Study of the occlusion effect induced by an earplug : modelling and experimental validation

Despite existing limits for occupational noise exposure, professional hearing loss remains a high priority problem both in Québec and worldwide. Several approaches exist to protect workers from harmful noise levels. The most frequently employed short term solution includes the distribution of hearing protection devices (HPD) such as earplugs and ear muffs. While HPDs offer an inexpensive (e.g. direct cost) and efficient means of protection workers often only tend to wear HPDs for limited amounts of time and, thus, remain at risk of developing professional hearing loss. Discomfort while using HPDs contributes to HPD underutilization and non-use. Two more general categories of discomfort can be distinguished. The category physical discomfort includes, for instance, problems such as heating of the ear and irritation of the ear canal that that occur upon earplug insertion. The category auditory discomfort refers to alterations in the auditory perception of sounds and one’s own voice as well as hindered workplace communications. One important auditory discomfort that promotes HPD non-use is the occlusion effect. The occlusion effect occurs upon earplug insertion and describes sound amplification phenomena in the occluded ear canal at the low frequencies. The sound amplification is both perceivable and measurable (e.g., open and occluded sound pressure levels, hearing threshold shift). Additionally, the occlusion effect causes the HPD wearer to perceive his/her own voice as being distorted (e.g. hollow sounding) and physiological noises (e.g. respiration, blood circulation) are amplified also subsequent to earplug insertion. Reducing the occlusion effect has the potential to increase the auditory comfort of HPDs and could help preventing occupational hearing loss in the future. In order to improve this and other shortcomings observed with currently existing HPDs a large research collaboration between the Robert-Sauvé research institute in occupational health and safety (IRSST) and the École de technologie supérieure (ÉTS) has been launched. The present study represents a part of this collaboration and aims at studying the occlusion effect of the system earplug – ear canal through the development of novel numerical models and experimental methods. A 3D (complex geometry) and an axisymmetric (simplified geometry) linear elasto-acoustic finite element model are presented in this study to simulate the objective bone conduction earplug occlusion effect. The 3D model of complex geometry is used to predict the occlusion effect induced by a silicone earplug at several insertion depths. Power balance computations are used to explain how the ear canal walls and the medial earplug surface contribute to observed occlusion effect magnitudes at varying occlusion depths. The numerical occlusion effect predictions are validated with experimental reference data that were retrieved from the literature. The 3D model is used to investigate two well established qualitative occlusion effect models using power balance computations. The axisymmetric occlusion effect model is used to predict open and occluded transfer function levels as well as the occlusion effect across three different excitation scenarios. First, only structure borne excitation is considered. Next, airborne noise is added incoherently to the structure borne excitation to study the effect of a mixed excitation. The mixed excitation is considered (i) for a leak free (perfect seal) earplug insertion and (ii) Under the presence of small earplug leaks. Each stimulation scenario is examined across four different boundary and load conditions. All predicted transfer function levels and occlusion effects are compared to experimental data. An adapted version of the axi-symmetric occlusion effect model is employed to investigate the contribution of the earplug type to the occlusion effect magnitude. First, the numerical model is validated with the help of experimental occlusion effect data which were measured in two independent human reference groups which each use a different earplug type (silicone earplug and foam earplug). Second, the numerical model is further validated through comparison with two gold standard lumped element occlusion effect models which were drawn from the literature. Third, power balance computations are employed to investigate the power flow inside the occluded ear canal cavity as well as the earplug body (coupled to the ear canal walls) of the numerical external ear model. The power balances are computed both for the foam and the silicone earplug models at medium and very shallow earplug insertion depths. A prototype of a novel artificial external ear test fixture for objective and standardized measurement of the occlusion effect is developed. Details on the implementation of the artificial external ear and the assembly of the occlusion effect test fixture are presented. The experimental test fixture is used to investigate the contribution of the structure and airborne sound transmission pathways. Experimental data is provided to demonstrate that the test fixture is functional and that it can be used to measure the occlusion effect of a foam earplug. The occlusion effect measurement is repeated for several mechanical stimulation levels to study the system’s linearity

Enzymatic Synthesis of Trimethyl-ϵ-caprolactone : Process Intensification and Demonstration on a 100 L Scale

Optimization and scaling up of the Baeyer-Villiger oxidation of 3,3,5-trimethyl-cyclohexanone to trimethyl-ε-caprolactones (CHLs) were studied to demonstrate this technology on a 100 L pilot plant scale. The reaction was catalyzed by a cyclohexanone monooxygenase from Thermocrispum municipale that utilizes the costly redox cofactor nicotinamide adenine dinucleotide phosphate (reduced form), which was regenerated by a glucose dehydrogenase (GDH). As a first stage, different cyclohexanone monooxygenase formulations were tested: cell-free extract, whole cells, fermentation broth, and sonicated fermentation broth. Using broth resulted in the highest yield (63%) and required the least biocatalyst preparation effort. Two commercial glucose dehydrogenases (GDH-105 and GDH-01) were evaluated, resulting in similar performances. Substrate dosing rates and biocatalyst loadings were optimized. On a 30 mL scale, the best conditions were found when 30 mM h-1 dosing rate, 10% (v/v) cyclohexanone monooxygenase broth, and 0.05% (v/v) of glucose dehydrogenase (GDH-01) liquid enzyme formulation were applied. These same conditions (with oxygen instead of air) were applied on a 1 L scale with 92% conversion, achieving a specific activity of 13.3 U gcell wet weight (cww)-1, a space time yield of 3.4 gCHL L-1 h-1, and a biocatalyst yield of 0.83 gCHL gcww-1. A final 100 L demonstration was performed in a pilot plant facility. After 9 h, the reaction reached 85% conversion, 12.8 U gcww-1, a space time yield of 2.7 g L-1 h-1, and a biocatalyst yield of 0.60 gCHL gcww-1. The extraction of product resulted in 2.58 kg of isolated final product. The overall isolated CHL yield was 76% (distal lactone 47% and proximal lactone 53%)

Ketoisophorone synthesis with an immobilized alcohol dehydrogenase

Altres ajuts: Authors also thank COST Action CM 1303-Systems Biocatalysis for financial support.The monoterpenoid α-isophorone is sourced from the available and renewable plant dry matter, as well as a waste recovery operation from acetone. This compound, can be hydroxylated to 4-hydroxy-isophorone which is the main precursor for the synthesis of ketoisophorone. On its turn, ketoisophorone is a key intermediate for the production of carotenoids and Vitamin E. Here, the enzymatic oxidation of 4-hydroxy-isophorone to ketoisophorone is demonstrated employing an alcohol dehydrogenase (ADHaa) from Artemisia annua and a NADPH oxidase (NOX), as a cofactor regeneration enzyme. After 24 h of reaction and an initial substrate concentration of 50 mM, 95.7 % yield and a space time yield of 6.52 g L⁻¹ day⁻¹ could be obtained. Furthermore, the immobilization of the alcohol dehydrogenase was studied on 17 different supports. An epoxy-functionalized agarose resulted in the highest metrics, 100±0% immobilization yield and 58.2±3.5 % retained activity. Finally, the immobilized ADHaa was successfully implemented in 4 reaction cycles (96 h operation) presenting a biocatalyst yield of 23.4 g product g⁻¹ of enzyme. It represents a 2.5-fold increase compared with the reaction with soluble enzymes

Trimethyl-e-caprolactone synthesis with a novel immobilized glucose dehydrogenase and an immobilized thermostable cyclohexanone monooxygenase

An often associated drawback with Baeyer-Villiger monooxygenases, is its poor operational stability. Furthermore, these biocatalysts frequently suffer from substrate/product inhibition. In this work, a thermostable cyclohexanone monooxygenase (TmCHMO) was immobilized and used in the synthesis of trimethyl-ε-caprolactone (CHL). As a cofactor regeneration enzyme, a novel and highly active glucose dehydrogenase (GDH-01) was used immobilized for the first time. MANA-agarose was the carrier chosen since it presented an immobilization yield of 76.3 ± 0.7% and a retained activity of 62.6 ± 2.3%, the highest metrics among the supports tested. Both immobilized enzymes were studied either separately or together in six reaction cycles (30 mL; [substrate] =132.5 mM). A biocatalyst yield of 37.3 g g−1 of TmCHMO and 474.2 g g−1 of GDH-01 were obtained. These values represent a 3.6-fold and 1.9-fold increase respectively, compared with a model reaction where both enzymes were used in its soluble form

Ketoisophorone synthesis with an immobilized alcohol dehydrogenase

Altres ajuts: Authors also thank COST Action CM 1303-Systems Biocatalysis for financial support.The monoterpenoid α-isophorone is sourced from the available and renewable plant dry matter, as well as a waste recovery operation from acetone. This compound, can be hydroxylated to 4-hydroxy-isophorone which is the main precursor for the synthesis of ketoisophorone. On its turn, ketoisophorone is a key intermediate for the production of carotenoids and Vitamin E. Here, the enzymatic oxidation of 4-hydroxy-isophorone to ketoisophorone is demonstrated employing an alcohol dehydrogenase (ADHaa) from Artemisia annua and a NADPH oxidase (NOX), as a cofactor regeneration enzyme. After 24 h of reaction and an initial substrate concentration of 50 mM, 95.7 % yield and a space time yield of 6.52 g L⁻¹ day⁻¹ could be obtained. Furthermore, the immobilization of the alcohol dehydrogenase was studied on 17 different supports. An epoxy-functionalized agarose resulted in the highest metrics, 100±0% immobilization yield and 58.2±3.5 % retained activity. Finally, the immobilized ADHaa was successfully implemented in 4 reaction cycles (96 h operation) presenting a biocatalyst yield of 23.4 g product g⁻¹ of enzyme. It represents a 2.5-fold increase compared with the reaction with soluble enzymes

Trimethyl-e-caprolactone synthesis with a novel immobilized glucose dehydrogenase and an immobilized thermostable cyclohexanone monooxygenase

An often associated drawback with Baeyer-Villiger monooxygenases, is its poor operational stability. Furthermore, these biocatalysts frequently suffer from substrate/product inhibition. In this work, a thermostable cyclohexanone monooxygenase (TmCHMO) was immobilized and used in the synthesis of trimethyl-ε-caprolactone (CHL). As a cofactor regeneration enzyme, a novel and highly active glucose dehydrogenase (GDH-01) was used immobilized for the first time. MANA-agarose was the carrier chosen since it presented an immobilization yield of 76.3 ± 0.7% and a retained activity of 62.6 ± 2.3%, the highest metrics among the supports tested. Both immobilized enzymes were studied either separately or together in six reaction cycles (30 mL; [substrate] =132.5 mM). A biocatalyst yield of 37.3 g g−1 of TmCHMO and 474.2 g g−1 of GDH-01 were obtained. These values represent a 3.6-fold and 1.9-fold increase respectively, compared with a model reaction where both enzymes were used in its soluble form

Enzymatic Synthesis of Trimethyl-ϵ-caprolactone : Process Intensification and Demonstration on a 100 L Scale

Optimization and scaling up of the Baeyer-Villiger oxidation of 3,3,5-trimethyl-cyclohexanone to trimethyl-ε-caprolactones (CHLs) were studied to demonstrate this technology on a 100 L pilot plant scale. The reaction was catalyzed by a cyclohexanone monooxygenase from Thermocrispum municipale that utilizes the costly redox cofactor nicotinamide adenine dinucleotide phosphate (reduced form), which was regenerated by a glucose dehydrogenase (GDH). As a first stage, different cyclohexanone monooxygenase formulations were tested: cell-free extract, whole cells, fermentation broth, and sonicated fermentation broth. Using broth resulted in the highest yield (63%) and required the least biocatalyst preparation effort. Two commercial glucose dehydrogenases (GDH-105 and GDH-01) were evaluated, resulting in similar performances. Substrate dosing rates and biocatalyst loadings were optimized. On a 30 mL scale, the best conditions were found when 30 mM h-1 dosing rate, 10% (v/v) cyclohexanone monooxygenase broth, and 0.05% (v/v) of glucose dehydrogenase (GDH-01) liquid enzyme formulation were applied. These same conditions (with oxygen instead of air) were applied on a 1 L scale with 92% conversion, achieving a specific activity of 13.3 U gcell wet weight (cww)-1, a space time yield of 3.4 gCHL L-1 h-1, and a biocatalyst yield of 0.83 gCHL gcww-1. A final 100 L demonstration was performed in a pilot plant facility. After 9 h, the reaction reached 85% conversion, 12.8 U gcww-1, a space time yield of 2.7 g L-1 h-1, and a biocatalyst yield of 0.60 gCHL gcww-1. The extraction of product resulted in 2.58 kg of isolated final product. The overall isolated CHL yield was 76% (distal lactone 47% and proximal lactone 53%)