts at predicting one person’s performance from group data can easily err by up to 20 dB.

The question is how to select or assign a hearing protection device (HPD) to an individual user, taking into account his or her noise exposure, the amount of protection that is required and potentially his or her hearing sensitivity as well.

These issues lead to the desire to fit test the individual to ascertain the actual performance that he or she gets using the HPD in question. Historically, the method of fit testing used is to replicate as closely as possible the laboratory test procedure. In the lab, we ask listeners to track their hearing threshold levels (the quietest sounds they can hear), much like when they take a conventional audiogram to measure their hearing sensitivity. The difference in the two thresholds is the noise reduction of the device. This procedure is called ‘real-ear attenuation at threshold’ (REAT), since the attenuation of the HPD is computed from differences in the threshold of hearing, with and without the hearing protector in place.

REAT fit testing in the field uses speakers in large circumaural cups and, in so doing, is limited to being only able to measure earplugs. However, earplugs are the type of HPD that is most variable in fit and therefore most in need of fit testing.

A principal disadvantage of field REAT is it is time-consuming - each frequency tested takes at least 30 seconds; a minimum of at least one minute to test the fit in each ear, much longer if multiple frequencies are to be tested. Furthermore, there is an inherent variability since the data rely on the listener’s ability to track his or her own threshold - this has an imprecision of approximately +5 dB for typical employees. And finally, accurate REAT measurements require low background noise so that the open-ear thresholds are not masked and contaminated - care must be exercised to be sure that the environment in which the tests are conducted is adequately quiet.

An alternative solution for field fit testing uses a technology called ‘field microphone-in-real-ear’, or F-MIRE. 3M has developed a product using F-MIRE called E-A-RFit, which utilises a miniature dual-element microphone to simultaneously measure the sound pressure levels in the ear canal under the hearing protector as well as those outside the HPD. These values, termed ‘noise reduction’ (NR), can be used with suitable correction factors, to estimate the hearing protector’s attenuation for the particular individual and fitting that was tested.

By developing special probed versions of E-A-R earplugs, we can now test in a matter of moments the attenuation that is being obtained, regardless of the fit of the HPD that is being evaluated. The actual measurement for any one fit test in one ear takes about 10 seconds to obtain data from all standard seven test frequencies from 125 Hz to 8 kHz, as well as an overall noise reduction rating called the PAR (personal attenuation rating). The exact amount of variability in PAR is defined and explicitly provided with the measurement. In addition to the brevity of the test, it can be conducted in substantially higher noise levels than can a field REAT measurement.

Individual fit testing of HPDs serves many purposes in a hearing conservation program but, first and foremost, it is a tool to train and motivate employees to wear their hearing protectors. A common shortcoming in hearing conservation programs (HCPs) is lack of training; even when training is attempted, lack of ability to demonstrate that the training has accomplished its goals presents a problem. With a quick and accurate fit-testing system like E-A-RFit, the hearing conservationist has a valuable tool to select the proper hearing protector in terms of fit, and then to work with the employee to make sure he or she has the knowledge and skill to repeatedly insert the product correctly. In turn, this helps motivate the employee, since they come to believe in the efficacy of their own behaviour, ie, it is worth the effort to fit the product properly.

Training the trainers

Not only do the employees need training in the use of their HPDs, but so do the trainers themselves need to learn how to train others. This train-the-trainer application is a key feature of the E-A-RFit approach. Now trainers can learn what makes a difference and how to direct users to get the most from their hearing protection.

Beyond correct fitting, the hearing conservationist may wish to assign HPDs based on noise exposure levels and/or the need to communicate clearly. Without individual fit test data, this matching of HPD to noise exposure/communication scenario is a folly. With the reliable octave-band attenuation and PAR, such HPD matching is now feasible and reasonable.

When employees experience a significant threshold shift (STS), one of the follow-up actions is to refit and retrain employees in the use of their HPDs and to provide more protective devices if needed. Now this can be done with a degree of reliability to determine if in fact employees need this retraining, to assess the protection they can obtain and whether more is needed.

Companies are also often concerned if they’re meeting regulatory requirements for adequate protection. The noise regulations direct them to meet certain noise exposure criteria - fit testing means they can demonstrate that a product and an individual are achieving sufficient attenuation to meet the requirements of the regulations.

Other applications

A final application of fit testing is for documentation for legal purposes. The concept of documenting training and validating that an employee was able to demonstrate correct and adequate use of a hearing protector would likely be valuable evidence in workers compensation proceedings.

For over a quarter century since the advent of modern hearing conservation regulations, we have been saddled with the knowledge that many hearing conservation programs are ineffective. Too often this is due to the simple fact that the key component of those programs, the object between the employee and a hazardous noise exposure, namely the hearing protector, is not doing its job. Worn correctly and consistently, HPDs can prevent noise-induced hearing loss in virtually all cases. A giant step in the resolution of this problem is individual fit testing, which represents the next step forward in hearing conservation. It is not a panacea but, applied judiciously and consistently in one’s overall hearing conservation efforts, the utility of such a tool is undeniable. As its use becomes more widespread, it is quite possible that such testing may become the standard for judging effective hearing conservation programs.

*Terry Gorman, Senior Occupational Hygienist, 3M Australia