On the area-averaged effective sound absorption coefficient of porous materials excited by a monopole.

The area-averaged effective sound absorption coefficient (SAC) of a rigid-backed homogeneous porous material subjected to a monopole excitation is calculated as the absorbed-to-incident sound power ratio. Using Allard's model to describe the sound propagation above the porous material, an analytical model for this power-based SAC is proposed and proves to give a good approximation of the sound absorption performance under monopole excitation of sufficiently large areas of material. The impact of factors on the power-based SAC, such as monopole height, material radial dimension used to calculate the sound powers, and material properties is discussed. The power-based SAC frequency-dependent behavior is analyzed through sound intensity field assessments at the material surface and is compared to normal incident plane wave and diffuse field SACs. The sound absorption behavior of sound absorbers under monopole excitation exhibits notable distinctions and peculiar results compared to those observed under plane wave and diffuse fields, particularly at low frequencies and for sources close to the material.

Impact of the ear canal motion on the impedance boundary conditions in models of the occlusion effect.

The occlusion effect (OE) denotes the increased low-frequency perception of bone-conducted sounds when the ear canal (EC) is occluded. Circuit and finite element (FE) models are commonly used to investigate the OE and improve its prediction, often applying acoustic impedances at the EC entrance and tympanic membrane (TM). This study investigates the sound generation caused by the structural motion of the EC. In addition to the EC wall vibration, it accounts for the motions of the EC entrance and TM, resulting from nondeforming motion of the surrounding structures. A model extension including these motions with the impedances is proposed. Related mechanisms are illustrated based on a circuit model. Implications are discussed by using an EC motion extracted from a FE model of a human head. The results demonstrate that the motions of the EC entrance and TM, addressed by the proposed extension, affects the TM sound pressure and may lead to a reduction of the OE at lower frequencies compared to solely considering the EC wall vibration. Accordingly, this phenomenon potentially reconciles differences between experimental data and OE simulations at frequencies below about 250 Hz, highlighting the importance to discern between multiple contributing mechanisms to the TM sound pressure.

Passive earplug including Helmholtz resonators arranged in series to achieve broadband near zero occlusion effect at low frequencies.

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.

Acoustic imaging with spherical microphone array and Kriging.

Acoustic imaging can be performed using a spherical microphone array (SMA) and conventional beamforming (CBF) or spherical harmonic beamforming (SHB). At low frequencies, the mainlobe width depends on the SMA radius for CBF and on the order of the spherical harmonics expansion for SHB, which is related to the number of microphones. In this letter, Kriging is used to virtually increase the SMA radius and/or the number of microphones. Numerical and experimental investigations show the effectiveness of Kriging to reduce the mainlobe width and thus improve the acoustic images obtained with a SMA and CBF or SHB.

Morphologic clustering of earcanals using deep learning algorithm to design artificial ears dedicated to earplug attenuation measurement.

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.

Reduction of the occlusion effect induced by earplugs using quasi perfect broadband absorption.

Passive earplugs are used to prevent workers from noise-induced hearing loss. However, earplugs often induce an acoustic discomfort known as the occlusion effect. This phenomenon corresponds to an increased auditory perception of the bone-conducted part of physiological noises at low-frequency and is associated with the augmentation of the acoustic pressure in the occluded earcanal. In this work, we report a new concept of passive earplugs for mitigating the occlusion effect between 100 Hz and 1 kHz. The strategy consists in reducing the input impedance of the earplug seen from the earcanal by using quasi-perfect broadband absorbers derived from the field of meta-materials. The proposed "meta-earplug" is made of 4 critically coupled Helmholtz resonators arranged in parallel. Their geometry is optimized using an evolutionary algorithm associated with a theoretical model of the meta-earplug input impedance. The latter is validated against a finite-element approach and impedance sensor measurements. The meta-earplug is manufactured by 3D printing. Artificial test fixtures are used to assess the occlusion effect and the insertion loss. Results show that the meta-earplug induces an occlusion effect approximately 10 dB lower than foam and silicone earplugs while it provides an insertion loss similar to the silicone earplug up to 5 kHz.

Towards a practical methodology for assessment of the objective occlusion effect induced by earplugs.

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.

A finite element model to predict the double hearing protector effect on an in-house acoustic test fixture.

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.

A critical review of the literature on comfort of hearing protection devices: analysis of the comfort measurement variability.

Objective. This article proposes a comprehensive literature review of past works addressing hearing protection device (HPD) comfort with the aim of identifying the main sources of variability in comfort evaluation. Methods. A literature review of study samples was performed: 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: (a) use a multidimensional construct of comfort and derive a comfort index for each comfort dimension;, (b) use exhaustive and valid questionnaires; (c) quantify as many triad characteristics as possible and use them as independent or control variables; (d) assess the quality of the earplug fitting and the attenuation efficiency.

Simulation of the objective occlusion effect induced by bone-conducted stimulation using a three-dimensional finite-element model of a human head.

The occlusion effect (OE) refers to the phenomenon that more bone-conducted physiological sounds are transmitted into the earcanal when it is blocked and may cause discomfort on users of hearing protection devices. Models have been proposed to study the OE as they can help understand the physical mechanisms and can be used to evaluate the individual contribution on the OE of the factors that may affect it (i.e., occlusion device, ear anatomy, and stimulation). The existing finite element models developed to study the OE are limited by their truncated ear geometries. In order to progress in the understanding of the OE, the goal of this paper is to develop a finite element model of an entire head to predict the sound pressure field in its earcanals, open or occluded by earplugs. The model is evaluated by comparing the computed input mechanical impedances and OEs in various configurations with literature data. It is able to reproduce common behavior of the OE reported in the literature. In addition, the model is used to assess the effects on the simulated OEs of several parameters, including the modeling of the external air, the boundary condition at the head base and the material properties.

Numerical investigation of the earplug contribution to the low-frequency objective occlusion effect induced by bone-conducted stimulation.

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.

Assessing the comfort of earplugs: development and validation of the French version of the COPROD questionnaire.

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 (COnfort des PROtections auDitives/COmfort of hearing PROtection Devices) 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 moulded 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. Abbreviations: COPROD: COnfort des PROtections auDitives/COmfort of hearing PROtection Devices; HPD: hearing protection devices; SEM: structural equation modeling; CFA: confirmatory factor analysis; GOF: goodness of fit; RMSEA: root mean square error of approximation; CFI: comparison fit index; SRMR: standardised root mean square residual.

Theoretical investigation of the low frequency fundamental mechanism of the objective occlusion effect induced by bone-conducted stimulation.

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.

Use of magnetic resonance image registration to estimate displacement in the human earcanal due to the insertion of in-ear devices.

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.

A critical review of the literature on comfort of hearing protection devices: definition of comfort and identification of its main attributes for earplug types.

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.

On the use of modified phase transform weighting functions for acoustic imaging with the generalized cross correlation.

The generalized cross correlation (GCC) is an efficient technique for performing acoustic imaging. However, it suffers from important limitations such as a large main lobe width for noise sources with low frequency content or a high amplitude of side lobes for noise sources with high frequencies. Prefiltering operation of the microphone signals by a weighting function can be used to improve the acoustic image. In this work, two weighting functions based on PHAse Transform (PHAT) improvements are used. The first adds an exponent to the PHAT expression (ρ-PHAT), while the second adds the minimum value of the coherence function to the denominator (ρ-PHAT-C). Numerical acoustic images obtained with the GCC and those weighting functions are compared and quantitatively assessed thanks to a metric based on a covariance ellipse, which surrounds either the main lobe or the side lobes. The weighting function ρ-PHAT-C provides the smallest surface ellipses especially when the arithmetic of the GCC is replaced by the geometric mean (GEO). Experimental measurements are carried out in a hemi-anechoic room and a reverberant chamber where two loudspeakers were set in front of microphone array. The acoustic images obtained confirm that the ρ-PHAT-C with the GEO outperforms the GCC, GCC-PHAT, and GCC ρ-PHAT.

How reproducible is the acoustical characterization of porous media?

There is a considerable number of research publications on the characterization of porous media that is carried out in accordance with ISO 10534-2 (International Standards Organization, Geneva, Switzerland, 2001) and/or ISO 9053 (International Standards Organization, Geneva, Switzerland, 1991). According to the Web of ScienceTM (last accessed 22 September 2016) there were 339 publications in the Journal of the Acoustical Society of America alone which deal with the acoustics of porous media. However, the reproducibility of these characterization procedures is not well understood. This paper deals with the reproducibility of some standard characterization procedures for acoustic porous materials. The paper is an extension of the work published by Horoshenkov, Khan, Bécot, Jaouen, Sgard, Renault, Amirouche, Pompoli, Prodi, Bonfiglio, Pispola, Asdrubali, Hübelt, Atalla, Amédin, Lauriks, and Boeckx [J. Acoust. Soc. Am. 122(1), 345-353 (2007)]. In this paper, independent laboratory measurements were performed on the same material specimens so that the naturally occurring inhomogeneity in materials was controlled. It also presented the reproducibility data for the characteristic impedance, complex wavenumber, and for some related pore structure properties. This work can be helpful to better understand the tolerances of these material characterization procedures so improvements can be developed to reduce experimental errors and improve the reproducibility between laboratories.

On the use of geometric and harmonic means with the generalized cross-correlation in the time domain to improve noise source maps.

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.

Low frequency finite element models of the acoustical behavior of earmuffs.

This paper compares different approaches to model the vibroacoustic behavior of earmuffs at low frequency and investigates their accuracy by comparison with objective insertion loss measurements recently carried out by Boyer et al. [(2014). Appl. Acoust. 83, 76-85]. Two models based on the finite element (FE) method where the cushion is either modeled as a spring foundation (SF) or as an equivalent solid (ES), and the well-known lumped parameters model (LPM) are investigated. Modeling results show that: (i) all modeling strategies are in good agreement with measurements, providing that the characterization of the cushion equivalent mechanical properties are performed with great care and as close as possible to in situ loading, boundary, and environmental conditions and that the frequency dependence of the mechanical properties is taken into account, (ii) the LPM is the most simple modeling strategy, but the air volume enclosed by the earmuff must be correctly estimated, which is not as straightforward as it may seem, (iii) similar results are obtained with the SF and the ES FE-models of the cushion, but the SF should be preferred to predict the earmuff acoustic response at low frequency since it requires less parameters and a less complex characterization procedure.

A hybrid finite element-transfer matrix model for vibroacoustic systems with flat and homogeneous acoustic treatments.

Practical vibroacoustic systems involve passive acoustic treatments consisting of highly dissipative media such as poroelastic materials. The numerical modeling of such systems at low to mid frequencies typically relies on substructuring methodologies based on finite element models. Namely, the master subsystems (i.e., structural and acoustic domains) are described by a finite set of uncoupled modes, whereas condensation procedures are typically preferred for the acoustic treatments. However, although accurate, such methodology is computationally expensive when real life applications are considered. A potential reduction of the computational burden could be obtained by approximating the effect of the acoustic treatment on the master subsystems without introducing physical degrees of freedom. To do that, the treatment has to be assumed homogeneous, flat, and of infinite lateral extent. Under these hypotheses, simple analytical tools like the transfer matrix method can be employed. In this paper, a hybrid finite element-transfer matrix methodology is proposed. The impact of the limiting assumptions inherent within the analytical framework are assessed for the case of plate-cavity systems involving flat and homogeneous acoustic treatments. The results prove that the hybrid model can capture the qualitative behavior of the vibroacoustic system while reducing the computational effort.