According to the World Health Organization, worldwide hearing loss estimates increased from 120 million people in 1995 to 250 million in 2004. Since it is often difficult, for technological or economic reasons, to reduce noise at its source, the most commonly used solution to protect workers from noise exposure consists in using hearing protection devices (HPDs).
Unfortunately, workers do not always wear HPDs as required. One problem is the difficulty in providing an appropriate attenuation level required by an individual’s work environment. Another problem is that the occlusion of the ear canal induces a modification of the wearer’s voice perception, which creates a discomfort that sometimes leads people to remove their HPD.
Both of these problems exist because present methods of occlusion effect (OE) measurement and attenuation measurement have limitations. Objective measurements using a microphone in the ear canal do not take into account the bone conducted sounds directly transmitted to the cochlea and psychophysical measurements at hearing threshold are biased due to the low frequency masking effects from test subjects’ physiological noise.
The main objective of this doctorate thesis work is to improve the measurement of HPDs attenuation and the OE induced by wearing HPDs. The general approach is to: (i) ensure that it is possible to measure HPDs attenuation by using auditory steady state responses (protocol 1), (ii) adapt this methodology to measure the OE induced by wearing HPDs (protocol 2), and (iii) validate each protocol trough measurements performed on human subjects.
Results of protocol 1 shows that auditory steady state responses (ASSRs) can be used to objectively measure HPDs attenuation: results at 500 Hz and 1 kHz report that the physiological attenuations are relatively similar to the REAT values. This finding is in agreement with what was expected since the low-frequency masking effects from test subjects’ physiological noise is negligible at these frequencies.
Results of protocol 2 shows that ASSRs can be also used to objectively measure the OE induced by wearing HPDs: the average physiological OE measured at 500 Hz is significantly higher than the average psychophysical OE. This finding is in agreement with what was expected since, below 1 kHz, the low-frequency masking effects from test subjects’ physiological noise induces an overestimation of the occluded psychophysical hearing threshold. However, the results obtained at 250 Hz are inconsistent with what was expected.
From a scientific point of view, this PhD work has achieved two new innovative methods using electroencephalography for measuring HPDs’ attenuation and the induced OE.
From an occupational health and safety point of view, the advances described in this thesis can help to better design the HPDs. Indeed, if these two new objective methods were recommended by standards for characterizing HPDs, they may provide: (i) a better understanding of the effective attenuation of HPDs and (ii) a measurement of the discomfort induced by the occlusion of the ear canal. Providing HPDs with an effective attenuation adapted to an individual’s work environment and with an optimized comfort will, ultimately, improve working conditions by minimizing the risk of damage to the worker’s hearing.
Further research perspectives mentioned at the end of this thesis consist mainly in: (i) exploiting these two methods with an extended frequency range, (ii) investigating the intra-individual variability of each method, (iii) comparing the results obtained with these two methods with those obtained by using the «Microphone in Real Ear» (MIRE) method and (iv) verifying the assumption that this method is suitable for other kind of HPDs, such as earmuffs.
| Date | 3 Mar 2016 |
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| Original language | French |
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| Awarding Institution | - École de technologie supérieure
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| Supervisor | Frédéric Laville (Supervisor) |
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Valentin, O. (Author),
Laville (Supervisor),
3 Mar 2016Student thesis: Doctoral thesis › Doctorate in Engineering: Engineering