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Objective Evaluate the ability of an extended version of the 3 MTM Eargage to estimate the earcanal size and assess the likelihood that a particular earplug can fit an individual’s earcanal, ultimately serving as a tool for selecting earplugs in the field. Design Earcanal morphology, assessed through earcanal earmolds scans, is compared to earcanal size assessed with the extended eargage (EE) via box plots and Pearson linear correlations coefficients. Relations between attenuation measured on participants (for 6 different earplugs) and their earcanal size assessed with the EE are established via comparison tests. Study sample 121 participants exposed to occupational noise (103 men, 18 women, mean age 47 years). Results The earcanal size assessed with the EE allows for estimating the area of the earcanal’s first bend cross-section (correlation coefficient r = 0.533, p < 0.001). Extremely large earcanals (12.7% of earcanals in our sample) lead to significantly lower earplug attenuation (potentially inadequate) than smaller earcanals. Conclusions The EE is a simple and inexpensive tool easily deployable in the field to assist earplugs selection. When extended with sizes larger than the maximum size of the commercial tool, it allows for detecting individuals with extremely large earcanals who are most likely to be under-protected.

A technique for estimating the sound absorption of materials under oblique incidence plane wave and diffuse field excitations is proposed. It requires a mobile loudspeaker and a pair of fixed microphones above a layer of absorbing material. Starting from sound pressure measurements made above the material surface for multiple source positions and by inverting Allard’s propagation model, the proposed method allows for the identification of two effective (or equivalent) parameters, namely, the complex effective density and the complex wave number. Obtaining these parameters makes it possible to estimate the sound absorption coefficient for a plane wave with any angle of incidence or under a diffuse acoustic field by summation over the angles. The results are compared to theoretical values calculated using the Johnson-Champoux-Allard model and to reference measurements obtained using an impedance tube, a small cabin, a large reverberant room, and a sound field synthesis method. One of the limitations of this method lies in the assumptions associated with the Allard model to describe the sound field above the material (assumed to be isotropic, homogeneous, of constant thickness, and to behave like an equivalent fluid). The main advantages are (1) that the sound absorption coefficient can be estimated under oblique incidence plane wave and diffuse acoustic field with values that are always in a physical range (between 0 and 1), and (2) that samples of the order of the square meter without specific preparation can be tested which reduces the constraints associated with impedance tube or reverberant room measurements

Longterm exposure to noise in the workplace can lead hearing loss and other psychosocial effects. Among the most effective noise reduction methods is to apply efforts directly at the source. To do so, each source must be characterized by their spatial location and contribution. In the workplace, a Spherical Microphone Array (SMA) can be used. When the microphones are held on a wireframe structure or on thin rods, the SMA is considered acoustically transparent, and the Conventional Beamforming in the Frequency domain (CBF) algorithm can be used. The CBF, however, does not compensate for the scattering effect of a diffracting object. On the other hand, rigid SMAs are usually made of a solid sphere with flush-mounted microphones and may scatter the acoustic waves. Although the literature has shown the advantages of using a rigid SMA with the spherical harmonics decomposition, the influence of the scattering effect on the acoustic image when not accounted for remains under-examined. This study aims to assess the influence of the uncompensated scattering effects on acoustic images obtained with a SMA. Images obtained with a rigid SMA are compared to those obtained from a theoretical perfectly transparent SMA using the CBF. First, a spherical wave field is simulated using a finite element analysis model to generate the microphone signals for both SMAs. Then, the images are generated using the CBF and three image quality criteria are used to assess the scattering effects, i.e., the ellipse area ratio, the mainlobe-to-sidelobe ratio and the mainlobe level. Results show that the influence of the scattering effect, if not corrected, will reduce the width of the mainlobe and amplify the level of the sidelobes. The effect on the estimated source level is not significant in this case.

The quality of an acoustic image is evaluated based on criteria such as the source level, the mainlobe-to-sidelobes ratio and the spatial resolution. Spatial resolution can be evaluated by the mainlobe area. For spherical microphone arrays, polar projection and ellipse area can be used. These methods are however applied in a plane and not in a spherical plane. In particular, the covariance ellipsemethod may exceed the spherical coordinate frame of the acoustic image when the mainlobe is large, typically at low frequencies, or when there are numerous sidelobes at high frequencies. In addition, the mainlobe may not be centered in the acoustic image and may be distorted around the poles. In order to overcome these limitations, a criterion based on the solid angle is introduced in this paper.The performance of the method based on the solid angle computation is compared with that based on the ellipse. Numerical simulations are performed using a spherical microphone array with a rigid t-design geometry, and spherical harmonics are used to generate the acoustic images. The results show that the solid angle method provides a more stable value compared to the covariance ellipse method when the source is not centered. When the source is centered, the results are similar except when sidelobes occur. In conclusion, the solid angle method provides a robust approach for evaluating the spatial resolution of acoustic images obtained with spherical microphone arrays.

Earplugs are extensively used as a preventive measure against noise-induced hearing loss. Nevertheless, their effectiveness is negatively impacted by the discomfort they induce, thereby influencing their consistent and proper use. In the context of earplug use, the comfort is defined as a multidimensional construct characterized by four dimensions: ‘physical’, ‘acoustical’, ‘functional’ and ‘psychological’. The (dis)comfort results from complex interactions between the triad constituted of the person wearing the earplug, his/her earplug, and his/her work environment. The objective of this paper is to enhance the understanding of the functional (dis)comfort induced by earplugs by identifying which triad characteristics can significantly influence the two attributes of this dimension: “functionality” (i.e., a general attribute) and “efficiency” (i.e., a specific attribute). Field assessments were conducted to evaluate the (dis)comfort of earplugs, involving 173 participants who tested different earplug models over seven weeks and provided a comprehensive comfort feedback through questionnaires. Triad characteristics were evaluated both on the field mainly through questionnaires, and in laboratory using comfort testers. Statistical analyses enabled the identification of key triad characteristics that influence functional comfort. Attributes of the functional comfort considered in this study are mainly governed by characteristics of the wearer: his/her previous experience with earplugs, his/her earcanal girth and a characteristic of the work environment: the interference of the earplug with other protective equipment The findings of this study hold the potential to provide manufacturers with tools for designing earplugs considering comfort aspects, as well as to help preventionists to recommend workers with earplugs that are best suited to individuals and work environment’s characteristics.

The use of passive earplugs is often associated with the occlusion effect: a phenomenon described as the increased auditory perception of one’s own physiological noise at low frequencies. As a notable acoustic discomfort, the occlusion effect penalizes the use and the efficiency of earplugs. This phenomenon is objectively characterized by the increase in sound pressure level in the occluded ear canal compared to the open ear canal. Taking inspiration from acoustic metamaterials, a new design of a three-dimensional printed “meta-earplug,” made of four Helmholtz resonators arranged in series, is proposed for achieving near zero objective occlusion effect measured on artificial ear in a broadband frequency range (300 Hz to 1 kHz). For this purpose, the geometry of the meta-earplug is optimized to achieve a null occlusion effect target based on an analytical model of the phenomenon. It results from the optimization process that the input impedance of the meta-earplug medial surface approximately matches the input impedance of the open ear canal, weighted by the ratio of volume velocity imposed by the ear canal wall to the ear canal cavity between open and occluded cases. Acoustic properties of the meta-earplug are also shown to significantly improve its sound attenuation at the piston-like mode of the system.

Designing earplugs adapted for the widest number of earcanals requires acoustical test fixtures (ATFs) geometrically representative of the population. Most existing ATFs are equipped with unique sized straight cylindrical earcanals, considered representative of average human morphology, and are therefore unable to assess how earplugs can fit different earcanal morphologies. In this study, a methodology to cluster earcanals as a function of their morphologies with the objective of designing artificial ears dedicated to sound attenuation measurement is developed and applied to a sample of Canadian workers’ earcanals. The earcanal morphologic indicators that correlate with the attenuations of six models of commercial earplugs are first identified. Three clusters of earcanals are then produced using statistical analysis and an artificial intelligence-based algorithm. In the sample of earcanals considered in this study, the identified clusters differ by the earcanal length and by the surface and ovality of the first bend cross section. The cluster that comprises earcanals with small girth and round first bend cross section shows that earplugs induced attenuation significantly higher than the cluster that includes earcanals with a bigger and more oval first bend cross section.

The occlusion effect (OE) occurs when the earcanal becomes occluded by an in-ear device, sometimes leading to discomforts experienced by the users due to the augmented perception of physiological noises, or to a distorted perception of one's own voice. The OE can be assessed objectively by measuring the amplification of the low-frequency sound pressure level (SPL) in the earcanal using in-ear microphones. However, as revealed by methodological discrepancies found in past studies, the measurement of this objective occlusion effect (OEobj) is not standardized. With the goal of proposing a robust yet simple methodology adapted for field assessment, three experimental aspects are investigated: (i) stimulation source and the stimulus's characteristics to induce the phenomenon, (ii) measurement method of the SPL in earcanal, (iii) indicator to quantify the OEobj. To do so, OEobj is measured on human participants in laboratory conditions. Results obtained with a specific insert device suggest using the participant's own voice combined with simultaneous measurements of the SPLs based on the noise reduction method and using a single value indicator leads to a simple yet robust methodology to assess OEobj. Further research is necessary to validate the results with other devices and to generalize the methodology for field assessment.

The sound attenuation of double hearing protectors (DHPs), earplugs combined with earmuffs, generally falls short of the sum of each single protector's attenuation when used independently. This phenomenon, referred to as the DHP effect, is found to be related to structure-borne sound transmission involving the outer ear and can also be observed on acoustic test fixtures (ATFs). At present, it still remains not fully understood, and no available model can help demonstrate the associated sound transmission mechanisms. In this work, a finite element model is proposed to study the DHP effect on an ATF between 100 Hz and 5 kHz. Power balances are calculated with selected configurations of the ATF in order to (i) quantify the contribution of each sound path, and study the effects of (ii) the artificial skin and (iii) acoustic excitation on the ATF exterior boundaries. The DHP effect is shown to originate from the structure-borne sound power injected from the ATF boundaries and/or earmuff cushion. The important influence of earcanal wall vibration is highlighted when the skin is accounted for. The simulation results allow for gaining more insight into the sound transmission through a DHP/ATF system.

Objective: This paper proposes a comprehensive literature review of past works addressing Hearing Protection Devices (HPDs) comfort with the aim of identifying the main sources of variability in comfort evaluation. Design: Literature review. Study samples: Documents were hand searched and Internet searched using ‘PubMed’, ‘Web of Science’, ‘Google Scholar’, ‘ProQuest Dissertations and Theses Professional’, ‘Scopus’ or ‘Google’ search engines. While comfort constructs and measurement methods are reviewed for both earplugs and earmuff HPD types, results and analyses are provided for earplugs only. Results: The literature shows that the multiple sources of the perceived comfort measurement variability are related to the complexity of the concept of comfort and to the various physical and psychosocial characteristics of the triad ‘Environment/Person/Earplug’ which differ from one study to the other. Conclusions: Considering the current state of knowledge and in order to decrease comfort measurements variability, it is advised to (i) use a multidimensional construct of comfort and derive a comfort index for each comfort dimensions, (ii) use exhaustive and valid questionnaires, (iii) quantify as much as triad characteristics as possible and use them as independent or control variables, and (iv) assess the quality of the earplug fitting and the attenuation efficiency.

The use of earplugs is commonly associated with an increased perception of the bone-conducted part of one's own physiological noise. This phenomenon is referred to as occlusion effect and is most prominent at low frequencies. Several factors influence the occlusion effect, such as the ear anatomy; the bone-conducted stimulation; and the type of occlusion device and its fit, insertion depth, and material properties. The latter factor is of great interest to potentially reduce the occlusion effect of passive earplugs. This paper investigates the mechanism(s) of contribution of earplugs to the objective occlusion effect. A two-dimensional axi-symmetric finite element model of the outer ear is used and investigated in an electro-acoustic framework. Simulation results are shown to compare reasonably well with measurement data, which qualifies the model to study the influence of earplugs on the occlusion effect. Two mechanisms are highlighted: (i) a Poisson effect induced by the normal component of the earcanal wall vibration and (ii) a longitudinal motion caused by the tangential component of the earcanal wall vibration. By varying the geometry of the surrounding tissues, the spatial distribution of the earcanal wall vibration is shown to influence the contribution of the earplug to the occlusion effect.

Earplugs are a common form of protection for workers exposed to hazardous noise levels. Their comfort directly impacts the effective protection by influencing their consistent and correct use. Nevertheless, comfort definition may vary according to the studies. Thus, a previous review of the literature has shown that to improve our understanding of perceived comfort and to reduce measurement variability, it is advisable to consider comfort through a multidimensional construct (physical, acoustical, functional and psychological). On this basis, the COPROD questionnaire was developed. It is intended for people working in noisy environments. Nine earplug models were evaluated by 118 participants over a six-week period. This paper presents the successive analyses that were used to validate the structure of the questionnaire and confirm the relevance of the proposed dimensions and of the addressed items. First results suggest a preference for custom molded earplugs. Practitioner summary: Earplugs comfort conditions the hearing protection of the users. As the definition of comfort can vary between studies, the COPROD questionnaire was developed to jointly evaluate all its dimensions. Nine earplugs models were evaluated by 118 participants during six weeks. This paper presents the validation process of the questionnaire.

L’effet d’occlusion est le nom donné à un inconfort acoustique couramment rencontré par les utilisateurs de prothèses auditives ou de protecteurs auditifs contre le bruit. Il est décrit comme la perception accrue des bruits physiologiques générés par le corps, notamment la voix, et dont les vibrations se propagent par conduction osseuse jusqu’à l’oreille interne. Pour expérimenter ce phénomène, rien de plus simple! Il vous suffit de lire ces lignes à voix haute en vous bouchant les oreilles. Dans cet article, l’histoire scientifique de l’effet d’occlusion est relatée depuis ses prémices jusqu’à la compréhension que nous en avons aujourd’hui, et nous plonge au cœur du processus par lequel se construit la connaissance. Mots clés : Effet d’occlusion, inconfort acoustique, appareil auditif

The occlusion effect is an acoustic discomfort commonly experienced by users of hearing aids or hearing protectors (i.e., earplugs or earmuffs). This phenomenon refers to the increased perception of physiological noise generated by the body, particularly the voice, when vibrations are propagated by bone conduction to the inner ear. Experimenting this phenomenon is very easy! Simply read these lines aloud while covering your ears. In this article, the scientific history of the occlusion effect is covered from its beginnings to our present understanding, and takes us to the heart of the process by which knowledge is built. Keywords: Occlusion effect, acoustic discomfort, hearing aid