The type test of hearing protectors (HPD) for certification purposes will be conducted in laboratory at room temperature. Optionally, the mechanical durability of HPDs will be tested in cold environment by a drop test. The purpose of this study was to find out the relevance of the drop test, the change of performance in HPD protection, and finally to estimate the possible change of protection efficiency against noise in cold environment. In total, 22 HPDs were selected to the measurements: 18 earmuffs, and 4 earmuffs attached to an industrial helmet. Attenuation of each earmuff cup was measured by applying insertion loss method for the test subjects in cold. The change of attenuation and temperature of cushion ring was followed up to nine minutes using 30-second intervals for sampling. Three HPDs were damaged in the test. The replaceable cushion was broken in two earmuffs and in one helmet-mounted HPD. The replaceable parts were replaced, and the HPD with attachment failure was removed from insertion loss measurement. In nine HPDs the relative change was less than 3 dB, and was at worst 10 dB. This change was typically at low frequencies, 125 Hz at the beginning when cooled HPDs were placed. In various HPDs the time to get the attenuation levelled varied from 1.5 minutes to 8 minutes. The recovery was dependent on the temperature of the cushion ring. In all cases the temperature of the full attenuation was achieved when the cushion ring reached 7oC. This temporary decrease in attenuation will have a minor effect to the protection efficiency, when the HPD is used full time during the whole exposure duration. A typical group of forest workers will have their exposure interrupted. The chain saws have to refuel, and the chain needs to be sharpened about every 40 minutes. During 6 hour daily operational time there will be about 9 - 10 minute break, long enough to cool the cushion ring back to below zero at -10oC, if the helmet mounted earmuffs are placed in stand-by position. In the worst case this will cause 1.6 dB increase in daily exposure level to noise.
Keywords: Forest work, chain saw noise, earmuffs, cushion temperature, insertion loss, helmet liners
|How to cite this article:|
Starck J, Toppila E, Laitinen H. Effects of coldness on the protective performance of earmuffs. Noise Health 2005;7:47-53
| Introduction|| |
The general requirements for personal protective equipment (PPE) will be set in the directive 89/686/EEC (89/686/EEC, 1986). Hearing protectors (HPD) can be tested by using the harmonised standard (EN 352) series for various types of HPDs. EN 352-1 (EN352-1, 1993) is for earmuffs, and EN 352-3 (EN352-3, 1996) for earmuffs attached to an industrial helmet. These standards define the tests used for certification purposes. One of these tests concerns the ability of the HPD to stay intact when the hearing protector is cooled to -10oC, and dropped onto a metal plate. The drop test is classified as 'optional one', and there is no recommendation as to when this test should be taken. Since the test causes extra costs, it is often neglected.
The drop test in cold does not give evidence that the hearing protector provides the same protection in cold as in warm environments. Research on this topic has been minimal. However, 2-10 dB decreases of attenuation in cooled earmuffs have been measured in a study conducted using an artificial head (Toppila et al., 1994). Thus the lack of warming effect from a human head has not been included. That may cause an overestimation to the result.
The purpose of this study was to find out if the drop test has any practical importance. Furthermore, the aim was to establish possible changes needed to standards EN 352-1, EN 3523, and EN 458 (EN 458, 1993). More specifically, it was conducted to take into account the effect of coldness in practical use, and to provide a suggestion on necessary additional tests, changes to the selection guidance for earmuffs, and changes to existing tests.
| Material and methods|| |
The selection of the earmuffs used in the tests was made to extensively cover all existing earmuffs on the market. In total, 22 HPDs were used, of which 18 were earmuffs, and 4 were earmuffs attached to an industrial helmet. Two tests were performed: the resistance to damage in low temperatures when dropped (EN 13819-1, 2002), and attenuation measurements for test subjects in the cold applying miniature microphone method to measure insertion loss (IL) (EN 13819-2, 2002).
Resistance to damage in low temperature when dropped
For the drop test, the earmuffs spent four hours in a chamber (Arctest 500) that was cooled to -20+3oC. After this conditioning, the earmuffs were dropped onto a steel plate within 10 seconds of the removal from the chamber. The drop height was 1500 mm, ±10 mm. The earmuffs attached to an industrial helmet were dropped by using bifilar suspension, as stated in EN 352-3 (EN 352-3, 1996).
Attenuation measurements for test subjects in cold
The attenuation measurements were performed in the climatic chamber where the temperature was adjusted to -10oC. We restricted the temperature to -10oC instead of -20oC, used in optional drop test, because exposing an immobile test person without cold protection of head to -20oC would cause too much discomfort. For the insertion loss measurement a pink noise leveled to 78-80 dB was produced at the subjects head location [Figure - 1]a. The insertion loss was measured by placing microphones into the outer auditory canal entrances of both ears and by measuring the sound levels with and without earmuffs. The temperature of the cushion rings was registered simultaneously with the sound level signals [Figure - 1]b.
Two measurement series were performed. The attenuation performance of all 22 earmuffs was measured from both ears with two test subjects. The change of attenuation performance was followed during six to nine minutes, with 30 seconds averaging time and 30 second sample intervals. Insertion loss was registered at the () octave band frequencies, from 125 Hz to 8 kHz. The reference level was set to be the result from the first measurement and the relative change in attenuation was recorded. The measurement was stopped when no change in attenuation was observed.
| Results|| |
Resistance to damage in low temperature when dropped
Out of 22 earmuffs, 3 sets were damaged in the test. The replaceable cushions were broken in two earmuffs and in one set of helmet mounted earmuffs. The broken parts were replaced to continue the measurements. In the helmet mounted earmuff the attachment was broken and removed from the insertion loss measurement.
Insertion loss measurements for test subjects in the cold
In nine earmuffs the relative change of insertion loss (IL) was less than 3 dB, and was at worst 10 dB, typically at 125 Hz octave band. The time to reach full attenuation varied from 1.5 minutes to 8 minutes [Figure - 2],[Figure - 3],[Figure - 4]. This time was dependent on the temperature of the cushion ring. In all cases the temperature of the full attenuation was achieved when the 7oC temperature of the cushion ring was reached [Figure - 5].
| Discussion|| |
The cushion temperatures leveled in few minutes to their final values of over 10oC. To get the full attenuation a cushion temperature of 7oC was required. Depending on the earmuff, this took from 1.5 to 6 minutes when the starting temperature of the cushion ring was -10oC. The corresponding maximum change in IL was from 3 to 10 dB during the warm-up time, depending of the earmuff.
This temporary drop in IL caused by the warming of the cushion ring will have a negligible effect to the protection performance when earmuffs are used full-time during the whole exposure duration. However it can be significant in occupations with an intermittent exposure pattern that may cause breaks in the wearing of earmuffs. The effect will increase in occupations where earmuffs will be placed on and removed several times in a day. A typical example is forest work in winter, when the operation of the chainsaw is interrupted for refueling and sharpening every 40 minutes. During 6 hour daily operational time, there will be about nine breaks of 10 minutes, long enough to cool the cushion ring to below zero if the ambient temperature is below -10oC, and if the helmet mounted ear muffs used by forest workers are placed in stand by position.
In this example the effect on the protection efficiency will be calculated for a HPD with constant attenuation as follows:
The sound power Pin inside the HPD in each frequency is:
Where SPL is noise level outside the HPD and Amax is the attenuation level of HPD. The total sound energy Etot is given by:
Where T k is time (hours), n time periods (here, there is only one time period, and it is assumed that HPDs are always worn when necessary).
The average sound power Pin,aver is:
The average sound level Lin,aver is given by:
L in ,aver = 10 log(P in,aver )
If we consider the temperature dependence of attenuation, and the time period required for the HPD to cool down, it is possible to calculate the sound power (Pin,m) inside the HPD as follows:
Where m is the measurement period, m=1, . . ., s and A m the attenuation for the measurement period m. The total sound energy Etot is calculated by:
t k,m =1/120 h, r = the number of time periods However, mt k mr cannot be greater than 8 hours. If mt k m r is less than 8 hours, the rest of the total energy is calculated by:
E tot , rest= Pin (8- mt k,m r)
The average sound power Pin,aver is therefore:
And the average sound level Lin,aver inside the HPD is:
L in er =10 log(Pin aver )
The change in attenuation caused by the cooling of the HPD against the chain saw noise is the difference between inside noise levels calculated for HPDs that were not affected by cold, and for HPDs affected by cold.
The decrease in IL varied between 0 and 1.6 dB when the number of 10 minutes breaks during the working day was 10. The attenuation deteriorated from 0 to 2.8 dB for 20 breaks, correspondingly. Even though the differences seem small, even 3 dB decrease in attenuation doubles the received sound energy inside the HPD. In this test arrangement the temperature rose quickly when the HPDs were placed on. However, if the cushion and the material inside are wet from the sweating of the worker, they will get frozen during the breaks, and the temperature will rise more slowly. Therefore the attenuation performance will be worse than observed in the present study. Furthermore, lower temperatures than those used in the study can increase the decrease in attenuation performance. The measurements for helmet mounted earmuffs were taken without wearing helmet lines. In a cold environment, for instance forest work, helmet liners are commonly used. Earlier we investigated the helmet liners' effect to the attenuation performance, and found that fur helmet liners decreased the protection efficiency of HPD almost completely, when the HPD was placed on the fur liner. If a split was made in the liner so that the earmuff could be placed directly on the skin, the attenuation was increased by 1-2 dB (Godenhielm et al., 1979).
| Conclusions|| |
For earmuffs, the test for damage resistance in low temperatures when dropped does not provide useful information for hearing protector selection. The results indicate that a test for damage resistance in low temperatures when dropped will not provide information for the selection of HPDs that are attached to an industrial helmet. The susceptibility for breakage may be greater in some individual cases.
The testing of the deterioration of attenuation with the above-described method is not suitable to be used for standards. The test arrangement is difficult to organise, requires special equipment, is expensive and is unpleasant for test subjects. Instead, the effects of cold should be taken into consideration in the standard when dealing with the hearing protector selection. The standard EN 458 gives the following values for the selection of HPD.
If the selection of a HPD is derived from the above table without taking into consideration the effects of coldness, protection can be inadequate. With a regulation stating that the sound levels inside a HPD should always be below 80 dB in cold conditions, the possible effects of cold could be compensated by a safety marginal of 3 dB. That should be subtracted from the values in [Table - 2].
| Acknowledgements|| |
Ministry of Social Affairs and Health in Finland funded this study. We would like to thank Mr. Kauko Konttinen for the constructive comments on the manuscript.
| References|| |
|1.||89/686/EEC. (1986) Council directive on the approximation of the laws of the Member States, relating to personal protective equipment, European Commission, Brussels. |
|2.||EN 13819-1:2002. (2002) Hearing protectors. Testing. Part 1: Physical test methods. European Committee for Standardization CEN, Brussels. |
|3.||EN 13819-2:2002. (2002) Hearing protectors. Testing. Part 2: Acoustic test methods. European Committee for Standardization CEN, Brussels. |
|4.||EN 458:1993. (1993) Hearing protectors - Recommendations for selection, use, care and maintenance - Guidance document, European Committee for Standardization CEN, Brussels. |
|5.||EN352-1:1993. (1993) Hearing protectors - Safety requirements and testing - Part 1: Earmuffs, European Committee for Standardization CEN, Brussels. |
|6.||EN352-3:1993. (1993) Hearing protectors - Safety requirements and testing - Part 3: Earmuffs attached to an industrial safety helmet, European Committee for Standardization CEN, Brussels. |
|7.||Godenhielm B., Perkio K., Starck J. (1979) The effect of helmet liners to the attenuation performance of earmuffs. (In Finnish with English Summary). Research of Finnish Institute of Occupational Health 148, Helsinki, 37 p. |
|8.||Toppila E., Starck J., Pekkarinen J. (1994) Hearing protection efficiency in forestry. In: Blomback P, Heikkonen J, Jokiluoma H, et al, eds. Proceedings of the Seminar on Clothing and Safety Equipment in Forestry; 1994 Jun 27-Jul 1; Kuopio. Kuopio: University Printing Office, pp 183-187. |
Director of the Department of Physics, Finnish Institute of Occupational Health, Topeliuksenkatu 41 a A 00250 Helsinki
Source of Support: None, Conflict of Interest: None
[Figure - 1], [Figure - 2], [Figure - 3], [Figure - 4], [Figure - 5]
[Table - 1], [Table - 2]