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Year : 2005  |  Volume : 7  |  Issue : 26  |  Page : 39-45
The influence of aging on the noise attenuation of ear-muffs

Central Institute for Labour Protection - National Research Institute, Department of Acoustic and Electromagnetic Hazards, Warsaw; Warsaw University of Technology, Institute of Radioelectronics, Warsaw, Poland

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  Abstract 

Ear-muffs are commonly used as personal protectors against the effect of noise. The methods of calculation the A-weighted sound pressure level under the cups of ear-muffs are based on the results of laboratory measurements of noise attenuation, which are carried out in the certification process of the product on brand new samples. Hearing protectors are usually stored for certain periods of time. Next, workers use them in different ambient outdoor conditions as long as there are no signs of their physical damage. The question is. What is the influence of ambient outdoor conditions, usage and storage time of ear-muffs on their attenuation? To answer this question, a three-year study has been undertaken. Four types of ear-muffs, most popular in the Polish work environment, made in Europe, meeting the certification requirements and granted a certification mark, were used in this study. Sixty samples of ear-muffs were worn by workers at noisy workplaces, 40 samples were stored and another 40 were exposed to ambient outdoor conditions. The workers were asked to evaluate subjectively the noise attenuation of the ear-muffs. After one, two and three years time of usage and storage, the sound attenuation of ear-muffs was measured. After the two years the headband force and the cushion pressure of tested samples were measured. The results of sound attenuation measurements were used to calculate the attenuation against high- (H), medium- (M) and low- (L) frequency noise and single number rating (SNR) of the tested ear­muffs. The results of the study showed that the attenuation of ear-muffs can be significantly reduced as a function of usage, storage, and exposure to ambient outdoor conditions. The observed decrease of the ear-muffs attenuation corresponded to decrease of the cushion contact area but did not correspond to the subjective workers' assessment.

Keywords: aging, ear-muffs, hearing protectors, noise protection, sound attenuation

How to cite this article:
Kotarbinska E. The influence of aging on the noise attenuation of ear-muffs. Noise Health 2005;7:39-45

How to cite this URL:
Kotarbinska E. The influence of aging on the noise attenuation of ear-muffs. Noise Health [serial online] 2005 [cited 2019 May 22];7:39-45. Available from: http://www.noiseandhealth.org/text.asp?2005/7/26/39/31641

  Introduction Top


In noisy workplaces, ear-muffs are commonly used to protect workers against noise-induced hearing loss. According to EU Directives 86/188/EEC (1986) and 2003/10/EC (2003) hearing protectors should reduce the noise level at the wearer's ears to below the exposure limit value. To be effective, hearing protectors have to be properly selected based on the noise characteristics. The standard EN 458 (2004) recommends four methods of assessing A­ weighted sound pressure level effective to the ear when hearing protectors are worn: the octave band method, the HML method, the HML check method and the SNR method. The octave band method is based on the sound attenuation data of hearing protectors and octave band sound pressure levels of the workplace noise. The HML method is based on high (H), medium (M), low (L) frequency attenuation values of the hearing protectors as well as the A-weighted sound pressure level and the C-weighted sound pressure level of the workplace noise. The HML check method needs high (H), medium (M), low (L) frequency attenuation value of hearing protectors as well as the A-weighted sound pressure level of workplace noise and subjective decision between two noise classes using examples of noise sources. The SNR method is based on the single number rating (SNR) of the hearing protectors as well as the A-weighted sound pressure level and the C-weighted sound pressure level of the workplace noise. According to the standard EN ISO 4869-2 (1995) H, M, L and SNR values of hearing protectors are calculated from the measured values of sound attenuation. In the "real world" situation the ear­muffs are usually used as long as there are no signs of any physical damage. The questions are. What is the influence of usage and storage time of ear-muffs on their attenuation? How long are the H, M, L and SNR values, defined in the certification process for brand new samples, valid? Does the absence of the visible damage of ear-muffs mean stability in their attenuation values? To answer these questions, a three-year study was carried out to identify changes in ear­muff attenuation due to their usage, storage and influence of ambient outdoor conditions typical for Central Europe. Four models of ear-muffs of normal size, produced in Europe and commonly used in the Polish work environment, were tested for three years. The ear-muffs fulfilled the standard EN 352-1 (1993) requirements and were granted a certification mark.


  Methodology Top


Thirty five brand new samples each of four models (I, II, III, IV) of ear-muffs were tested for three years.

Fifteen samples of each model were supplied to 60 workers for their usage. Before it, the workers had used the hearing protectors of other models than the tested ones. Noisy workplaces with comparable atmospheric conditions were chosen at four different industrial companies. The workers used the tested ear-muffs every working day. The cushions were changed according to the instructions of producers. After the first and the second year of usage, the workers were asked to evaluate subjectively the noise attenuation of ear-muffs answering the question: "How do you assess the attenuation of your ear-muffs - is it poor, fair, good, or excellent?"

Ten samples of each model of ear-muffs were exposed to ambient outdoor conditions 8 hours every working day. The samples were mounted on special head simulators [Figure - 1].

The air temperature and humidity were checked twice a day. During the other 16 h, the samples were stored in the laboratory room where the producer's requirements were fulfilled (temperatures above +55oC, not impaired by chemical substance). Histograms of air temperature and humidity in the environment, where the tested samples of ear-muffs were exposed to ambient outdoor, are presented in [Figure - 2].

The remaining ten samples of each tested model of ear-muffs were stored following the manufacturer's recommendations.

After one and two years and then after three years of usage and exposure to ambient outdoor, the four samples of each model of ear-muffs, taken at random, under went laboratory test. The stored samples were tested after the two and the three years. The sound attenuation of ear-muffs (for four samples) was measured with sixteen subjects, in accordance with EN 24869-1 (1992) standard. Prior to the measurements the cushions were changed for new ones in all samples. The high-frequency attenuation value H, medium­frequency attenuation value M, low-frequency attenuation value L and SNR of ear-muffs were determined. For each calculated value H, M, L and SNR uncertainties were determined at 95% confidence interval (Laitinen, 1998).

After two years of usage, exposure to ambient outdoor and storage the headband force and cushion pressure of ear-muffs were tested (EN 352-1, 1993).


  Results and Discussion Top


[Figure - 3],[Figure - 4], [Figure - 5] and [Figure - 6] show the differences between the estimated attenuation data of H, M, L and SNR of ear-muffs when testing them after one, two and three years and the corresponding values estimated for certification process and given in the wearer information. On these figures the uncertainties of the calculated differences arising from the uncertainties for sound attenuation measurements in different laboratories (EN 24869-1, 1992), (Laitinen, 1998) are presented.

After the one year of usage all the estimated attenuation values for model I were significantly lower by 4 - - 5 dB on average as compared to those given in the wearer information. After the three years of usage, the high-frequency attenuation H decreased by 9.1 dB. The exposure to ambient outdoor conditions affected samples of model I very strongly; after the three years SNR was lower than the catalogue value by 10.1 dB. After the three-year storage the decrease in attenuation values H, M, L and SNR did not exceed 4 dB.

The average headband force of two years tested samples of model I for which the sound attenuation was measured decreased: 8.3 % for used samples, 23.6 % for samples exposed to ambient outdoor conditions and 1.2 % for stored samples. When cushion pressure was tested for samples of model I the significant decrease of area of the impression of the cushion contact area was observed. In case of all used and stored sample the cushion pressure was greater than 4500 Pa. In case of ear-muffs exposed to ambient outdoor conditions for two samples the barrier between the inside and the outside perimeter was broken.

The attenuation values of samples of model II were stable during the three years of usage, exposure to natural atmospheric conditions and storage. The average headband force of the two years tested samples of model II decreased not more then 0.87 N. The decrease of area of the impression of the cushion contact area was observed for some samples of model II but the cushion pressure was not greater than 4500 Pa.

Only slight changes in attenuation were also observed for samples of model III; after the two years, the average attenuation values was lower by no more than 2.3 dB and after the three years a maximum decrease of 4.7 dB was observed for the attenuation against low (L) frequency noise.

The decrease in attenuation of the two years used samples corresponds to observation of broken barrier between the inside and the outside perimeter of the cushions for all tested samples. The areas of the impressions of the cushion contact area were not significantly smaller.

In model IV, a significant decrease in high- (H) frequency attenuation was observed. After the first year of usage and exposure to atmospheric conditions, value H was about 6 dB lower than that given in the wearer information. The two­ year storage also reduced high- (H) frequency attenuation by about 6 dB. The observed change in attenuation against high- (H) frequency is difficult to explain because the headband force and cushion pressure were very stable. For the two-year tested samples the average headband force decreased: 0.12% for used samples, 8.2 % for samples exposed to ambient outdoor conditions, 0.07 % for stored samples and the average cushion pressure increased not more than 8.5%.

The ambient outdoor conditions mostly affected the attenuation values of ear-muffs of model I. After the first year of exposure a maximum decrease - - 4.8 dB was observed for the middle­(M) frequency attenuation, compared with the manufacturer's data. After the two years of exposure, attenuation M decreased by 9.2 dB. These changes correspond to the significant decrease of the area of the impression of the cushion contact area observed during the cushion pressure tests. Significant changes were also observed for model IV after one year of exposure, the estimated high (H) attenuation value was 6.4 dB lower than that given in the wearer information. The results of the testing of headband force and cushion pressure do not explain these observation. The influence of ambient outdoor conditions on the attenuation of ear-muffs of models II and III was insignificant.

[Figure - 7] summarizes the results of a two year survey in a group of workers using the tested ear­muffs.

It is observed that after two years of usage of ear­muffs the number of positive assessments of their attenuation is higher then after one year of usage. In case of model I these opinions do not correspond to the results of the tests in the laboratory.


  Conclusions Top


Attenuation properties of ear-muffs, which meet the EN 352-1 standard requirements and are granted a certification mark may decrease significantly as a function of time of usage and storage and exposure to ambient outdoor conditions when the visible damages of the sample are not observed. The changes in attenuation values H, M, L, and SNR may differ significantly between products.

The time factor was the most important in model I. After the one-year of usage the attenuation values H, M, L and SNR were lower than those given in the wearer information by 4 - 5 dB on average. After the three years of usage, the high ­frequency attenuation H decreased by 9.1 dB. After the three years of exposure to ambient outdoor conditions the single number rating SNR was lower by 10.1 dB. The two-year storage caused a decrease in attenuation H, M, L and SNR by about 3 - 4 dB.

The most stable attenuation properties were observed in case model II. During the three years, the decrease in H, M, L, and SNR, as compared to the values given in wearer information, was not higher than 2 dB.

The decrease of the attenuation of ear-muffs corresponded to the significant decrease of the contact area of the ear-muffs' cushion which was observed during the testing of cushion pressure. The subjective assessment of the noise attenuation for the ear-muffs of model I did not correspond to the objective changes in their attenuation. After the second year of testing, the number of subjective positive assessments of the ear-muffs attenuation increased for all the four models. It suggests that the wearers accept more the hearing protectors which they are used to wear than the new ones.


  Acknowledgement Top


State Committee for Scientific Research and Ministry of Economy and Labour in Poland funded this study. I would like to thank Mr. Rafal Mlynski for the help in preparing this manuscript[9].

 
  References Top

1.Council Directive 86/188/EEC of 12 May 1986 on the protection of workers from the risks related to exposure to noise at work, Official Journal of the European Communities, No L 137: 28-34  Back to cited text no. 1    
2.Directive 2003/10/EC of 6 February 2003 on the minimum health and safety requirements regarding the exposure of workers to the risks arising from physical agents (noise), Official Journal of the European Union, No L 42: 38-44  Back to cited text no. 2    
3.Draft standard EN 458, (2004) Hearing protectors - Recommendations for selection, use, care and maintenance - Guidance document, European Committee for Standardization, Brussels  Back to cited text no. 3    
4. EN 352-1, (1993) Hearing protectors - Safety requirements and testing - Part 1: Ear-muffs, European Committee for Standardization, Brussels  Back to cited text no. 4    
5. EN 24869-1, (1992) Acoustics - Hearing protectors - Subjective methods for the measurements of sound attenuation, European Committee for Standardization, Brussels  Back to cited text no. 5    
6. EN ISO 4869-2, (1995) Acoustics - Hearing protectors - Part 2: Estimation of effective A-weighted sound pressure levels when hearing protectors are worn, European Committee for Standardization, Brussels  Back to cited text no. 6    
7. Kotarbinska E., (2003) Attenuation properties of hearing protectors in versus time factor, International Journal of Occupational Medicine and Environmental Health, 16 (1): 67-72  Back to cited text no. 7    
8.Kotarbinska E., (2002) Long term investigations of sound attenuation changes in ear-muffs: preliminary results, Proceedings Forum Acusticum 2002, Sevilla: CD-ROM  Back to cited text no. 8    
9.Laitinen H., (1998) Uncertainty in harmonized standards. Master thesis of Helsinki University of Technology, Vantaa, Finland  Back to cited text no. 9    

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Correspondence Address:
E Kotarbinska
Central Institute for Labour Protection - National Research Institute, Department of Acoustic and Electromagnetic Hazards, Czerniakowska 16, 00 701 Warsaw
Poland
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Source of Support: None, Conflict of Interest: None


PMID: 16053604

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    Figures

  [Figure - 1], [Figure - 2], [Figure - 3], [Figure - 4], [Figure - 5], [Figure - 6], [Figure - 7]

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