Research on the optimization method of selecting hearing protectors in power station

In order to explore the noise reduction effect and application of different hearing protectors in different workplaces in a power station, so as to facilitate to select differently suitable hearing protectors to staff based on needs. This paper uses principal component analysis and evaluation according to the insertion loss test results of 112 kinds of hearing protectors. Combined with the field test results of 10 workplaces in a power station, it is found that 112 kinds of hearing protectors mostly are suitable in 7 working places. People need to select suitable hearing protectors specifically in other 3 places. At the same time, this paper can provide reference for the selection of hearing protection in other different working places.


Introduction
With the deepening of reform and opening up and the rapid development of the national economy, the construction of power station has also entered a period of rapid development. Because the power station has the characteristics of high water head, large capacity, high speed of the unit, two-way operation of water flow and changing frequency of working conditions, the noise is quite obvious, which will have a great impact on the personnel working in the factory area. Long-term work is highly likely to lead to permanent hearing loss and even severe occupational deafness in a high-noise environment without effective protection [1,2]. At present, many countries have listed occupational deafness as one of the important occupational diseases [3,4]. Therefore, people working in the high noise area of power station need hearing protection to avoid the risk of hearing loss. In this paper, we tested the insertion loss of 112 kinds of hearing protectors on the market, then we picked out the more suitable hearing protectors in 10 kinds of workplaces in the power station using principal component analysis method. At present, there are many kinds of hearing protectors that can be purchased, workers have some confusion when choosing them, and they often cannot find the most suitable hearing protectors. In order to help workers make the best choice according to the needs of the workplace and protect their hearing, this research work has been carried out.

Characteristics and analysis of power station noise
In this paper, there are different workplaces in this power station, the environment noise is measured separately in order to fully understand the noise situation of power station and select the best noise protection equipment. The noise exposure value in the working place is mainly measured, and the test time is from March 21th to March 23th in 2018. Under the operating condition of each unit, the ventilation facilities should operate normally. The noise level of the operating condition of each unit are shown in Table 1.
From Table 1, it can be seen that there are 10 testing places which are more than 85 dBA. The spectrum measurement and analysis of the 10 workplaces under operating conditions are shown in Fig. 1.

Preliminary screening of hearing protectors
We tested 112 kinds of hearing protectors insertion loss, they are all selected randomly from the market. During the test, we referred to the following standards: ISO 4869-3:2007 "Acoustics-Hearing protectors -Part 3: Measurement of insertion loss of ear-muff type protectors using an acoustic test fixture" and ISO 4859-2:1994 "Acoustics-Hearing protectors -Part 2: Estimation of effective A-weighted sound pressure levels when hearing protectors are worn" [5,6].
We tested using the instruments by B&K 4128C Head and Torso Simulators, using the pink noise as the noise source. The sound pressure levels were tested with [1] the hearing protector and without the hearing protector, respectively. The results of errors are insertion loss, that is: where , is the equivalent continuous A-weighted sound pressure level after wearing hearing protectors when workplaces are tested. , is the actual test of each frequency band sound level in different workplaces. , is the insertion loss of each frequency band sound level of different kinds of hearing protectors, is 33 frequency bands. The steps of testing are: (1) The sound pressure level at the microphone is tested using PLUSE 7758 when Head and Torso Simulators are not inserted the hearing protector.
(2) Place the hearing protectors in the Head and Torso Simulators' ear canal in sequence, and ensure the hearing protectors is located in the center of the microphone (as shown in Fig. 2).
(3) After about 30 s, the sound pressure level is tested again using PLUSE 7758 when Head and Torso Simulators are inserted the hearing protector.
(4) Repeat steps (1)-(3) three times with the same hearing protectors, the average value of three times is used to calculate the insertion loss.   Table 2.

Statistical analysis
Principal component analysis (PCA) is a multivariate statistical method that transforms multiple indexes into a few comprehensive indexes based on the idea of dimensionality and the principle of minimizing the loss of data information [7].
In this paper, the insertion loss of each central frequency band of 24 kinds of hearing protectors is analyzed by PCA method, as shown in Table 3 and Table 4.  According to Table 3, it can be obtained that the principal component coefficients extracted in each frequency band are all more than 0.5, which indicates that the extracted principal components have a higher degree of interpretation of each variable. According to Table 4, two principal components were extracted in this calculation analysis. The characteristic root of the first principal component is 6.990, the variance contribution rate is 69.899 %. And the characteristic root of the second principal component is 1.349, the variance contribution rate is 13.492 %. The cumulative variance contribution rate of the two principal components is 83.391 %. It is further indicated that the extraction of the two principal components is appropriate.  Table 5, the correlation coefficient between the first principal component and frequencies from 63 Hz to 8000 Hz are all close to 1, this indicate that there are more hearing protectors with noise reduction range from 63 Hz to 8000 Hz in the tested hearing protectors. Similarly, there are more hearing protectors with noise reduction range with frequency bands of 31.5 Hz and 16000 Hz in the second principal component.
Each principal component can be weighted and summed according to the characteristic root of each principal component: where and are two principal component functions respectively, and the characteristic roots of the two principal components respectively. According to Eq. (2), the ranking of principal component score and comprehensive score of 24 kinds of hearing protectors can be determined, as shown in Table 6.
We can see from the Table 6 that in the comparison of each brand of hearing protection in this study, the hearing protection of Xing Gong brand is more representative among 63-8000 Hz, and Pluggerz-sleep brand's hearing protection is more representative in 31.5 Hz and 16000 Hz.
Combined with the above principal component analysis, it can be concluded that among the 112 kinds of hearing protection devices tested, most of the hearing protection devices are more suitable for the 7 sites studied: #1 generator layer, #1 outdoor water turbine, #1 indoor water turbine, #2 outdoor draft tube cone, #2 indoor draft tube cone, #3 outdoor draft tube cone and #3 indoor draft tube cone.