Hearing protectors: State of art and emerging technologies

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PROCEEDINGS of the 22 nd International Congress on Acoustics Plenary Lectures: Paper ICA2016-895 Plenary presentation Hearing protectors: State of art and emerging technologies Samir Gerges Federal

1 PROCEEDINGS of the 22 nd International Congress on Acoustics Plenary Lectures: Paper ICA Plenary presentation Hearing protectors: State of art and emerging technologies Samir Gerges Federal University of Santa Catarina, Mechanical Engineering, Florianopolis, SC, Brazil, Federal Institute of Santa Catarina, Mechatronic, Florianopolis, SC, Brazil Abstract In many industrial and military situations, it is not practical or economical to reduce ambient noise to levels that present neither a hazard to hearing nor annoyance. In these situations, personal hearing protection devices are capable of reducing the noise by up to around 35 db. Although the use of a hearing protector is recommended as a temporary solution until action is taken to control the noise, in practice, it ends up as a permanent solution in most cases. Therefore, hearing protectors must be both efficient in terms of noise attenuation and comfortable to wear. Comfort in this case is related to the acceptance of the user to wear the hearing protector consistently and correctly at all times. The purpose of this paper is to review the stat of art for the need to develop methods to address three important topics not sufficiently treated in the published literatures: Detection of outliers and their effect on the noise attenuation measurements, uncertainty in the measurement results of Hearing Protectors Noise Attenuation and quantification of comfort by measurement the contact pressure between the users and earmuff. Keywords: hearing protectors, measurement uncertainty noise attenuation

2 Hearing protectors: State of art and emerging technologies 1 Introduction When workers are exposed to sound levels in excess of occupational levels or action limits, the first action is to reduce the noise at the source and or pathway (engineering controls). When sound levels cannot be reduced to less than 85 dba for an 8-hour Time Weighted Average (TWA) or through source and or pathway control(s), hearing protection should be used to protect workers from occupation-related hearing loss. Choosing the right kind of hearing protective device and correct use are essential to protection from hazardous noise exposures. Hearing loss is a function of exposure time, the average sound level, and the peak level of very loud sounds. Exposure to excessive noise from industrial machinery, heavy construction equipment and vehicles, power tools, aircraft, gunfire, motorcycle and auto race tracks, dental drills, sporting events, fireworks, rock concerts, marching bands, and music from a player's own instrument or nearby instruments can cause hearing loss depending on the level and duration of the noise. Some persons are more susceptible to hearing loss from high-level sound than others. Some workers obviously need high-attenuation earplugs. Shipbuilders, flight crew who stand behind jet aircraft on the flight deck, and army tank operators usually fall in this category. Such individuals can't get enough attenuation for proper protection even with plugs and earmuffs combined. But, many industrial workers can be adequately protected with as little as 10 db of attenuation: the majority of eight-hour equivalent noise exposures fall between 85 and 95 db. Therefore, hearing protectors must be both efficient in terms of noise attenuation and comfortable. The purpose of this paper is to review the stat of art for the need to develop methods to address three important topics not sufficiently treated in the published literatures: Detection of outliers and their effect on the noise attenuation measurements, uncertainty in the measurement results of Hearing Protectors Noise Attenuation and quantification of comfort by measurement the contact pressure between the users and earmuff cushion. 2 Detection and contribution of outliers for subjective measurements of noise attenuation of hearing protectors by REAT method Measuring the noise attenuation of hearing protector devices (HPDs) using the REAT Real-ear Attenuation at Threshold method [1,2] is based on subjective measurements, where each subject determines their open (without HPD) and closed (with HPD) threshold levels. The subjective determination of the threshold levels shows a high variation between subjects even when they are qualified and familiarized with the method used to determine these threshold levels, as required by the relevant standard. Some subjects pay greater attention and can 2

3 determine their threshold with more accuracy than others. Some subjects simply do not pay attention and answer randomly depending on their mood and mental condition on that day. This section shows a methodology to observe the statistical distribution and quantify the contribution of each subject to the final single number Noise Reduction Ratio NRRsf. Eliminating a few subjects (the outliers) increases the NRRsf and reduces the variability of the measurements (from around ± 4 to ±1). The results for the measurement of 20 different brands of pre-molded earplugs are reported as a case study. This section describes how to identify these outlier subjects [3], that is, those with very different results compared with most of the subjects, and investigates the effect of eliminating them on the final NRRsf value. In a real situation in the field, most HPD users receive training on each type of device and they are aware of the risk of permanent hearing loss if the HPD is not properly fitted and used throughout all work shifts. Therefore, the presence of these outliers can inhibit an evaluation of the real situation and it may be useful to consider their elimination from the final results in order to obtain a truly representative sample. The Real-Ear Attenuation at Threshold (REAT) method is the gold-standard method, most commonly used and accepted worldwide for the measurement of hearing protector noise attenuation. This is a subjective measurement where the subjects determine their own threshold levels (with and without an HPD). The accuracy of this measurement is strongly dependent on the subject s perception of the sound level at the ear and each subject has to concentrate to determine their own threshold level. Considering that the subjects are paid, earning between 10 to 50 USD for each test, there is no guarantee that the subject has properly determined their threshold level. Some subjects pay greater attention than others and some may have work and/or educational experience which allows them to provide better results. Therefore for each hearing protector brand measurement, especially for plug-type devices (which are more difficult to fit than earmuffs) there are sometimes a few subjects (generally not more than five out of twenty) who show low accuracy in determining their threshold levels and this can result in large variations in the NRRsf value. In this paper, the results obtained for a pre-molded earplug brands, using the subject fitting method (B) of ANSI S , based on the evaluations of 20 subjects, are reported and analysed. Figure 1 shows the results for the bootstrap statistical analysis, considering 10 subjects, with 100,000 repetitions. The statistical distribution for the NRRsf shows a complex distribution, with four peaks for a range of NRRsf values of 11 to 20. In this case NRRsf=15 with a standard deviation of ±3 db. 3

Figure 1: The statistical distribution of NRRsf and contribution of subject 3 and 14 The Crystal Ball software was used to evaluate the sensitivity of the result with respect to each subject.

On removing Subjects 3 and 14 and recalculating the statistical distribution, a new distribution, which is very close to Gaussian, is obtained, as shown in figure 2, and the NRRsf value increased

4 Figure 1: The statistical distribution of NRRsf and contribution of subject 3 and 14 The Crystal Ball software was used to evaluate the sensitivity of the result with respect to each subject. Figure 1 shows that Subject 3 contributes 71.7 % to the NRRsf value and Subject 14 contributes 14%. On removing Subjects 3 and 14 and recalculating the statistical distribution, a new distribution, which is very close to Gaussian, is obtained, as shown in figure 2, and the NRRsf value increased from 15 to 19 db, while the standard deviation decreased from ±3 to ±1 db. It shows clearly that by observing the statistical distribution, calculated for each group of 10 subjects and repeated 100,000 times, it is possible to detect the extent to which the results deviate from a Gaussian distribution. Crystal Ball software was then used to identify the contribution of each subject to the NRRsf. With the removal of only two subjects, the NRRsf increased from 15 to 19 db and the standard deviation decreased from 3 to 1 db. 4

Figure 2: After removing the outliers (subject 3 and 14), showing that the NRRsf increased from 15 to 19 and the standard deviation decreased from 3 to 1 db 3 Uncertainty of hearing protectors noise

5 Figure 2: After removing the outliers (subject 3 and 14), showing that the NRRsf increased from 15 to 19 and the standard deviation decreased from 3 to 1 db 3 Uncertainty of hearing protectors noise attenuation measurements by REAT method All measurement results have an associated with uncertainty value. In general terms, this is known as measurement uncertainty, and is attributed to factors that influence the final results of the measurements. Measurement uncertainties can come from the measuring instrument, from the item being measured, from the environment, from the operator, from subjects used and from other sources. The errors of measurement can be expressed by the measurements uncertainty value. This value can be used to quantify the confidence limits of the measured results and allows comparison of measurements carried out by same or different laboratories and for same or different products. The uncertainty calculated here is based on GUM1 and other publications [4]. Uncertainty calculation for Hearing Protector Device (HPD) noise attenuation measurement can be carried out in different situations such as: a) One specified brand of HPD measured in one laboratory: which is the most important case that should be reported by that laboratory at the final results report. Usually a hearing protector s manufacturer or user asks a laboratory to make measurements for noise attenuation of a certain model of HPD. Also this can be extended to measurements of 5

6 several times the same HPD, in the same laboratory, using different subjects, which is very common case of periodic measurements of the same HPD in the same laboratory; b) Specified type of HPD: in this case the laboratory measured the noise attenuation of different model of HPD (for example a number of different model of Earmuff HPD) and should combine the results to give the uncertainty of all these models measured. Figure 1 show different types and models of HPD; c) Inter-laboratories measurements: for example inter-laboratories round measurements of noise attenuation of a specific model of HPD. In this case a specific model of HPD goes around different laboratories, where the noise attenuation is carried out using the same standard. Most of publications and also at the standards like ISO [3] or ISO [4] and ANSI S12.6 [6] do not consider these different cases separately. The first case is the most important one since we are usually interested to know for a specific model measured in a specific laboratory, what is the uncertainty of the noise attenuation measurement of this HPD. This paper shows detailed calculation of uncertainty of measurements results of noise attenuation of one specific model of HPD measured in one laboratory and extend that to repeated periodic measurement of the same HPD in the same laboratory several time. This paper is refinement of our previous paper published [5], where calculation is presented in more details with justifications[6]. The sources of uncertainty are test subject response for determining threshold of hearing, measurements parameters, hearing protector type, equipment used and test acoustics room. A typical especial case for Earplug show that [6] an error in the noise attenuation results can be up to 5 db. The largest contribution of uncertainty in hearing protector noise attenuation measurements is the subject response variation and standard deviation between subjects. Therefore, for each measurement of a HPD in one laboratory, the uncertainty calculation should be carried out. If the whole measurements are repeated for the same HPD, even for the same subjects, calculation should be carry out again because of the different in the subject responses used. This calculation should be presented in the final report of measurements. The subject response variation distribution need to be study more since it is not clear if it have a rectangular distribution. The subject response variation distribution may have a bimodal distribution since all the peak are near each other and all the valleys are near each other in the trace of the subject. Therefore, is important to determine what kind of distribution this type of uncertainty has. 4 Hearing protector comfort Hearing protector comfort can be measured subjectively by conducting perception experiments on a number of users via questionnaires. Subjective perception is influenced by many physiological and psychological factors. Based on a literature review summarized in [7] it appears that most studies on comfort have been focused on the total force of the head band or 6

7 the average pressure (dividing total force by contact area) and subjective evaluations based on the responses of a group of users who subjectively evaluate the comfort. However, a large number of the studies published on hearing protectors show that there is often a lack of correlation between comfort and total headband force or average pressure. Some published results, as previously discussed by the author, even indicate, unexpectedly, that a strong headband force is more comfortable than a weak headband force. Although comfort may initially appear to be a secondary requirement, it is important to note that an uncomfortable hearing protector device (HPD) may become intolerable after prolonged wear time and is typically removed or refitted for comfort, and not for best attenuation, leaving gaps which allow noise leakage. Pressure exerted by an HPD on the skin and underlying tissue and bone is probably one of the most common direct causes of discomfort. If the contact pressure is strong and continues for a relatively long period of time, the pain may become unbearable. Two factors are involved in this scenario: the total force of the hearing protector against the skin and the distribution of the contact pressure. The pressure exerted by earmuffs varies proportionally with the force applied by their means of support. When the total force is well distributed over a large area the resulting contact pressure is lower than when it is concentrated at a few smaller contact points and the protector is more comfortable. In order to ensure a large area of contact with the skin, earmuff cushions should not only be of a size and shape compatible with the ear and head anatomy, but they should also be made of a compliant material. Besides the most important parameter of contact pressure distribution, other parameter so lesser importance can affect the comfort of earmuffs including [7]: a) Total force of the headband: This is recommended to be below 14 Newton; b) Earmuff weight: The weights of 69 earmuffs were between 140 and 380 g, with an average value of 220 g and standard deviation of 57 g. Earmuffs with less than 245 g are acceptable and comfort is weakly related to earmuff weight; c) Contact area: For the same headband force, the larger the contact area the lower the pressure value and the better the comfort will be, but a large contact area can also result in leakage. Consequently, earmuffs should cover the smallest possible area of the head surface, while still accommodating the pinna, which conflicts with the need for a homogeneous pressure distribution; d) Noise attenuation: This is high for a strong headband force, which may reduce the level of comfort; e) Temperature and humidity: Ambient temperature and humidity can affect both the acoustic performance and the comfort of an earmuff. In some cases, a moderate softening of the material at body temperature may improve the conformability. This paper represents a continuation of a study [1] published by the same author, which concentrated on the contact pressure distribution, with two new contributions in relation to the measurement technique and the calculation methodology. Firstly, the new measurement system used in this paper is more robust and has permanent sensors fixed on a dummy head and a flat surface at the same time (in the previous paper the sensors gave false signals due to the 7

8 curves on the dummy head). Secondly, the comfort indices calculated are modified to cover a larger range with better resolution. A subjective evaluation is also carried out to verify the measurement results. This approach is under discussion at the ISO, ANSI and ABNT (Brazilian Standards Organization) working group on hearing protectors in relation to the preparation of a guide for earmuff comfort evaluation. The contact pressure between two surfaces can be measured by capacitive tactile sensors which store an electrical charge proportional to the contact pressure value. The sensors are made of conductive cloth (conformable), Kapton (industrial), Lycra (stretchable) or a combination of conductive cloth and Kapton (hybrid). Resistive tactile sensing is the primary competing technology, where the resistance of a conductive material, such as an elastomer, foam, or conductive ink, is used to detect and measure the pressure. The use of capacitive sensors is more appropriate for applications that require high levels of accuracy and sensitivity. These sensors are easier to calibrate, better at providing repeatable readings, and less susceptible to wear and tear than resistive sensors, which have reduced performance over time as the ink ages. The measurement system used in this paper consists of highly sensitive conformable tactile sensors (manufactured by Pressure Profile Systems - PPS) with thresholds down to pressures of less than 2 [kpa], which are mounted on a measurement system (see Figure 3). The sensor surface is 122 x 122mm with32x32sensors, each sensor having an area of 3.8x 3.8 mm. A measurement test system of variable width was constructed, with a flat surface on one side and half human dummy head on the other side, as shown in Figure3. The half dummy head adheres to the ANSI S3.36 standard. The open earmuff width is adjusted to 145±1 mm, as given in ANSI S [1] for the measurement of the headband force. The advantage of the flat surface measurements is that they enable an absolute comparison between the earmuff comfort indices to be performed without interference due to the complex geometry of the human (or dummy) head. The geometry of the human head is very complex and comfort index values will show a large variability. Therefore, a measurement taken on a flat surface is an absolute value without the interference of human variability. The dummy head should be able to provide a useful comfort index taking into account that in this case the human head geometry is standardized and through comparison with the flat surface measurements the effect of this human geometry can be determined. Two equations were used for the calculation of the comfort index. The first equation provided Comfort Index 1 (CI1), which is used in [7] and is given by: { [ ( ) ]} (1) 8

where: is the average pressure is the total pressure is the total number of contact points This index varies between 100 for zero variation in the contact pressure (very comfortable muffs) to 0for a

The second equation provided Comfort Index 2 (CI2),which is given by: [ ] [ [ ( ) ( ) ( ) ] ] (2) These two equations are based on the standard deviation of the contact pressure distribution.

9 where: is the average pressure is the total pressure is the total number of contact points This index varies between 100 for zero variation in the contact pressure (very comfortable muffs) to 0for a highly concentrated contact pressure or large standard deviation (very uncomfortable muffs). The CI1values for the eight muffs obtained with this equation are very close to each other. The second equation provided Comfort Index 2 (CI2),which is given by: [ ] [ [ ( ) ( ) ( ) ] ] (2) These two equations are based on the standard deviation of the contact pressure distribution. The standard deviation shows the degree of variation or dispersion from the average pressure distribution. A low standard deviation indicates that the data points tend to be very close to the mean (comfortable muff), while a high standard deviation indicates that the data points are spread over a large range of values (uncomfortable muff). Other equations were also investigated, but they provided very similar results. Figures 3, 4 and 5 show the values obtained for the two indices(ci1 and CI2), for the dummy head and the flat surface, for the eight earmuffs (A a H) considered. For each index the average value(± standard deviation) was calculated from the 9 measurements(three samples measured in triplicate) for each of the 8 earmuffs. Note that the comfort index for the flat surface is always larger than that for the dummy head, since the latter has curved surfaces and the contact pressure variation is greater. Figure 3: Variable width measurement system with flat surface on one side and half human dummy head on the other side. 9

10 Figure 4 : Contact pressure between earmuff and human manikin (left) and between earmuff and flat surface (right) for the 8 earmuffs. 10

11 A B C D E F G H Dummy head CI2 Flat CI2 Dummy Head CI1 Flat CI1 Subjective Figure 5: Comfort indices for the eight earmuffs, forthe dummy head and the flat surface, calculated usingthe two equations: CI1 (Equation 1) and CI2 (Equation2). The results obtained applying this novel technique show that the contact pressure distribution between the earmuff cushions and dummy head or flat surface is directly related to comfort. A more uniformed distribution gives more comfort even for a higher total headband force. Therefore, the design of the headband point of attachment, type of headband arc and flexibility of the cushions are very important factors in relation to earmuff design. A comparison between the values for the measured indices and the subjective evaluation showed a good correlation for all earmuffs (above 75%).Comfort indices do not provide calibrated values with absolute number, but rather the relative values obtained for the different brands (A to H). Using either the flat surface or the dummy head and also using either Equation 1 (CI1) or Equation 2 (CI2), the results gave almost parallel curves with the same comfort ranking. Further work is needed to quantify the uncertainty of the measurements. The uncertainty values for the sensors used are not available from the manufacturer. This technique for comfort evaluation can be also used to detect leakage as shown in [7]. Conclusions Hearing Protectors Devices (HPD) are the salvation of workers in high level noise to avoid permanent hearing loss. HPD should provide sufficient noise attenuation. Avoiding over protection they should be worn during all working period and therefore should be comfortable. In this paper novel method is 11

12 presented for evaluating earmuff comfort by measurement contact pressure between Earmuff and user face. Acknowledgments This research was carried out with the support of the Brazilian funding bodies (CNPq, CAPES and FINEP) and Laboratory of Personal Hearing Protector Equipment (LAEPI) of NR Consultancy Ltda. The guidance and orientation of Prof. Armando Albertazzi for the uncertainty calculation used her is very much appreciated. References [1] ANSI S Method for measuring the Real Ear Attenuation of Hearing Protectors. [2] ISO , Acoustics-Hearing Protectors. [3] Gerges, S. N. Y.; Dias, R.A.; Geges, R.N.C. Detection and contribution of outliers for subjective evaluation of sound. Accepted for IJSV, [4] BIPM,IEC, IFCC, ISO, IUPAP, OIML, Guid to express of Uncertainty in Measurements,Geneva, 1995 [5] Lima, F; Gerges, S.; Zmijevski, T.; Bender D.; Gerges, R. Uncertainty Calculation for Hearing Protectors Noise Attenuation Measurements by REAT Method. Journal of the Brazilian Society of Mechanical sciences and Engineering. Vol.32(1), 2010, pp [6] Gerges, R.; Gerges, S. and Vergara F. Uncertainty of hearing protector noise attenuation based on REAT method. ICA 2016, Buenos Aires. [7] ] Gerges S.N.Y. Earmuff comfort. Applied acoustics. 2012; 73,

3M Center for Hearing Conservation

3M Center for Hearing Conservation Key Terms in Occupational Hearing Conservation Absorption A noise control method featuring sound-absorbing materials that are placed in an area to reduce the reflection