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|Year : 2004 | Volume
| Issue : 25 | Page : 41--47
Instruction and the improvement of hearing protector performance
National Acoustic Laboratories, Australia
Senior Research Engineer, National Acoustic Laboratories, 126 Greville Street, NSW 2067
Too often, in spite of the encouragement of those who advocate the removal of noise from the workplace as the preferred solution to noise exposure, hearing protectors are provided as the first line of defence against noise. Unfortunately hearing protectors are too often supplied with no real instruction or education in their use. This degrades their performance considerably.
In this project the attenuation performance of one particular model of earplug was compared with and without instructions to test subjects. The instructions given were the most basic as commonly supplied on the plastic packaging containing the plugs. The attenuation performance (SLC80) of the plugs with the instructions was 16 dB greater than without the instructions. Overall performance was improved in every octave band with smaller associated standard deviations. The comparison showed that even with a very modest amount of instruction attenuation performance can be significantly improved.
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Williams W. Instruction and the improvement of hearing protector performance.Noise Health 2004;7:41-47
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Williams W. Instruction and the improvement of hearing protector performance. Noise Health [serial online] 2004 [cited 2021 Feb 25 ];7:41-47
Available from: https://www.noiseandhealth.org/text.asp?2004/7/25/41/31648
The use of hearing protectors with an appropriate education and instruction program along with regular updated training has been advocated as part of the occupational noise management process for many years (AS 1269:1989; NIOSH:1996; WHO: 2001). This includes training on why noise is a health hazard, noise exposure levels, noise injury and hearing protector selection, use and maintenance (AS/NZS 1269.3: 1998). "The success of an HLPP [Hearing Loss Prevention Program] depends largely on effective worker education regarding all aspects of the program" (NIOSH: 1998, p 52).
This is even more important when the prevention of noise exposure relies almost solely on the provision and wearing of hearing protectors. "[B]before personal hearing protectors are issued, the need for their use should be fully explained" (NOHSC: 2000, Section 9.13). As discussed by Berger (2000) if subjects are not trained in the fitting of hearing protectors we can never be sure of their actual performance. In fact
we can never be sure of the real performance of a hearing protector in practice unless measurements are made using a microphone placed appropriately in the ear. In practice the assumption tends to be made that when hearing protectors are being worn they are working at or around their rated capacity.
Thus to ensure or increase individual hearing protector performance good instruction should be provided. As noted by Berger (2000) "this is a key area where programs [ie hearing protector fitting] must be upgraded" (p 398).
Many hearing protector acoustic attenuation performance measures, including the Noise Reduction Rating (NRR) (ANSI S 12.6 - 1997) and the Sound Level Conversion (SLC 80 )
(AS/NZS 1270: 2002), utilize a figure that is a direct function of the mean attenuation and standard deviation measured at several Octave Bands. In the case of NRR this figure is a function of the mean attenuation minus two standard deviations while in the case of the SLC 80 it is a function of the mean minus one standard deviation. Thus for an NRR or SLC80 that most accurately represents the potential of the device it is particularly advantageous to have a high mean and small standard deviation.
Measurement of the attenuation of the selected ear plugs was carried out using the National Association of Testing Authorities (NATA) Certified facilities of the National Acoustic Laboratories (NAL), Chatswood, Australia. The procedure followed was that as required by the then Australian Standard AS 1270: 1988. [Note: This Standard has undergone subsequent changes the main one being that testing is now required to use pink noise of third octave band width, at octave band centre frequencies. Previous testing was carried out utilising a pure tone, Beckesy audiometric technique, using open ear and occluded ear thresholds. "For practical purposes pure-tone and 1/3-octave band measurements of ear-protector attenuation are identical" (Waugh: 1974, p 1868)]
Subject recruitment and instruction required the use of 'inexperienced' test subjects, similar to ANSI S12.6 - 1997 Method B: Subject fit, draft ISO 4869-7 and the current combined Australian/New Zealand Standard AS/NZS 1270: 2002 Acoustics - Hearing protectors.
The distinguishing feature of this form of testing, "the subject-fit method", is that the experimenter (tester) is not permitted to directly instruct the subject(s) on how to fit the device (ear plug, earmuff, etc). Any form of one-to-one instruction is excluded except in the case where it is inherently built in to the supply of the device, for example in the case of personally moulded ear plugs. Instructions may only come from the information typically supplied to the consumer by the manufacturer/distributor. This may be in the form of a written and/or pictographic presentation.
Two series of attenuation tests were carried out on the one particular ear plug which was supplied in a single size only. The first series of tests involved fifteen test subjects and there were no fitting instructions supplied in any form. The best method of fit was entirely the responsibility of the test subject. Fitting noise was supplied to allow the test subject to optimize the plug fit. The second series of tests involved sixteen test subjects. This time instructions were issued as supplied by the manufacturer/distributor. These instructions are illustrated in [Figure 1] and can be seen to be typical of those provided on the back of the small plastic 'pocket' in which many companies supply ear plugs.
The descriptor used for comparing the performance of the attenuation of hearing protectors is the SLC80, expressed in dB. This descriptor, the Sound Level Conversion, is deemed to offer the amount of attenuation to approximately 80% of the wearers at any one time. The SLC 80 is a direct function of combination of the mean attenuation minus the standard deviation at the seven octave bands tested. The detailed method of calculation is given in AS/NZS 1270: 2002, Appendix A and is described more fully in Waugh (1984).
For a hearing protector to have an overall high SLC 80 rating it should ideally have a high mean attenuation and low standard deviation. The worst case is a low mean attenuation with a corresponding wide standard deviation resulting in a 'negative' attenuation. This would mean in effect that the device could be considered to be 'enhancing' the effect of the noise at a particular octave band. Hence for a well behaved device that offers high or low attenuation, the standard deviation at all octave bands should be as small as possible.
The overall test results for the two series of tests without and with manufacturer's/distributor's supplied instructions are provided in [Table 1] and [Table 2] respectively. As can be seen the results with no instructions have much greater variation resulting in greater variance and an overall lower SLC 80 rating, 8 dB compared to 24 dB as would be more usually expected for this type of plug.
An examination of the increased attenuation performance is shown in [Figure 2] for the results from the 4k Hz 1/3 Octave Band attenuation tests. The decrease in the variance of the results and the increase in the value of the mean are clearly illustrated. The Kurtosis of the results increased from 0.61 to 6.15 a marked improvement when the rating of the device depends on the mean minus the standard deviation (mean - SD).
[Figure 3] and [Figure 4] show the convergence of the mean attenuations for all test bands. The mean attenuations converge more rapidly and to higher values. Similarly the standard deviations also converge more rapidly and to significantly lower values [Figure 4] and [Figure 5].
[Figure 5] and [Figure 6] show the convergence of the standard deviation respectively for all test frequencies.
As can be readily seen from the two tables of results, the overall performance of the plugs increased significantly with the provision of very simple instructions. For all frequencies all of the means increased and the standard deviations decreased resulting in an overall increase in SLC 80 rating of 16 dB, from 8 to 24 dB.
The comparisons of the convergence of the mean attenuation values and the standard deviations [Figures 3] & [Figure 4] and [Figure 5] & [Figure 6] respectively) show that the convergence of the results to the final value is much more rapid in the case of 'with instructions' compared to 'no instructions'.
Not all subjects experienced attenuation performances at the same level. For example in [Table 2], subjects 5 and 11 provided attenuation figures well below the mean, while subjects 4 and 16 provided values, in general, well above the mean. Thus while some or better instruction may provide some of the driver for decreasing the probability of a "bimodal distribution" of results (Williams: 2003) it may not necessarily provide the complete solution. The extrapolation that if a little information is good therefore more will be better may not necessarily be the case. Providing more detail and sometimes complexity has been shown not to increase preventative action in noise exposed individuals (Williams, Purdy, Murray, Dillon, LePage,
Challinor and Storey: 2004). Any instruction must be tailored to the situation and the individuals' needs.
If hearing protectors are to be utilized as the first line of defence against noise exposure then wearers should be instructed and educated in their use in order to ensure rated and consistent performance is experience. With the very simple information provided in this case the increase in performance of the earplugs tested was significant.
|1||ANSI S 12.6 - 1997 American National Standard Methods for measuring the real-ear attenuation of hearing protectors, Acoustical Society of America, New York, NY|
|2||Australian Standard AS 1269: 1989 Acoustics - Hearing conservation, fourth edition, Standards Australia, Sydney|
|3||Australian Standard AS 1270: 1988 Acoustics - Hearing protectors, third edition, Standards Australia, Sydney|
|4||Australian/New Zealand Standard AS/NZS 1269.3:1998 Occupational noise management Part 3 Hearing protector program, Standards Australia, Sydney|
|5||Australian/New Zealand Standard AS/NZS 1270: 2002 Acoustics - Hearing protectors, Standards Australia, Sydney|
|6||Berger, EH (2000) Hearing Protection Devices, in The Noise Manual, edited by EH Berger, LH Royster, JD Royster, DP Driscoll and M Layne, fifth edition, American Industrial Hygiene Association, Fairfax, VA|
|7||Draft ISO 4869-7 Acoustics - Hearing protectors-Part 7: Subjective method for measurement of sound attenuation - Subject-fit method, International Organisation for Standardisation, Geneva|
|8||NIOSH (1996) Preventing Occupational Hearing Loss - A Practical Guide, Edited by JR Franks, MR Stephenson, and CJ Merry, National Institute for Occupational Safety and Health, Cincinnati, Ohio|
|9||NIOSH (1998) Occupational Noise Exposure, Revised Criteria 1998, National Institute for Occupational Safety and Health, Cincinnati, Ohio|
|10||NOHSC (2000) Occupational Noise; national Code of Practice[NOHSC: 2009(2000)], second edition, National Occupational Health and Safety Commission, Australia, Canberra, July 2000|
|11||Waugh, R (1974) Pure-tone, third-octave, and octave-band attenuation of ear protectors, JAcoust Soc Am, vol 56, No 6, December 1974|
|12||Waugh, R (1984) Simplified Hearing Protector Ratings an International Comparison, Journal of Sound and Vibration, 1984, 93(2): 289 - 305|
|13||Williams, W (2003) An explanation for the apparent poor performance of some hearing protectors, Acoustics Australia, Vol 31, August, 2003, No 2: 59 - 62|
|14||Williams, W, Purdy, SC, Murray, N, Dillon, H, LePage, E, Challinor, K and Storey, L (2004) Does the presentation of audiometric test data have a positive effect on the perception of noise and noise exposure avoidance?, in press with Noise & Health|
|15|| World Health Organization (2001) Occupational Exposure to Noise: Evaluation, Prevention and Control, Edited by B Goelzer, C Hansen and G Sehmdt, Special Report S64, published on behalf of the WHO by Wirtschaftsveralg NW, Verlag f?r neue Wissenschaft GmbH, Bremerhaven|