Objective and subjective assessment of sound attenuation efficiency by individual hearing protectors with various acoustic filters – a preliminary study
Background: Hearing loss caused by excessive noise levels is one of the most common health risks for employees. One solution for noise reduction is the use of hearing protectors, which is one of the most effective methods for protecting hearing from noise at the workplace. In order to obtain different attenuation efficiency, individual hearing protectors can be equipped with a suitable acoustic filter. The effectiveness of the hearing protectors attenuation is based on real measurement of hearing thresholds for normal-hearing people with and without hearing protectors. However, this is a time-consuming process and the obtained values are characterized by quite large inter-individual variability. The optimal solution is to measure the attenuation characteristics based on the objective method (without the presence of the subject), the results of which will be in accordance with the results of subjective tests. Therefore, the main purpose of the research in this work was to measure the attenuation characteristics of individual hearing protectors with acoustic filters through the use of subjective and objective methods, and to compare the results in terms of the research methods.
Methods: Measurements of the acoustic attenuation obtained by individual hearing protectors with designed F1, F2 and F3 acoustic filters, as well as full insert earplugs (without any acoustic filters) were carried out using two methods: objective and subjective. The objective measurements were carried out in an anechoic chamber. The artificial head (High-frequency Head and Torso Simulator Brül & Kjær Type 5128) was located at a distance of 3 m, directly opposite the loudspeaker. The test signal in the measurements was pink noise - in the frequency range up to 12.5 kHz and the level 85, 90 and 95 dB. The hearing protectors with and without acoustic filters were mounted in the Head and Torso Simulator which was connected with Pulse System Brül & Kjær. Five normal hearing subjects participated in the subjective measurements. A pink noise signal was used for one-third octave bands: 125, 250, 500, 1000, 2000, 4000 and 8000 Hz. The attenuation value was defined as the difference (in dB) between the hearing threshold of the test signal with a hearing protector and the hearing threshold determined without a hearing protector.
Results: The results of the objective method proved that in addition to the significant impact of frequency on the attenuation values, the type of filter used also had a significant effect. The objective measurement method showed that different levels of stimulation of the test signal did not significantly affect the attenuation efficiency for both the full earplugs and the earplugs with the F1, F2 and F3 filters. In addition, the results of the objective method showed that in the whole frequency range the highest attenuation values are shown by the full earplugs, achieving slightly above 45 dB for frequency of 8 kHz. The attenuation values obtained from subjective measurements also confirmed that both the frequency and type of filter significantly affect the attenuation values of the tested hearing protectors. Unlike the results of the objective method, the subjective method did not indicate significant differences in attenuation when using F1 and F2 filters.
Conclusions: The comparison of the average attenuation values obtained from the objective and subjective methods showed that in general the measurement method does not significantly affect the average attenuation values. In turn, the analysis of variance broken down into subgroups according to the types of filters used in the earplugs showed that the influence of the measurement method on the attenuation values is statistically significant when the F1 filter and full earplug are used. The results of this study partly confirmed the hypothesis that there is no significant impact of the measurement method on the attenuation characteristics of the earplugs with different types of acoustic filters.