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The objective occlusion effect induced by bone-conducted stimulation refers to the low frequency acoustic pressure increase that results from occluding the ear canal opening. This phenomenon is commonly interpreted as follows: the bone-conducted sound “leaks” through the earcanal opening and is “trapped” by the occlusion device. This instinctive interpretation misrepresents the fundamental mechanism of the occlusion effect related to the earcanal impedance increase and already highlighted by existing electro-acoustic models. However, these models simplify the earcanal wall vibration (i.e., the origin of the phenomenon) to a volume velocity source which, in the authors' opinion, (i) hinders an exhaustive comprehension of the vibro-acoustic behavior of the system, (ii) hides the influence of the earcanal wall vibration distribution, and (iii) could blur the interpretation of the occlusion effect. This paper analyzes, illustrates, and interprets the vibro-acoustic behavior of the open and occluded earcanal using an improved finite element model of an outer ear in conjunction with an associated electro-acoustic model developed in this work. The two models are very complementary to dissect physical phenomena and to highlight the influence of the earcanal wall vibration distribution, characterized here by its curvilinear centroid position, on the occlusion effect.

The IEC 60318-4 ear simulator is used to measure the insertion loss (IL) of earplugs in the ear canal of an acoustical test fixture (ATF) and is designed to represent an average acoustic impedance (in a reference plane) of the human ear. The ear simulator is usually modeled using a lumped parameter model (LPM) which has frequency limitations and inadequately accounts for the thermo-viscous effects in the simulator. The simulator numerical models that can better deal with the thermo-viscous phenomena often lack essential geometric details. Most related studies also suffer from the lack of experimental validation of the models. Therefore, a transfer matrix (TM) model of the IEC 60318-4 simulator is proposed based on a direct assessment of its geometric dimensions. Such a model is of particular interest for designing artificial ear simulators. The variability in the simulator impedance due to the geometric uncertainties is quantified using the Monte Carlo method. The TM model is validated using i) a finite element (FE) model of the simulator and ii) impedance measurements with a sound intensity probe. It is found to better describe the simulator impedance above 3 kHz compared to the LPM. The TM model is then coupled to a FE model of an occluded ATF ear canal to simulate the IL of an earplug in the frequency range [100 Hz, 10 kHz]. In the model, the simulator is considered as a cylindrical cavity terminated by an equivalent tympanic impedance which is determined from the TM model to simulate the sound pressure measured at the real microphone position (not at the reference plane) in the ATF ear canal. The simulated IL is validated against i) that obtained with a complete FE model of the corresponding system and ii) measurements using an ATF. The TM model is shown to better agree with the simulator FE model than the LPM above 6 kHz regarding the earplug IL simulated using this method.

In-ear devices are used in a wide range of applications for which the device's usability and/or efficiency is strongly related to comfort aspects that are influenced by the mechanical interaction between the device and the walls of the earcanal. Although the displacement of the earcanal walls due to the insertion of the device is an important characteristic of this interaction, existing studies on this subject are very limited. This paper proposes a method to estimate this displacement in vivo using a registration technique on magnetic resonance images. The amplitude, the location and the direction of the earcanal wall displacement are computed for four types of earplugs used by one participant. These displacements give indications on how each earplug deforms the earcanal for one specific earcanal geometry and one specific earplug insertion. Although the displacement due to a specific earplug family cannot be generalized using the results of this paper, the latter help to understand where, how much, and how each studied earplug deforms the earcanal of the participant. This method is revealed as a promising tool to investigate further acoustical and physical comfort aspects of in-ear devices.

Occlusion effect is a known phenomenon affecting in-ear device (e.g., hearing protection devices HPD, hearing aids) wearers. This psychoacoustical annoyance felt when the ears are occluded is perceived under various types of natural excitations such as physiological noises (chewing, breathing, heart beats) and one’s own voice or under an artificial excitation such as a bone oscillator. A common objective measurement of this occlusion effect (denoted OE) is defined as the difference between sound pressure levels (SPL) measured inside the occluded ear and the unoccluded ear. Two factors may cause this indicator to be sensitive to the type of excitation: (i) the repeatability of the excitation, because occluded and unoccluded SPLs are commonly measured at two different moments and (ii) the contribution of airborne sound emitted during the excitation process and mainly captured during unoccluded ear measurements. To investigate the impact of these factors, SPLs are measured with miniature microphones placed in occluded and unoccluded ears of various participants subjected to 5 different excitations, namely chewing, bone oscillator and speech at three intensities. Measurements are done in three ear configurations: (i) both ears unoccluded, (ii) one ear occluded and contralateral ear unoccluded, (iii) both ears occluded. This allows for the calculation of the aforementioned OE, but also a much less common indicator called the Real-Time Occlusion Effect (RTOE). Results are presented and discussed to compare the occlusion effect induced by the three excitations and objectively measured using the OE and RTOE indicators.

Hearing protection devices (HPD) are widely used to prevent noise-induced hearing loss (NIHL). In a noisy environment, wearing a correctly fitted HPD during all the time exposure is the sine qua non condition to prevent NIHL. However, this condition is often unfulfilled due to the discomforts in-duced by HPDs. Although this is a well-known fact, it remains challenging to quantify HPD com-fort since it is a multidimensional concept related to subjective feelings of the users. Thus, it is nec-essary to use objective indicators correlated to subjective attributes of HPD comfort to help manu-facturers designing efficient protectors. For earplugs, the insertion loss (IL), attenuation and occlu-sion effect (OE) seem good candidates to objectively describe attributes belonging to the acoustical dimension of comfort. However, most of the current ear simulators are not adapted to evaluate the physical variables related to these indicators since they do not consider important features of the ex-ternal ear such as the complex earcanal geometry. As part of an ongoing project aiming at develop-ing augmented artificial heads for measuring indicators of the acoustical comfort induced by ear-plugs, the goal of this study is to find parameters of the ear that significantly affect its vibro-acoustical behavior and evaluate a 3D vibro-acoustic finite element model of an artificial ear pre-sented in a companion paper. The artificial ear is placed in anechoic conditions and excited both acoustically and mechanically. Sound pressure is measured at the eardrum location when the earca-nal is open or occluded with a specially designed steel earplug and measurement are compared with numerical simulations. This study provides solid bases to the elaboration of augmented artificial ears for earplug comfort assessment.

Objective: This article presents a comprehensive literature review of past works addressing Hearing Protection Devices (HPD) comfort and to put them into perspective regarding a proposed holistic multidimensional construct of HPD comfort. 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 types, results and analyses are provided for the earplug type only. Results: This article proposed a multidimensional construct of HPD comfort based on four dimensions: physical, functional, acoustical and psychological. Seen through the prism of the proposed holistic construct of HPD comfort, the main comfort attributes of earplugs have been identified for each comfort dimension. Conclusions: The observed lack of consensus on the definition of HPD comfort in the scientific community makes it difficult to prioritise the importance of comfort attributes yet necessary for future design of comfortable earplugs.

The generalized cross correlation (GCC) is a standard technique for estimating time delay between microphone signals. A prefiltering operation by a weighting function may be included to whiten the cross spectrum of the microphones signals. The expected result is a narrow cross correlation function and a more accurate estimation of the time delay. Among the classic weighting functions, the most known is the PHAse Transform (PHAT). The ability of the PHAT weighting function to whiten the cross spectrum of the microphone signals can be improved by adding an exponent or the minimum value of the coherence function to the denominator. Both approaches have shown promising results for time delay estimation. In this work, the aforementioned modifications of the PHAT weighting function are considered for performing acoustic imaging with the classic GCC and the GCC based on the geometric mean. Numerical and experimental measurements are carried out in the case of two acoustic sources in front of a regular microphones array.

Hearing protection devices (HPD) are widely used to prevent noise-induced hearing loss (NIHL). In a noisy environment, wearing a correctly fitted HPD during all the time exposure is the sine qua non condition to prevent NIHL. However, this condition is often unfulfilled due to the discomforts in-duced by HPDs. Although this is a well-known fact, it remains challenging to quantify HPD comfort since it is a multidimensional concept related to subjective feelings of the users. Thus, it seems nec-essary to use objective indicators correlated to subjective attributes of HPD comfort to help manufac-turers to design efficient protectors. Those indicators would also help an adequate HPD selection by users. For earplugs, the insertion loss (IL), attenuation and occlusion effect (OE) are good candidates to objectively describe attributes belonging to the acoustical dimension of comfort. However, most of the current ear simulators are not adapted to evaluate the physical variables related to these indi-cators since they do not consider important features of the external ear such as the earcanal geometry or the tissues surrounding it. As part of an ongoing project aiming at developing augmented artificial heads for measuring indicators of the acoustical comfort dimension induced by earplugs, this study presents an artificial ear to assess objective indicators related to attributes of the acoustical comfort dimension of earplugs. This testing device has a realistic geometry and includes synthetic tissues mimicking the real human ear’s bone, cartilage and soft tissues. Its design and fabrication steps are presented. The ability of the device to assess objective indicators related to comfort attributes of ear-plugs is investigated by comparing attenuation and occlusion effect measurements performed on the artificial ear, a subject and an acoustic test fixture. Limits of the device such as its material properties, the absence of tympanic membrane and boundary conditions are discuss

The use of in-ear devices such as hearing aids or earplugs can be accompanied by an amplification of physiological noises upon their insertion in the earcanal, also called occlusion effect. Classical lumped elements-based prediction models for the occlusion effect consider the source of this effect solely due to the ear canal walls vibration. In most of these models, the in-ear device is considered as a simple infinite acoustic impedance which modifies the ear canal cavity length and thereby the ear canal walls vibrating surface depending on the insertion depth. The contribution of the earplug sound radiation to the occlusion effect is not considered as a significant mechanism. However, some authors have recently demonstrated using a finite element (FE) model that the earplug affect the vibration pattern of the ear canal walls but also may play the role of sound source in the unobstructed ear canal cavity thereby affecting the occlusion effect. In the most sophisticated lumped models, the in-ear device acts additionally as a loading on the ear canal wall which in turn modifies the ear canal free walls vibration. However the skin/in-ear device mechanical coupling is simplified and the earcanal cavity/in-ear device vibroacoustic coupling is ignored. Other physical effects such as leaks between in-ear device and skin may affect the occlusion effect at low frequencies but this has been rarely tackled in the literature. In this paper, a 2D axisymmetric FE model of the human open and occluded external ear including or not the presence of leaks is proposed to calculate the occlusion effect induced by earplugs. This model is used to study the influence on the occlusion effect of the various couplings skin/earplug/earcanal cavity for various insertion depths and leak diameters.

In Quebec (Canada), the most prevalent and expensive occupational disease is noise-induced hearing loss (NIHL). Noise reduction at the source or during the propagation path is not always possible and is often costly, resulting in a frequent use of hearing protection devices (HPDs), such as earplugs, to prevent workers from NIHL. Multiple studies have shown that workers are not protected to the level expected by the labelled attenuation given by the HPDs manufacturers. One of the causes for this discrepancy is the lack of training about HPDs insertion. This study aims to document the noise-exposed worker’s ability to adequately insert three different earplug models over a 5-week follow-up. To date, eighteen workers exposed to a high continuous noise level completed a questionnaire about their noise exposure history and HPDs experience. They were then group trained to insert the earplugs. A calibrated 3M™ E-A-Rfit™ Dual-Ear Validation System was used to measure the per-sonal attenuation rating (PAR) and train them individually for proper insertion during five weeks. During each fit test procedure, the earplug insertion was adapted by the worker until a PAR≥10 was obtained, and the worker wore the tested earplug model in the noisy workplace for a week. Workers followed this PAR measurement procedure and tested roll-down foam model for two weeks, push-to-fit model for two weeks, and premolded model for one week. All the quantitative data were analysed using SAS. Different levels of analyses were conducted. Results about the workers’ experience to insert the three different earplug models and the number of PAR trials will be presented considering the worker HPD experience and training prior to and during the study.

Acoustic test fixtures (ATFs) can be used to measure the insertion loss (IL) of earplugs. Reliable numerical modeling of the occluded artificial ear canal of the ATF could be helpful for optimizing the earplug performance during the design phase. This requires the simulation of the IEC 60318-4 occluded ear simulator which is usually based on the classical lumped parameter model (LPM) or complete 3D numerical model. Lumped models are generally accepted to have inherent frequency limitations and cannot properly deal with the thermal and viscous phenomena in certain areas of the simulator. 3D numerical models based on the finite element (FE) or boundary element method are capable of accurately describing the simulator behavior taking thermo-viscous effects into consideration but many dimensional details related to the published numerical models of the IEC 60318-4 simulator remain unspecified. This study proposes a transfer matrix (TM) model of the IEC 60318-4 simulator whose geometry is determined using Computed Tomography scan images. The specific acoustic impedance of certain elements in the simulator model is deduced using the low reduced frequency (LRF) model which has been proved satisfactory to account analytically for thermo-viscous energy losses. The TM model is validated using a 3D FE model of the simulator based on the same geometric dimensions. It is then coupled to a 2D axisymmetric FE model of an ATF ear canal occluded or not by a silicone earplug to simulate the earplug IL. The coupled FE/TM method is found to provide satisfactory IL prediction compared to a complete 3D FE model of the corresponding system. The proposed TM model is also shown to better capture the simulator behavior compared to the classical LPM.

The occlusion of the human earcanal is commonly associated with the so-called occlusion effect. This phenomenon is particularly uncomfortable at low frequency when wearing an intra-aural occlusion device in shallow insertion. In low frequency, the occlusion effect is objectively quantified as the acoustic pressure increase in the occluded earcanal compared to the open one. Fundamentally, the occlusion effect is governed by the change of earcanal acoustic impedance seen by its wall due to its occlusion. However, several factors influence the occlusion effect such as the occlusion device (e.g., type, fit, position in the earcanal), the stimulation (e.g., position and nature) and the earcanal anatomy (e.g., geometry and material properties of earcanal surrounding tissues). Compared to the first two aforementioned aspects, the latter has not been investigated yet because of experimental difficulty. However, it is important since the earcanal anatomy influences the earcanal wall vibration, which is the origin of the acoustic pressure generated in the earcanal. In this work, the influence of the earcanal surrounding tissues geometry on its vibration distribution is numerically studied. For this purpose, a full factorial design of experiment is conducted using a 2D axi-symmetric finite element model of an open and occluded outer ear. The occlusion is simplified to an infinite impedance defined at the ear-canal entrance. The influence of the earcanal vibration distribution on the occlusion effect and its fundamental mechanism is then investigated. In conjunction, an eletro-acoustic model is proposed to explain in a simplified way the trend of the finite element model results.

One of the reasons why hearing protectors are not fully efficient at protecting against noisy environments is the acoustical discomfort they induce. This discomfort may incite an individual to wear his protector incorrectly or to remove it, thereby reducing its performance. The acoustical discomfort depends on the acoustic pressure value at the eardrum of the protected ear. To reduce it efficiently, a tool for the acoustical design of the hearing protector device (HPD) allowing for determining this acoustic pressure, may prove to be useful. This paper presents the methodology to develop a unique and realistic numerical Vibroacoustic Model (VM) in the audible frequency range of a human head wearing an HPD together with an associated instrumented Experimental Anatomical Phantom (EAP). The VM consists of a 3D finite element model of the head of a living subject wearing an HPD reconstructed from medical images. This model will be validated and calibrated step by step against an EAP made up of synthetic materials with average mechanical properties. It will then be calibrated against the human subject from which the VM is the replica. A new EAP will be fabricated based on the updated mechanical properties of the calibrated VM. Finally, it will be exploited to study a variety of earplugs and earmuffs together with the sound transmission mechanisms through the head/HPD system. Preliminary results on some stages of the VM development are also presented. A registration method applied on medical images of a simplified EAP of the external ear without and with earplugs show to which extent the latter can deform the ear canal. The deformed shape of earplugs can also be identified. Comparisons between the predicted sound attenuation of a commercial earmuff and experimental data in a simplified configuration are presented to evaluate an improved earmuff finite element model.

Many workers are exposed on a daily basis to noise levels that can cause hearing problems. The risk of hearing loss then depends on both the level and the duration of exposure. Although it should ideally remain the last choice in terms of noise exposure reduction, hearing protection devices (HPDs) are the most commonly used noise control solution. Nevertheless, practically, discontinuous use, misuse or refusal to wear are frequently observed. Thus, while the "comfort" dimension (even though this notion for HPDs still turns out to be unclear and plural in the literature) may a priori seem to be secondary compared to the provided attenuation, it impacts the effective protection by influencing the HPD consistent and correct use. This paper focuses on earplugs which are the most commonly used category of HPDs. Despite the fact that the question of their use (and consequently of the protection) cannot be dissociated from the comfort one, they still remain exclusively characterized by the acoustic attenuation they provide. In a general way, it raises the question of identifying and understanding the comfort attributes and therefore, more fundamentally, the question of the definition of the comfort of earplugs itself. This last point proves to be major because this definition will finally impact the protocol of any research dedicated to comfort. Consequently, in order to progress on this crucial point, this paper presents different comfort attributes identified in a recent literature review (from the 1950s to the present) and proposes a "global" definition of comfort which includes the following four dimensions: physiological (irritation, pain, etc.), auditory (attenuation, occlusion effect, etc.), functional (ergonomics, easy-to-fit, etc.) and psychological (habituation, acceptance, etc.)

Microphone array techniques are an efficient tool to detect acoustic source positions. The delay and sum beamforming is the standard method. In the time domain, the generalized cross-correlation can be used to compute the noise source map. This technique is based on the arithmetic mean of the spatial likelihood functions. In this study, the classical arithmetic mean is replaced by the more standard generalized mean. The noise source maps provide by the arithmetic, geometric and harmonic means are compared in the case of numerical and experimental data obtained in a reverberant room. The geometric and harmonic means provide the best noise source maps with no side lobes and a better source level estimation.

Microphone arrays have become efficient tools to localize sound acoustic sources. The standard method is the delay and sum beamforming which can be used in the frequency or time domain. In the frequency domain, the cross spectral matrix of the microphone signals is used to generate the noise source map. Therefore, the processing has to be done frequency by frequency. To improve noise source maps, deconvolution or inverse methods can be utilized. However, these methods increase the computation time and may require user-defined parameters. In the time domain, the generalized cross-correlation of the microphone signals is used to compute the noise source map. The generalized cross-correlation of a microphone pair signal provides the spatial likelihood functions. Then, these spatial likelihood functions are averaged to generate the noise source map. This map is composed of a main lobe which leads to the source position together with side and spurious lobes which may prevent the localization of the source. The spatial likelihood functions averaging process is commonly based on the arithmetic mean. In this study, averaging based on the generalized mean is considered to improve the source localization. Several source-microphone array configurations are numerically computed. Noise source maps obtained with geometric and harmonic means are compared with that based on the classical arithmetic mean. The geometric and harmonic means are shown to provide the best noise source maps with few side lobes. The effect of the number of microphones on the noise source map quality is also investigated.

To be efficient, an Hearing Protection Device (HPD) should be worn during the time the workers are exposed to high level noises in order to protect them against hearing losses or deafness hazards. Several reasons like the difficulty to communicate, aesthetic considerations or discomfort issues prevent the workers from wearing HPDs properly. In the past, some indicators of physical (static pressure on the ear canal walls) and auditory (insertion loss, occlusion effect) discomfort induced by earplugs have been assessed using numerical models. However, most of these models rely on the assumption that the shapes of the open and occluded ear canal are identical which has never been demonstrated experimentally. As the recent rise of medical imaging and numerical computing technologies allows for reconstructing the 3D geometry of the outer ear, this study proposes to investigate the validity of this assumption. The shapes of the open and occluded ear canal of a human subject were obtained using high-resolution magnetic resonance imaging. Several kinds of earplugs were tested. Their non deformed shapes were obtained thanks to preliminary measurements. In each case the qualitative (from a radiologist assessment) and quantitative (from deformation measurements) comparison between the reconstructed occluded and open ear canal was carried out to evaluate whether and how both earplugs and ear canal deform. The deformation of earplugs and tissues of the ear canal provides useful information to investigate how each earplug and skin should be modelled and characterized mechanically in order to predict the earplug sound attenuation and occlusion effect.

This year, the ÉTS-IRSST common infrastructure for research in acoustics (ICAR) celebrates its 4th year of activity. This is a joint laboratory between the École de technologie supérieure (ÉTS) and the Institut de recherche Robert-Sauvé en santé et en sécurité du travail (IRSST). When first created in 2011 at ÉTS, the lab included a semi-anechoic chamber coupled with a reverberation room. In 2014, an audiometric booth was added and a new laboratory for the characterization of acoustic materials, described in a companion paper [Doutres JCAA 2015], was added this year.

Researchers of /TS and IRSST decided to synergize again their efforts in order to expand the ICAR laboratory and include a facility dedicated to acoustical materials. Acoustic properties of porous materials such as the normal incidence sound absorption coefficient and the normal incidence sound transmission loss together with several physical intrinsic parameters required as input in the most commonly used associated models (i.e., porosity, airflow resistivity, Young's modulus...) can now be measured in ICAR. This new facility aims at (1) improving the knowledge about the physical phenomena associated with the dissipation of the acoustic energy in porous materials of various microgeometries, (2) developing new materials with dedicated or uncommon acoustical properties (e.g., metamaterial, metacomposite,...) for industrial applications such as hearing protection, building, aeronautic or aerospace and (3) providing the input parameters required in acoustical prediction software dealing with acoustical materials.

A method for estimating the sound absorption coefficient of a material under a synthesized Diffuse Acoustic Field was recently proposed, as an alternative to classical sound absorption measurements in reverberant rooms. Using sound field reproduction approaches and a synthetic array of acoustic monopoles facing the material, estimation of the sound absorption coefficient under a reproduced Diffuse Acoustic Field but in free-field conditions was shown to be feasible. The method was successfully tested on two samples of melamine foam of close thicknesses and areas. In this paper, the principle of the method will be first recalled. The effect of varying the reproduction array size on the calculated sound absorption coefficients is then studied. A comparative measurement between the suggested method and the reverberant room method on ceiling tiles is finally reported.

This work presents a new method for assessing the normal incidence insertion loss (IL) of earplug (EP) by the use of a classical impedance tube. The proposed technique allows for fast and straightforward IL measurements and thus is particularly suitable for earplug testing in a design phase. Indeed, compared to standard ATF measurements, the proposed method does not require cumbersome experimental facilities (i.e., reverberant or anechoic chambers). In the proposed method, the EP (≈ 10 mm outer diameter) is inserted in a rigid sample holder which is itself placed in a larger diameter impedance tube (commonly 29 mm inner diameter). The transfer matrix of the system is measured according to a standardized procedure. The transfer matrix of the EP alone is then recalculated by eliminating the effect of the sample holder. Finally, the IL is estimated by coupling the EP transfer matrix to a one-dimensional model of the occluded ear canal taking into account for the tympanic membrane. The proposed method is tested numerically using a two-dimensional acoustical finite element model (FEM) of a cylindrical impedance tube and is validated by comparison with IL simulations computed from another numerical model of an occluded ear canal.